Hilab volt system for electrolyte measurement: a portable solution for point-of-care testing

Abstract

Point-of-care (POC) diagnostic systems are designed to deliver rapid, reliable results directly at the site of patient care, whether in clinical settings, ambulatory units, field environments, or even at home. These systems are intended for use by non-specialized personnel and feature short turnaround times. Hilab has developed a miniaturized electrochemical platform that offers an integrated solution for POCT testing. In this study, we present the Hilab Volt system, an electrochemical diagnostic platform comprising dedicated hardware, disposable sensors, and user-friendly software that enables the accurate and efficient detection of key analytes. To validate the system’s analytical performance, we report results obtained for a panel of essential electrolytes, K⁺, Na⁺, Ca²⁺, and Cl⁻, demonstrating its potential for clinical application in decentralized testing environments. The recovery ranges (n = 67) were 84–112%, 94–101%, 80–125%, and 97–104% for K⁺, Na⁺, Ca²⁺, and Cl⁻, respectively. For all sensors, satisfactory results were obtained (r > 0.8 and p > 0.8) using control samples. The Hilab Volt system demonstrated reliable analytical performance for key electrolytes, with recovery values and correlation metrics supporting its suitability for point-of-care use. Its miniaturized design, ease of operation, and accurate detection capabilities make it a promising solution for decentralized clinical testing, enabling rapid and accessible diagnostics across diverse care settings.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-54077-z.

Introduction

Point-of-care testing (POCT) enables rapid diagnostic results to be obtained by non-specialized personnel directly at the patient site, whether in the field, at home, in ambulances, or within clinical settings. Traditionally, diagnostic tests have been performed in centralized laboratories using bench-top analyzers operated by trained professionals, often resulting in delays of several days before results are available^1–3. In response, there has been a growing shift toward diagnostic technologies that provide rapid, on-site results, enabling timely intervention in acute conditions and expanding access to testing in decentralized and home-based settings. It is important to distinguish between fully miniaturized systems, in which both the recognition element and the signal transduction components are integrated into a compact device, and partially miniaturized approaches, in which the assay is reduced in scale, but detection still depends on conventional, bulky instrumentation^4–6.

To overcome the limitations of conventional POCT systems, Hilab has explored several novel technologies to meet their specific requirements^7,8, including electrochemical technologies. Hilab Volt (ANVISA N° 80583710019, Fig. 1A-D) is a new technology that comprises Hilab’s technology portfolio. It is an electrochemical testing platform that uses a single analytical instrument to support multiple electrochemical techniques, enabling the measurement of a wide range of analytes, including electrolytes, transaminases, glucose, and COVID-19¹⁰. The Hilab Volt system operates based on calibrated electrochemical responses tailored to each target analyte. Analytical results are obtained within approximately 10 min using sample volumes of up to 40 µL. The device architecture is compact and integrates low-power electronic components, enabling operation under conditions with limited electrical and laboratory infrastructure. Sample measurements are performed directly after sample application, without additional chemical pre-treatment or complex handling steps. System operation follows a predefined, standardized workflow that minimizes user-dependent variability and does not require specialized technical training. The analytical unit interfaces with disposable sensor cartridges via a mechanical and electrical plug-in connection, ensuring consistent sensor positioning and signal acquisition. Signal processing is performed using embedded algorithms that analyze the measured electrode potentials and predefined quality criteria, such as signal stability and calibration conformity, before result reporting, thereby enhancing analytical robustness.

Fig. 1.

Schematic representation (A) of the Hilab volt, and (B) of the sensor used. Generated using internal rendering tools (software blender V5.0) and figma. Electrolytes, including K⁺, Na⁺, Ca²⁺, and Cl⁻, are essential for maintaining homeostasis in living organisms. Present in both intra- and extracellular compartments, their balanced distribution is critical for fundamental metabolic functions. Due to their ionic nature, electrolytes play a key role in regulating osmotic pressure and fluid balance^9–11.

Potassium (K⁺) is the primary intracellular cation, with a normal serum concentration of approximately 4.0 mM. Given its relatively low extracellular concentration, minor fluctuations can lead to significant physiological effects^9–11. Ionized Calcium (Ca²⁺), the biologically active form of calcium, circulates in extracellular fluids at a concentration of around 1.2 mM. It serves as a crucial cofactor in numerous enzymatic processes, including the coagulation cascade. Additionally, many intracellular and extracellular enzymes require Ca²⁺ for proper function^9–11. Sodium (Na⁺) is the predominant cation in the extracellular space, typically present at a concentration of approximately 140 mM. It is responsible for maintaining extracellular fluid volume and modulating membrane potential, making Na⁺ salts the principal determinants of extracellular osmolality^9–11. Chloride (Cl⁻) ions constitute approximately two-thirds of all extracellular anions, with normal serum levels ranging from 98 to 106 mM. Cl⁻ is essential for maintaining osmotic pressure, regulating fluid distribution, and balancing acid-base levels^9,12.

In this work, a point-of-care diagnostic method for electrolyte quantification is presented, based on proprietary screen-printed electrodes modified with a PVC-based ion-selective layer and operated on the portable Hilab Volt platform using potentiometry. The compact instrumentation and simplified analytical workflow enable electrolyte testing to be performed directly in near-patient settings, reducing reliance on centralized laboratories. This approach facilitates decentralized diagnostics and expands access to clinical testing, particularly in environments with limited laboratory infrastructure.

Methods

Materials

In this study, “control” refers to external commercial samples, not internal device controls.

K⁺ ionophore I (CAS N° 2001-95-8, selectophore reagents), Na⁺ Ionophore X (CAS N° 97600-39-0, selectophore reagents), Calcium Ionophore II (CAS N°74267-27-9, selectophore reagents), high-molecular-weight poly(vinyl chloride) (PVC, CAS N° 9002-86-2, selectophore reagents), Potassium-tetrakis(4-chlorophenyl)borate (KTPB, CAS N° 14680-77-4, selectophore reagents), cyclohexanone (CHA), Proclin-300, Bis(2-ethylhexyl) sebacate (DOS, CAS N° 122-62-3, selectophore reagents), and HEPES buffer (CAS N° 7365-45-9) were purchased from Sigma Aldrich. Potassium chloride (KCl), sodium chloride (NaCl), and calcium chloride (CaCl2) were analytical grade reagents. Commercial samples were purchased from Controllab. Proprietary screen-printed electrodes were used as the basis for the sensor design, with the working and counter electrodes composed of carbon and the reference electrode composed of Ag/AgCl. The electrode areas were precisely defined and limited using a dielectric ink layer, ensuring controlled electrochemical behavior and reproducible sensor performance.

The sensing membranes were deposited onto the electrodes with a volume of 2 µL per layer, resulting in membranes with an estimated thickness of a few micrometers (~ 2–10 μm), depending on solvent evaporation and membrane composition. After fabrication, the electrodes were conditioned for 1 h 30 min before use.

Due to confidentiality agreements and intellectual property restrictions, the detailed composition of certain reagents and specific aspects of the manufacturing process cannot be disclosed. However, all procedures were conducted in accordance with validated protocols, and the system’s performance was rigorously assessed using comparative and analytical validation, as described below.

The Hilab Volt System

Potentiometric measurements are based on potential variation, with quantification relying on the potential variation. To ensure accurate potential variation determination, a stable baseline is first established by applying a proprietary background solution to the electrode membrane. Subsequently, the sample or control is added to the same region, and the resulting potential change is recorded (Fig. 2; Panel A: Steps 3–5, Video S1). The potential variation is automatically calculated by a signal-processing algorithm that identifies the baseline and sample regions, although manual selection is also possible via the user interface (Fig. 2; Panel B). Further details on this process are provided in the Discussion section.

Fig. 2.

(A) Operational steps for the Hilab volt system. (B) Generated graphic from data analysis and AI selection. Generated using internal rendering tools (software blender V5.0) and figma.

Analytical behavior

Precision tests assess the degree of agreement among a series of measurements when a procedure is repeatedly applied to aliquots of a sample. These tests were performed using a repeatability and reproducibility model with different commercial control samples, stabilized serum-like aqueous matrices containing certified concentrations of Na⁺, K⁺, Cl⁻, and Ca²⁺ with preservatives and buffering agents, covering clinically relevant ranges (2.5, 4.5, and 6.0 mM for K⁺; 130, 140, and 150 mM for Na⁺; 1.2, 1.5, and 2.0 mM for Ca²⁺; and 101, 105, and 110 mM for Cl⁻). For each measurement, separate aliquots of the sample were used, and the electrodes were not reused. Repeatability was assessed by performing 10 consecutive measurements at each concentration level using 10 different electrodes. Reproducibility was evaluated by repeating the measurements with two different Hilab Volt devices, following the same procedure. Based on Clinical and Laboratory Standards (CLSI) EP15-A3¹⁷ analysis, average, standard deviation (SD), coefficient of variation (CV; Intra and Inter), random error, total error, and bias were calculated. For Gage R&R analysis, degrees of freedom (DF), sum of squares (SS), mean squares (MS), F value (F), P value (P), variance components, study Var (%SV), and SD were calculated and shown.

Linearity refers to the ability of an analytical method to produce signals that are directly proportional to analyte concentration within each concentration range, thereby exhibiting a linear relationship. Linearity was achieved by measuring increasing concentrations of commercial control samples, with lower concentrations obtained by appropriately diluting the highest-concentration solutions. The tested ranges were: 1 to 10 mM for K⁺, 50 to 200 mM for Na⁺, 1 to 3.6 mM for Ca²⁺, and 50 to 300 mM for Cl⁻. From the study, the detection limit (estimated from the intersection of two extrapolated linear segments of the calibration curve) and the linear dynamic range (LDR) of the methodology were determined^18,19.

An interference study was conducted to evaluate the potential impact of other ions on the measurement of the target analyte and the systematic error introduced under abnormal sample conditions. Unlike traditional ISE selectivity studies, which calculate a selectivity coefficient (K) for each interfering ion, this study aimed to assess whether the sensor’s response and measured value remained consistent when non-target ions were present at pathologically altered concentrations while the target ion remained within its normal physiological range. Measurements were performed under two conditions: (i) all ions at normal physiological concentrations and (ii) selected non-target ions at elevated or reduced concentrations outside their normal ranges. For the interference studies, the following concentrations of potentially interfering ions were added to the analyte solutions: Potassium interference study: 150 mM Na⁺, 1.35 mM Ca²⁺, and 110 mM Cl⁻; Sodium interference study: 6.50 mM K⁺, 1.35 mM Ca²⁺, and 110 mM Cl⁻; Ionized calcium interference study: 6.50 mM K⁺, 150 mM Na⁺, and 110 mM Cl⁻; Chloride interference study: 6.50 mM K⁺, 150 mM Na⁺, and 1.35 mM Ca²⁺. For each sensor, the coefficient of variation (CV) and bias were calculated and compared between these conditions to determine the influence of the altered ionic environment on the sensor’s performance.

An accelerated long-term storage stability study was performed by storing the ion-selective sensors in an incubator maintained at 45 °C (± 5 °C), a temperature selected to accelerate degradation processes relative to ambient storage conditions. Baseline measurements were recorded on day 0, followed by stability assessments on days 20, 39, 59, and 78. The final time point of 78 days was defined based on an accelerated aging model using the Q10 approach, which allows extrapolation of sensor shelf life at room temperature from elevated-temperature storage. According to this model, storage at 45 °C for 78 days corresponds to 12 months of storage under typical ambient conditions, assuming a Q10 value of 2.0. At each stability time point, measurements were performed for each sensor using the same solution at two concentration levels for each analyte: 4.2 and 6.0 mM for K⁺; 135 and 149 mM for Na⁺; 1.02 and 2.0 mM for Ca²⁺; and 100 and 110 mM for Cl⁻. Sensor performance was considered acceptable when the measured signal retained at least 90% of its initial response over the evaluated storage period.

The parameter equation for the study was as follows:

In which the Q10 = degradation rate was 2.0, D1 = shelf life or duration at temperature T1; D2 = shelf life or duration at temperature T2; T = temperature 1 (low) or 2 (high).

Spiked Commercial Samples

CLSI definitions supported the sample size calculation for the study. The Kolmogorov–Smirnov normality test was used to verify that the data met the assumptions of parametric tests. Commercial whole blood was used for all experiments. The Preliminary Clinical Evaluation study employed a recovery study and Passing-Bablok regression due to their robustness against measurement errors, which provided estimates of proportional and systematic differences. The significance level was set at p ≤ 0.05; no p-value adjustment for multiple tests was necessary, and all analyses encompassed the 95% confidence interval (95% IC). All tests were double-blinded, analyzed, and plotted using the R software (version 4.2.1) statistics package.

Results and discussion

The Hilab Volt - Device architecture and design considerations for point-of-care testing

The Hilab Volt was developed as a point-of-care testing (POCT) platform, with design choices guided by the ASSURED criteria proposed by the World Health Organization for decentralized diagnostics, particularly simplicity of operation, robustness, and suitability for use outside conventional laboratory environments. The device architecture and user interface were designed to minimize user-dependent variability and to support use by non-specialized personnel in primary care and near-patient settings. Figure 1A presents the external configuration of the device, and Fig. 1B illustrates the layered structure of the disposable electrochemical sensor. The hardware architecture consists of a WiFi-enabled microcontroller integrated with a dedicated analog front-end. The microcontroller is responsible for system control, communication with the central analytical platform, and power management. Battery-powered operation was selected to facilitate deployment in settings with limited infrastructure and to reduce susceptibility to electrical noise from mains-powered laboratory equipment, consistent with POCT deployment requirements. The analog front-end performs signal conditioning, acquisition, and analog-to-digital conversion of the potentiometric signals generated by the ion-selective electrodes. For each assay, analyte-specific measurement protocols are transmitted to the device, defining acquisition parameters and timing. The digitized signals are processed using predefined algorithms to extract stable potential values, which are subsequently transmitted for result calculation and reporting. This architecture enables standardized signal acquisition while remaining compatible with decentralized, near-patient testing workflows.

International health agencies emphasize that diagnostics intended for decentralized use in low- and middle-income countries (LMICs) must meet strict affordability thresholds to ensure scalability and sustainability. The WHO-endorsed ASSURED framework explicitly identifies affordability as a core requirement for point-of-care diagnostics²⁰, and WHO target product profiles for peripheral, instrument-based tests recommend capital costs below USD 2,000 and per-test consumable costs ideally ≤ USD 2²¹. In contrast, conventional benchtop electrochemical analyzers typically require initial investments of USD 20,000–50,000, in addition to laboratory infrastructure and maintenance, with consumable costs of USD 3–7 per test²². Likewise, many commercially available electrochemical POCT platforms remain priced between USD 5,000 and USD 12,000, with per-test consumable costs typically ranging from USD 5 to 8. When additional operational expenses are considered — such as trained personnel, routine maintenance, quality control procedures, and device calibration — the total cost per reported result increases substantially. These cumulative costs represent a significant barrier to widespread adoption and sustainable implementation of such technologies in low- and middle-income countries (LMICs)^22,23. Within this context, the Hilab Volt targets an instrument cost below USD 2,000 and disposable kits priced under USD 3 per test. Although slightly above the most stringent per-test targets proposed by WHO, this cost structure substantially lowers economic barriers compared with conventional laboratory systems and many existing POCT platforms, and aligns with WHO’s affordability recommendations^23,24.

Sensor, analysis, and system operation

The developed method is based on potentiometric measurements using ISE, in which the analytical signal arises from variations in electrical potential generated at the sensor–solution interface^25,26 These potential changes result from charge-separation processes driven by the target ion’s activity in the sample. To enable this measurement, the system comprises at least two electrodes: a reference electrode with a stable and constant potential, and a working ion-selective electrode coated with a selective membrane containing an ionophore specific to the analyte, K⁺ (ANVISA N° 80583710030), Na⁺ (ANVISA N° 80583710032), Ca²⁺ (ANVISA N° 80583710036) or Cl⁻ ions (ANVISA N° 80583710052), which will indicate the concentration of the analyte.^25,26. The resulting potential difference between the reference and ion-selective electrodes follows the Nernstian response and provides a quantitative measure of the analyte concentration in the sample^25,26. Video S1 illustrates the step-by-step process, and Fig. 2 Panel B shows the measurement result.

After insertion of the sensor into the Hilab Volt, the test is initiated by applying a defined background solution to the electrode region. This solution is formulated to condition and stabilize the electrode surface by establishing a controlled ionic strength and protein environment at the membrane–solution interface before sample addition. By preconditioning the electrode surface, this step ensures a predictable electrochemical response, as ionic strength directly influences the relationship between ion activity and measured potential, and a controlled background minimizes contributions from non-target ions, thereby reducing matrix-related interferences²⁶.

The background solution is retained within the absorptive pad integrated into the sensor architecture, where it equilibrates with the ion-selective membrane without causing dilution of the subsequently added sample. This controlled wetting of the electrode surface establishes a stable baseline (“zero”) potential, such that subsequent changes in potential predominantly arise from the target analyte introduced with the sample. This preconditioning step is critical for achieving stable, linear, and reproducible potentiometric responses when analyzing complex biological matrices.

Following surface conditioning, the sample is dispensed into the same well, and the Hilab Volt records the resulting potential variation (Fig. 2, Panel B). Because the internal composition of the sensor remains constant, the observed potential change reflects the activity of ions present in the sample. This relationship enables quantification of the analyte concentration in the patient sample based on the measured potential shift.

Signal variation is processed automatically by embedded algorithms that calculate the potential difference relative to the conditioned baseline and compare the result against reference values established using a gold-standard method. By minimizing deviation from these reference values, the algorithm enhances analytical accuracy and supports consistent sensor performance.

Thus, based on information from the system interface and dual-verified by the signal detection algorithm and an expert reviewer, the system delivers diagnostic results comparable to those obtained with conventional reference systems, as discussed in the following section.

Analytical performance

Analytical behavior was assessed through tests for linearity, precision, specificity, and long-term stability.

Precision tests

Repeatability and reproducibility of the Hilab Volt system were evaluated using commercial control solutions across three clinically relevant concentration levels. The performance metrics are summarized in Table 1 and were interpreted in accordance with the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) analytical performance specifications derived from biological variation data²⁷.

Table 1.

Results obtained for precision study.

	K⁺			NA⁺			CA²⁺			CL⁻
	Level 1	Level 2	Level 3	Level 1	Level 2	Level 3	Level 1	Level 2	Level 3	Level 1	Level 2	Level 3
Grand mean	2.36	4.70	5.86	131	139	148	1.07	1.51	1.81	102.4	105.1	107.7
Total standard deviation	0.29	0.38	0.44	1.77	0.36	2.97	0.09	0.12	0.23	0.24	0.94	1.80
Coefficient of variation inter	12.06%	7.53%	7.64%	0.96%	1.01%	1.91%	10.10%	9.08%	12.01%	0.96%	1.01%	1.91%
Coefficient of variation intra	10.72%	8.17%	7.46%	1.35%	1.64%	2,01%	8.75%	8.25%	12.51%	0.87%	0.90%	1.67%
Random error	13.25%	13.49%	12.31%	2.22%	2.70%	3.32%	14.44%	13.61%	20.64%	1.44%	1.48%	2.75%
BIAS	2.80%	0.07%	− 2.41%	1.00%	− 0.43%	− 0.95%	− 8.16%	− 5.53%	− 9.68%	1.39%	0.12%	− 2.07%

Among the evaluated sensors, Na⁺ and Cl⁻ sensors demonstrated the best performance, with low inter- and intra-assay CVs, minimal random error, and slight bias, indicating robust precision and accuracy within clinical acceptability. Both analytes met the Clinical Laboratory Improvement Amendments of 1988 (CLIA’88) total allowable error (TEa) limits for ISE methods (± 4 mM for Na⁺ and ± 5% for Cl⁻) and fell within the EFLM minimum performance specifications. The K⁺ sensor showed higher variability, with inter- and intra-assay CVs of 7.5–12%, respectively, and a BIAS of ± 2.4%, which exceeds the EFLM desirable limit for imprecision and approaches the CLIA TEa threshold (± 5%). The Ca²⁺ sensor demonstrated the weakest analytical performance, characterized by high imprecision (coefficients of variation ranging from 8.2% to 12.5%), elevated random error (14–20%), and a systematic negative bias of − 5.5% to − 9.7%. These performance metrics exceed the desirable analytical specifications proposed by the EFLM (≤ 1.3%) and also surpass the CLIA total allowable error (TEa) for calcium (± 0.05 mM).

A nested Gage R&R analysis (Tables S1–S4) was performed to evaluate the internal consistency of the measurement system, focusing on repeatability (within-device precision), reproducibility (operator-related variability), and part-to-part variation (biological variability among samples). As expected, part-to-part variation was the dominant source of variability for all analytes (76–93% of total variance), indicating that most observed variation reflects true biological differences rather than measurement system instability.

Repeatability and reproducibility contributed minimally for K⁺ and Na⁺ sensors, supporting adequate internal precision for POCT applications. In contrast, Ca²⁺ and Cl⁻ sensors showed higher measurement-related variability (Gage R&R ≥ 17%), primarily driven by within-device imprecision. While higher than typical laboratory-based systems, these values are within ranges reported for point-of-care devices and must be interpreted in the context of their intended use. Importantly, Gage R&R is a measure of internal reliability, and thus the critical requirement is that measurement variability remains sufficiently low to ensure consistent results within the device. In this regard, internal consistency was evaluated against clinically relevant thresholds. For the analytes studied, the observed variability corresponds to concentration deviations below approximately 8%, which does not compromise the ability to classify results relative to established clinical decision limits.

A statistically significant sample–operator interaction was observed for all analytes except Cl⁻, indicating that procedural factors may still contribute to variability. Further standardization, such as controlled sample volume application, defined timing between background solution and sample addition, fixed equilibration times, and consistent sensor handling, may reduce these effects and improve internal consistency.

While conventional analytical performance specifications are derived from centralized laboratory standards, these may not fully translate to POCT systems. As highlighted by Oosterhuis et al.²⁸, performance goals should be aligned with clinical decision-making requirements. In this context, internal consistency must be sufficient to ensure reliable classification of results rather than strict equivalence to laboratory-grade precision.

Overall, the Hilab Volt system demonstrates internal consistency compatible with its intended use, particularly for Na⁺ and K⁺, where measurement variability remains limited relative to biological variation. Although further improvements in precision are desirable, the current level of reproducibility supports reliable clinical interpretation in primary care and screening settings.

Linearity and limit of detection calculation

The CLSI EP06-A²⁹ guideline was used to evaluate the linearity of the Hilab Volt system. Serial dilutions of high-concentration commercial control solutions were prepared in the background solution to generate multiple levels across the expected clinical range for each analyte. For each analyte, a minimum of five concentration levels were analyzed in five replicates, each performed using distinct sensors, to assess linearity. The relationship between the sensor response (measured potential) and the logarithm of ion concentration was evaluated.

The results summarized in Table 2 demonstrate excellent linearity for all analytes, with regression analysis yielding strong correlation coefficients (R² > 0.99 for all sensors). The sensors exhibited near-Nernstian responses for the monovalent cations K⁺ (58.6 mV/dec) and Na⁺ (66.6 mV/dec). For Ca²⁺ (36.5 mV/dec) and Cl⁻ (79.1 mV/dec), slopes higher than the theoretical Nernstian values were observed, a behavior previously reported for ion-selective electrodes and commonly attributed to membrane composition, ion-exchange dynamics, or interfacial effects³⁰. Such deviations are frequently attributed to factors including membrane composition, ion-exchange equilibria, interfacial capacitance effects, and activity-related phenomena, rather than to fundamental flaws in sensor operation. Importantly, the observed slopes were highly reproducible, accompanied by excellent linearity, and did not compromise analytical accuracy, as confirmed by recovery, linearity, and agreement with the reference method. With respect to chloride, the slope is reported as a positive value for consistency with the calibration plots; however, this results from the inversion of the electrode configuration. In physical terms, the chloride-selective electrode exhibits the expected inverted (negative) potentiometric response, in accordance with the Nernst equation, in which the electrode potential decreases with increasing anion activity. Although slopes exceeding the theoretical Nernstian values were observed for Ca²⁺ and Cl⁻, such behavior has been previously reported for ion-selective electrodes employing polymeric membranes and solid-contact architectures. When the slope remains linear, reproducible, and within literature-reported ranges, it is generally considered acceptable for analytical applications, particularly in point-of-care devices. In this study, the observed slopes did not compromise sensor linearity, stability, or analytical accuracy, supporting their suitability for the intended use. Additionally, the limits of detection (LOD) were estimated at the intersection of two extrapolated linear segments of the calibration curve, as recommended by analytical performance evaluation guidelines¹⁹. The values of 0.33, 10, 0.55, and 30 mM were reached for K⁺, Na⁺, Ca²⁺, and Cl⁻ sensors, respectively.

Table 2.

Results obtained for the linearity study.

Analyte	Regression	LDR (mM)	LOD (mM)
K+	∆E = 209.5 + 58.6 pK	1.0–10.0	0.33
Na+	∆E = 118.6 + 66.6 pNa	50–200.0	10.0
Ca2+	∆E = 246.0 + 36.5 pCa	1.0–3.6	0.55
Cl−	∆E = 30.6 + 79.1pCl	50–300	30.0

These results confirm that the Hilab Volt system maintains linear response characteristics across clinically relevant concentration ranges for all tested electrolytes, meeting the requirements for quantitative use in clinical diagnostics.

Interference study

The use of a background solution specifically formulated for the tests described in this study ensures that the electrode interface mimics the physicochemical conditions of the sample matrix. This is particularly important because ionic strength directly influences the activity-to-concentration relationship in ISE measurements^30,31. By closely matching the ionic environment, the background solution helps stabilize the sensor interface and minimize potential interference from other ionic species in the sample³¹.

This stabilization contributes to a more reliable electrochemical response, allowing the sensor to maintain a linear correlation between the logarithm of analyte concentration and the measured potential, as described by the Nernst equation³². As presented in Table S5, the coefficient of variation (CV%) and bias (%) for all analytes remained below 10%, which is in line with analytical performance goals for imprecision based on biological variation and ISE method guidelines (BIAS < 5.7%; CV < 12%)^19,33. These findings indicate that non-specific ionic interference was minimal and that the background solution effectively supports consistent and accurate sensor performance.

Long-term storage stability

An accelerated stability study was conducted to assess the robustness of the test system under elevated-temperature conditions, following the recommendations of CLSI EP25³⁴ for evaluating the accelerated stability of IVD reagents. Control sensors were stored under stress conditions, and potential variations and concentration were monitored over time to assess signal preservation and sensor functionality.

As shown in Table S6, the variation in the signal obtained after 78 days at elevated temperatures for K⁺, Na⁺, and Cl⁻, and after 59 days for Ca²⁺, did not exceed ± 15%, indicating no statistically significant impact on the sensor responses and demonstrating strong stability and minimal degradation. Based on these findings and using standard extrapolation models described in EP25³⁴, the results support a real-time stability claim of up to 365 days at room temperature for K⁺, Na^+, and Cl⁻, assuming the product is stored under recommended conditions. For Ca²⁺, a 6-month real-time stability was achieved.

Although temperature fluctuations of ± 5 °C were observed during accelerated aging, the Arrhenius analysis was performed using the average recorded temperature. The resulting lifetime extrapolation should therefore be interpreted as an estimated trend rather than an absolute prediction, consistent with the intended use of accelerated aging models.

These results demonstrate that the Hilab Volt system maintains adequate analytical performance over time, meeting the necessary stability requirements for reliable clinical use throughout its intended shelf life.

Preliminary clinical evaluation using spiked control sample

A recovery study was conducted to evaluate the accuracy of the analytical method by measuring the extent to which known electrolyte concentrations can be correctly detected after being added (spiked) to a sample matrix. An acceptable recovery typically falls within 90–110%, depending on the analyte’s clinical relevance, the method’s precision, and the analyte’s concentration range. Control samples were fortified with known quantities of each analyte (KCl, NaCl, and CaCl₂) at clinically relevant concentrations. These spiked samples were then analyzed using the Hilab Volt system, and the measured values were compared to the expected (theoretical) values.

The results obtained in this study (Table 3; Table S7) showed recovery percentages within the acceptable range for all analytes, indicating that the system provides accurate and reliable quantification of electrolytes in blood-based matrices. For K⁺, recoveries < 90% were seen mainly at the lowest (2.5 mM), which aligns with the expectation of greater variability near extreme ranges (where signal/noise ratios drop). Ca²⁺ exhibited the widest variability, with recoveries from 80.0% to 125.0%, particularly at low concentrations (0.8–1.2 mM). This broad range reflects the greater analytical challenges of Ca²⁺ detection. The platform exhibits excellent performance for Na⁺ and Cl⁻, with narrow recovery ranges and low error rates.

Table 3.

Results obtained for the recovery study.

Analyte	Range of recovery (%)	Total error average (%)
K+	84–112	6.05
Na+	94–101	2.75
Ca2+	80–125	6.87
Cl−	97–104	2.12

When comparing the TEa to the strict limits set by the EFLM²⁷, especially for Na⁺ and Ca²⁺, it slightly exceeded the recommended total error thresholds. This is not uncommon in portable, POCT devices, which often prioritize speed and usability over laboratory-level precision. Despite these deviations, the system consistently demonstrated reliable results, supporting the suitability of the method for use in POCT environments.

Figure 3 (panels A–D) presents Passing–Bablok regression analyses comparing measured results and spiked concentrations for K⁺ (A), Na⁺ (B), Ca²⁺ (C), and Cl⁻ (D) using the Hilab Volt system.

Fig. 3.

Correlation graphs for preliminary clinical evaluation using a spiked control sample for: (A) K⁺ sensor; (B) Na⁺ sensor; (C) Ca²⁺ sensor; and (D) Cl⁻ sensor.For the K⁺ sensor (A), the regression equation (y = − 0.25 + 1.06x, r = 0.982) showed a slope close to unity, a small intercept, and strong correlation, with predicted bias ranging from − 1.14% to + 1.83% across clinically relevant concentrations (3.5–6.0 mM)³⁵. The Na⁺ sensor (B) exhibited a regression slope and intercept numerically close to unity and zero, respectively (r = 0.918), indicating minimal systematic deviation from the reference method within the limits of analytical resolution; however, confidence intervals widened slightly at higher concentrations³⁵. The Ca²⁺ sensor (C) demonstrated strong correlation (r = 0.971) and a slope near unity (1.04), with a small but consistent negative bias (− 4.00% to − 1.33%), suggesting a tendency toward underestimation across the tested range (1.0–2.0 mM)³⁰. The Cl⁻ sensor (D) showed the weakest correlation (r = 0.876) and evidence of both proportional and constant bias (intercept = 34.86), with predicted biases ranging from + 2.69% at lower concentrations to − 2.49% at higher concentrations, indicating a systematic deviation from the reference relationship³⁵.

The narrow confidence bands observed for K⁺ and Ca²⁺ indicate high consistency. In contrast, the broader intervals for Na⁺ and, especially, Cl⁻ suggest greater variability in measurement at the extremes of the tested range. Overall, the regression results confirm strong analytical agreement for the proposed sensors.

Table 4 compares the described work with other sensors reported in the literature^36–45. Compared with previously reported POCT systems for electrolyte determination, the Hilab Volt platform offers distinct advantages in sample compatibility, integration, and clinical applicability. While many existing devices rely on serum, sweat, or synthetic matrices and often remain in prototype or research-phase development, the Volt system enables accurate detection of key electrolytes directly from whole blood using minimal sample volume. Unlike optical or microfluidic methods, Volt uses a fully integrated electrochemical approach with disposable sensors and user-friendly software, enabling proper bedside or decentralized testing. Additionally, its performance demonstrates strong agreement and operational suitability in real-world clinical environments. This positions the Volt platform as a practical and scalable solution for POCT diagnostics, especially in settings where rapid, low-volume testing is critical.

Table 4.

Comparative table of literature reported sensors and the volt system.

Analyte	System	Detection method	Sample type	Sample volume	Linear range/LOD	Price/USD	Key challenges/limitations	Refs.
K⁺	Solid-state screen‑printed potentiometric ISE	Potentiometric ISE (valinomycin‑based)	Serum	NI	0.5–10 mM LOD 0.3 µM	NI	Stability > 50 days but requires calibration; not whole‑blood; reference electrode integration	36
	Impedance sensor	Impedance‑based ionophore membrane	Solution samples (model)	NI	2.7–18.7 mM LOD 1 µM	NI	Ex‑lab testing; no whole‑blood or clinical validation; needs further miniaturized hardware	37
	Commercial portable devices (i‑STAT, Epoc)	ISE-based (clinical benchtop portable)	Capillary/venous whole blood	~ 60–90 µL	Clinical range	Eq: 5000–8000 Test: 5–8	Requires > 60 µL → finger‑prick haemolysis; high cost; professional use only	38
	Microneedle‑based potentiometric patch (ex vivo)	Solid‑state potentiometric ISE on microneedles	Interstitial fluid (animal skin ex vivo)	Minimal (interstitial fluid)	1–8 mM range LOD 10⁻⁴ M	NI	In vivo performance, ISF‑blood correlation unknown; clinical validation pending	39
	VOLT K⁺	Solid‑state potentiometric ISE	Commercial control samples; Potential for whole blood and serum samples	40 µL	1–10 mM / LOD: 0.33 mM	Eq: 2000 Test: 3	Lithium-Heparin as a unique whole blood conservative	**
Na⁺	Radiometer Blood Gas Analyzer (BGA) in ED	Electrochemical ISE in blood gas analyzer	Venous blood (BGA)	70 µL	Clinical	NI	Requires reagents and micro-optical readout; serum only	40
	Wearable Sweat-based Na⁺ ISE sensor	Solid-state Na⁺-ISE on wearable substrate (amperometric)	Sweat (wearable)	NI	20–120 mM	NI	Tested in sweat only; ISF/blood correlation unverified; preclinical	41
	VOLT Na⁺	Solid‑state potentiometric ISE	Commercial control samples. Potential for whole blood and serum samples	40 µL	50 to 200 mM LOD 10 mM	NI	Lithium-Heparin as a unique whole blood conservative; Interference from sample dilution errors	**
Ca²⁺	i‑STAT CHEM8+ / Alinity	Ion-selective electrode potentiometry (electrochemical)	Whole blood () from a capillary or venous draw	~<100 µL	Clinical range	Eq: 5000–8000 Test: 5–8	Occasional slight bias depending on sample anticoagulant; requires cartridge-based single-use sensors	42
	EPOCT Blood Analysis System	ISE potentiometry, integrated BGEM panel	Whole blood via handheld test cards (~ capillary/venous)	< 90 µL	Clinical range LOD 0.10 mM	Eq: 10,000 Test: 5–8	Proprietary cards; cost per test, not for direct home use	43
	Reusable pocket-sized device	Portable electrochemical POCT device (prototype)	Venous & capillary blood, saliva		Clinical range	NI	Prototype stage; lower precision vs. clinical lab; saliva measurements are less reliable	44
	VOLT Ca²⁺	Solid‑state potentiometric ISE	Commercial control samples; Potential for whole blood and serum samples	40 µL	1–3.6 mM LOD 0.55 mM	Eq: 2000 Test: 3	Lithium-Heparin as a unique whole blood conservative; Strong pH dependence	**
Cl⁻	Paper‑based screen‑printed voltammetric sensor	Ag/AgCl working electrode on disposable paper SPE, reagent‑free voltammetry	Serum or sweat; low µL scale	~a few µL	10⁻⁶–1 mol/L LOD ~ 1 µM-level	NI	Serum/sweat only; not yet whole‑blood; optical/color interference minimal	45
	VOLT Cl⁻	Solid‑state potentiometric ISE	Commercial control samples. Potential for whole blood and serum samples	40 µL	50–300 mM / LOD 30 mM	Eq: 2000 Test: 3	Low signal amplitude compared to cations; Requires a stable baseline	**

** This work. NI: non-informed. ISE: Ion Selective Electrode. POC: Poínt of care. BGA: Blood Gas Analyzer. Eq: Equipment.

Conclusions

This study presents a convenient, sensitive, and rapid potentiometric system for POCT, based on the Hilab Volt platform. The system comprises compact instrumentation, ion-selective sensors, and an intuitive user interface that streamlines the entire analytical process. The quantification of electrolytes in commercial control samples showed strong agreement with the reference method and minimal deviation, supporting its applicability in clinical diagnostics. The developed sensors exhibited near-Nernstian behavior across a broad dynamic range, along with high sensitivity, good precision, and specificity. Additionally, the system showed acceptable accuracy and stability under long-term storage conditions, meeting key performance criteria for decentralized testing. The recovery and linearity studies conducted for K⁺, Na⁺, Ca²⁺, and Cl⁻ confirm the analytical accuracy and reliability of the Hilab Volt system for the determination of electrolytes in blood-based matrices. All analytes exhibited recovery rates within the acceptable range, supporting the method’s validity for clinical use. The sensors demonstrated strong linear correlations between spiked and measured concentrations, with slopes near unity and minimal intercepts, indicating low systematic bias and high analytical precision.

Despite its portability and ease of use, the Hilab Volt system presents certain limitations when compared to other POC and laboratory-based analyzers. One drawback is its reliance on single-sample processing, which limits throughput and may hinder its applicability in high-demand settings, such as emergency departments or mass screening scenarios. Another consideration is the reliance on embedded software and cloud-based data transmission, which, while innovative, introduces potential challenges in device synchronization, software updates, and quality assurance across multiple decentralized locations. Furthermore, each assay on the Hilab Volt has a defined analytical range, which may not cover critical values encountered in intensive care or neonatal patients, potentially requiring confirmatory testing by central laboratories. Additionally, although the device showed high agreement with a reference analyzer under controlled conditions, external validation with other reference devices and in diverse clinical environments is required to confirm robustness and reproducibility outside specialized centers.

It is also important to note that hemolysis effects were not evaluated in this study and are therefore considered outside its scope. Given the known influence of hemolysis on electrolyte measurements, particularly potassium and ionized calcium, future studies should systematically investigate the impact of hemolytic samples on sensor performance to further strengthen clinical applicability.

Importantly, this POCT device is not conceived to compete directly with the reference laboratory technique but to complement conventional methods by providing an immediate response when rapid clinical decisions are required. While analytical parameters such as sensitivity, limit of detection, and linear range are comparable between the two, the POCT device may yield less precise results. However, its rapid turnaround can significantly impact triage of life-threatening conditions. Thus, the relative importance of analytical performance must be evaluated in the context of intended use. The Hilab Volt system has the potential to facilitate the early detection of critical medical conditions, prevent complications, improve patient outcomes, and ultimately reduce healthcare costs through timely intervention.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

A. Gevaerd: Conceptualization, Investigation, Supervision, Formal analysis, Validation, Writing - Original draft. E.A. Carneiro, and J.L. Gogola: Investigation, Formal analysis, Validation. A. Gasparin: Supervision, Writing. (A) Timm, J.V. Predebon, L.F. Hartmann, and T. M. Lourenço: Device Investigation. (B) M.M. Almeida: Project administration, Writing. M. V. M. Figueredo and S. R. Rogal Júnior: Project administration, Resources, Writing. All authors have read and approved the final manuscript.

Funding

This study was supported by research funding from Hi Technologies to M.V.M. Figueredo and S.R. Rogal-Junior.

Data availability

All data generated are available from the corresponding author upon reasonable request.

Declarations

Competing interests

M. V. M. Figueredo is the CEO at Hilab; S. R. Rogal Júnior is the CTO at Hilab; A. Gasparin is R&D manager at Hilab; A. Gevaerd is electrochemical manager at Hilab; E. A. Carneiro and J. L. Gogola are health researchers at Hilab; L. F. Hartmann is head of R&D at Hilab; J. V. Predebon is engineering manager at Hilab; T. M. Lourenço and (A) Timm are R&D researchers at Hilab; (B) M. M. Almeida is medical director at Hilab.