Protocol for a laboratory-developed chromogenic anti-Xa assay for rivaroxaban monitoring

Abstract

Rivaroxaban monitoring is essential in specific clinical scenarios, such as renal impairment or emergency surgery. While LC-MS/MS is the gold standard, its high-cost limits accessibility. This study describes the development and validation of a cost-effective, laboratory-developed chromogenic anti-Xa assay suitable for automated platforms. The assay utilizes the competitive inhibition of Factor Xa, where residual enzyme activity is measured via the cleavage of a specific chromogenic substrate, with absorbance being inversely proportional to the rivaroxaban concentration.

Dual-Range Quantification: Validated for both routine therapeutic levels (30–500 ng/mL) and an extended range (500–1500 ng/mL) via automated dilution.

Kinetic Optimization: Employs a specific 180-second incubation and a stabilized measurement window (20–80 s) to maximize linearity.

Reference Comparability: Demonstrates high correlation with LC-MS/MS (r = 0.997) and meets CLSI precision requirements.

Graphical abstract

Specifications table

Subject area	Medicine and Dentistry
More specific subject area	Clinical Chemistry and Hematology — Hemostasis and Thrombosis
Name of your method	Laboratory-Developed Chromogenic Anti-Xa Assay
Name and reference of original method	PMID: 23,846,172 DOI: 10.1160/TH13–04–0274
Resource availability	Instrumentation: XL-3200c Automated Coagulation Analyzer (ZONCI). Reagents: Bovine Factor Xa and Chromogenic substrate S-2732 (Aglyco, China); Rivaroxaban standard (NIFDC, China). Software: IBM SPSS Statistics 27.0; Python 3.9 (Matplotlib/Seaborn) for regression analysis.

Background

Rivaroxaban is a direct Factor Xa inhibitor recommended as a first-line therapy for stroke prevention in atrial fibrillation and Venous Thromboembolism (VTE) management [1,2]. With the expiration of core patents and the introduction of generic formulations, its use is rapidly expanding in developing countries [[3], [4], [5]]. While routine monitoring is not mandatory, precise plasma concentration assessment is critical in specific scenarios, such as renal impairment, advanced age, severe bleeding, or emergency surgery [6]. However, resource-limited settings face diagnostic challenges: Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) is precise but expensive and time-consuming, while routine Prothrombin Time (PT) testing lacks quantitative accuracy [7]. In comparison, the chromogenic anti-Xa assay has emerged as the preferred clinical choice due to its balance of speed and precision [8].

To address the need for accessible diagnostics, this article describes a protocol for a cost-effective, laboratory-developed chromogenic anti-Xa assay. Based on the inhibition of Factor Xa activity, this method utilizes a specific chromogenic substrate where absorbance is inversely proportional to drug concentration [9]. The procedure details the establishment of a double-range measurement system, covering both low and high therapeutic concentrations. This protocol provides step-by-step instructions for reagent preparation, calibration, and assay execution, offering a rapid and precise alternative to commercial kits. It is designed to facilitate the implementation of individualized Rivaroxaban monitoring in clinical laboratories with limited budgets or access to high-end instrumentation.

Method details

Sample collection and plasma preparation

To ensure the pre-analytical quality of samples, strictly follow these procedures:

• •Venipuncture: Perform venipuncture in the antecubital fossa using a 21-gauge butterfly needle. Apply the tourniquet for less than 1 min to minimize hemoconcentration (Discard the initial 2–3 mL of blood to prevent tissue factor contamination from the puncture site).
• •Anticoagulation: Collect blood into vacuum tubes containing 0.109 mol/L (3.2 %) sodium citrate at a ratio of 1:9 (1 part anticoagulant to 9 parts blood). Gently invert the tubes 3–4 times to mix thoroughly.
• •Plasma Separation: Process samples within 1 hour of collection in accordance with Clinical and Laboratory Standards Institute (CLSI) guidelines [10]. Centrifuge at 2000 × g for 15 min at 15 °C to obtain Platelet-Poor Plasma (PPP).
• •Storage: Immediately aliquot the PPP and store at −80 °C. Thaw samples at 37 °C immediately before analysis.

Preparation of buffers and standards

• •Tris–HCl Buffer (50 mM, pH 7.4–8.0): Dissolve 3.0285 g of Tris base in 400 mL of ultrapure water. Adjust pH to 7.4–8.0 with HCl and dilute to 500 mL. Store at room temperature.
• •Rivaroxaban Standard Stock Solution: Dissolve Rivaroxaban standard (Purity 99.0 %, National Institutes for Food and Drug Control, China) in Dimethyl Sulfoxide (DMSO) (99 %) to 0.4 mg/mL. Store at −20 °C protected from light.
• •Rivaroxaban Standard Working Calibrators: Dilute the stock solution and mix with Normal Pooled Plasma (NPP).
• •NPP Specification: The NPP used for calibration and dilution was pooled from >80 healthy individuals to minimize individual matrix variation (Note: Ensure the final DMSO concentration is 0.05 % to avoid matrix interference. Aliquot and store at −80 °C. Do not refreeze).

Preparation of optimized reagents

Based on our system optimization, reagents were prepared using a custom formulation designed to maximize stability and linearity. Specific raw materials were sourced to ensure quality:

• •Buffer System: All reagents were prepared in a 50 mM Tris–HCl buffer (pH 7.4–8.0) containing NaCl to adjust ionic strength.
• •Stabilizers: The buffer was supplemented with 0.5 % Bovine Serum Albumin (BSA) and a non-ionic surfactant (e.g., PEG-6000,0.1 %−0.5 %) optimized for enzymatic kinetics.
• •Reagent 1 (R1 - Enzyme): Bovine Factor Xa (Purity 95 %, Aglyco, China) diluted to 1.0 IU/mL in the optimized buffer containing 150 mM NaCl (Optimization Note: 150 mM NaCl maintains the necessary ionic strength for the enzyme's active site).
• •Reagent 2 (R2 - Substrate): Chromogenic substrate S-2732 (Aglyco, China) diluted to 0.25 mmol/L.

Automated assay protocol & optimized kinetics

The assay was validated on the XL-3200c Automated Coagulation Analyzer (Beijing ZONCI Technology Development Co., Ltd., China). The parameters below are adaptable to other open-system platforms but strictly require precise temperature and timing control.

• •Temperature Control: Ensure the analyzer's incubation block is stabilized at 37 °C (Critical Tip: In laboratories with low ambient temperature [<20 °C], ensure reagents are equilibrated to room temperature before loading. If the analyzer cannot maintain the cuvette temperature strictly at 37 °C, the reaction time may need to be experimentally extended, as kinetics are highly temperature-dependent.)
• •Optical Shielding: Ensure the analyzer's cover is fully closed during measurement to prevent stray light interference with the Optical Density (OD) readings.

• Step 1: Mix 10 µL of plasma sample with 50 µL of R1. Incubate the mixture at 37 °C for 180 s.(Optimization Note: Gradient experiments (90–270 s) confirmed that a 180-second duration is necessary for the inhibition of Factor Xa by rivaroxaban to reach a stable plateau.)

• Step 2: Add 50 µL of R2 to initiate the colorimetric reaction (Volume Ratio Sample:R1:R2 = 10:50:50).

• Step 3: Monitor the absorbance at 405 nm. Calculate the change in absorbance (ΔOD) per minute using the data collected strictly within the 20–80 s window post-initiation (Technical Insight: This specific interval was selected as it represents the most stable phase of substrate cleavage).

Dual-range calibration strategy

To accommodate both routine monitoring and emergency overdose assessment, the method employs a dual-range strategy:

• •Direct Mode (Routine): Used for concentrations in the range of 30–500 ng/mL. Samples are analyzed directly without pre-dilution.
• •Automated Dilution Mode (Emergency/Overdose): For specimens exceeding the upper limit of quantification (>500 ng/mL), the system is programmed to perform an automated 1:2.5 dilution.
• •Diluent: Normal Pooled Plasma (NPP, pooled from >80 donors) is used as the diluent to maintain matrix consistency.

Analytical Performance Validation and Method Comparison Protocol

• •
  Precision Profile Construction (CLSI EP05-A3):Preparation: Aliquot high, medium, and low-level quality control (QC) plasma samples and store at −80 °C to ensure stability [11].

  • (1)
    Repeatability (Intra-assay): Thaw one set of aliquots and perform 20 consecutive measurements for each level within a single analytical run.
  • (2)
    Intermediate Precision (Inter-assay): Over 20 consecutive working days, thaw a fresh aliquot daily and perform 2 runs per day, with 2 replicates per run (n = 80 per level).
  • (3)
    Calculation: Calculate the arithmetic mean, standard deviation (SD), and coefficient of variation (CV=SD/mean × 100 %).
• •
  Sensitivity Limits Determination (CLSI EP17-A2) [12]

  • (1)
    Limit of Blank (LoB): Measure blank plasma samples 60 times over at least 3 separate runs. Calculate LoB = μ_B + 1.645 × σ_B (μ_B is the mean value of blank sample measurements, and σ_B is the standard deviation of blank sample measurements).
  • (2)
    Limit of Detection (LoD): Measure a low-concentration sample 60 times. Verify a detection rate 95 % and calculate LoD = LoB + 1.645 × σ_L (σ_L is the standard deviation of low-concentration sample measurements).
  • (3)
    Limit of Quantitation (LoQ): Test a dilution series daily for 5 days. LoQ is defined as the lowest concentration meeting CV ≤ 20 % and recovery of 80–120 %.
• •
  Linearity and Range Verification (CLSI EP06-Ed2) [13]

  • (1)
    Direct Range (30–500 ng/mL): Prepare 9 evenly distributed concentration levels. Measure each level 3 times and perform linear regression analysis.
  • (2)
    Extended Range (500–1500 ng/mL): Prepare 4 high-concentration levels using a manual 1:2.5 dilution with validated buffer/plasma. Measure each level 3 times.
  • (3)
    Switch-point Validation: Prepare 6 samples near the 500 ng/mL threshold (400, 450, 500, 550, 600, 750 ng/mL). Analyze each using both direct and dilution methods across 3 batches (n = 9) to ensure relative bias ≤ 10 %.
• •
  Method Comparison Study (CLSI EP09c) [14]

  • (1)
    Sample Selection: Collect 80 clinical plasma samples from patients receiving rivaroxaban therapy, covering the quantifiable range.
  • (2)
    Execution: Analyze all samples using the novel anti-Xa assay and reference LC-MS/MS within the same analytical batch to minimize inter-batch variation.
  • (3)
    Statistical Analysis: Perform Pearson correlation (r), Passing-Bablok regression, and Bland-Altman analysis. A paired t-test (α = 0.05) is applied to detect systematic differences.
  • (4)
    Statistical Analysis General statistical analyses were performed using IBM SPSS Statistics 27.0. Python 3.9, utilizing the Matplotlib and Seaborn libraries, was employed specifically for the execution of Passing-Bablok regression analysis and the generation of all analytical figures. For all tests, a two-tailed P < 0.05 was considered statistically significant.

Method validation

The analytical performance of the novel anti-Xa assay was systematically validated according to CLSI guidelines, as illustrated in the experimental design framework (Fig. 1). The comprehensive evaluation confirmed that the assay meets all pre-defined clinical and laboratory acceptance criteria, with the consolidated quantitative results summarized in Table 1.

Fig. 1.

Overview of the experimental design framework based on Clinical and Laboratory Standards Institute (CLSI) guidelines. The validation process encompasses: (Left) Precision and sensitivity assessment, including intra/inter-assay variability and Limit of Detection (LoD); (Middle) Linearity evaluation covering both direct and dilution modes; (Right) Method comparison study using 80 clinical plasma samples to validate the novel assay against the LC-MS/MS reference method. The bottom panel summarizes the pre-defined acceptance criteria for the final evaluation.

Table 1.

Summary of analytical validation results for the anti-Xa assay.

Validation Parameter	Acceptance Criteria	Experimental Result	Status
Intra-assay Precision	CV≤5 %	1.3 % – 2.5 %	Passed
Inter-assay Precision	CV≤10 %	1.5 % – 4.5 %	Passed
Linearity (R2) - Direct	R2≥0.99	0.9981	Passed
Linearity (R2) - Extended	R2≥0.99	0.9985	Passed
Sensitivity (LoQ)	CV≤20 %	30 ng/mL	Passed
Switch-point Bias	Relative Bias≤10 %	2.17 %	Passed
Method Comparison (r)	Pearson r ≥ 0.975	0.99	Passed

Method Comparison with LC-MS/MS: Following the analytical validation, the clinical accuracy of the assay was evaluated through a head-to-head comparison with the reference LC-MS/MS method using 80 patient samples. As shown in the multi-panel analysis in Fig. 2, the novel assay demonstrated good agreement with the LC-MS/MS.

Fig. 2.

Method comparison between the novel chromogenic anti-Xa assay and the reference LC-MS/MS method.

(A) Correlation Analysis: Scatter plot illustrating the linear relationship between the novel assay and LC-MS/MS (n = 72 quantifiable samples), showing a Pearson correlation coefficient of r = 0.9971 (P < 0.001). (B) Passing-Bablok Regression: Regression analysis evaluating systematic bias. The blue line represents the regression line (Y = 0.999X - 0.377), while the shaded area indicates the 95 % confidence interval. The slope near 1.0 and intercept near 0 indicate no significant proportional or constant bias. (C) Bland-Altman Bias Plot: Analysis of the difference between methods against their mean concentration. The solid red line represents the mean bias, and the dashed green lines represent the 95 % limits of agreement (LoA). (D) Distribution of Differences: Histogram with a kernel density estimate curve showing that the distribution of measurement differences is centered near zero, confirming the clinical accuracy of the assay.

Limitations

Since all collected clinical samples were <500 ng/mL, consistent with expected pharmacokinetic distributions, comparison for the diluted measurement range (500–1500 ng/mL) was not performed using patient samples. However, this range was strictly validated using spiked samples, demonstrating full compliance with U.S. Food and Drug Administration (FDA) bioanalytical guidelines [15].

Ethics statements

The study was approved by the Institutional Ethics Committee of The Affiliated Hospital of Qingdao University (Approval No.: QYFY WZLL 30,726) and conducted in accordance with the Declaration of Helsinki. Written informed consent was obtained from all participants.

Related research article

L. Sun, W. Lu, G. Jiang, C. Zhu, H. Rong, L. Lun, Validation of a Chromogenic Anti-Xa Assay for Rivaroxaban Monitoring: Concordance with LC-MS/MS and a Proposed Management Algorithm, Annals of Laboratory Medicine, (Under review).

CRediT author statement

Lin Sun: Validity tests, Data curation, Writing- Original draft preparation. Wenfei Lu: Investigation, Writing- Reviewing and Editing. Guohong Jiang: Visualization, Investigation. Chao Zhu: Software, Validation. Hui Rong: Funding acquisition. Limin Lun: Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Hui Rong is an employee of Beijing ZONCI Technology Development Co., Ltd. The sponsor was involved in study design but had no role in data collection, analysis, interpretation, or manuscript writing. The remaining authors declare no conflicts of interest.

Data availability

Data will be made available on request.