Abstract
Background
The most clinically useful blood ketone in the diagnosis, management, and recovery of diabetes ketoacidosis in both adults and children is 3‐hydroxybutyrate. In the absence of laboratory routine methods, several point‐of‐care methods are in use, but very few clinical evaluations are published.
Methods
This study evaluates linearity and reproducibility of two handheld point‐of‐care meters for blood 3‐hydroxybutyrate measurement for hospital use, Nova StatStrip, and FreeStyle Precision Pro. Whole blood from healthy volunteers was spiked with different concentrations of a 3‐hydroxybutyrate solution and tested on the point‐of‐care instruments. The results were compared with plasma 3‐hydroxybutyrate that was analyzed with a laboratory enzymatic end point spectrophotometric reference method.
Results
Blood 3‐hydroxybutyrate on StatStrip was linear with the reference method up to approximately 4 mmol/L, and FreeStyle was linear up to 6 mmol/L. At higher concentrations, the point‐of‐care instruments gave falsely too low results, especially the StatStrip meter. The FreeStyle meter had better precision and less bias than StatStrip.
Conclusion
In the acute setting of diabetes ketoacidosis, blood 3‐hydroxybutyrate in the higher ranges should be interpreted with caution as the point‐of‐care meters are less accurate there.
Keywords: 3‐Hydroxybutyrate, blood ketones, diabetes ketoacidosis, point‐of‐care testing
1. INTRODUCTION
Ketones, that is 3‐hydroxybutyrate, acetoacetate, and acetone, can be formed in small amounts during fasting or high‐intensity training and in larger amounts during alcohol ketoacidosis, salicylate poisoning, and most importantly during diabetes ketoacidosis.1 Diabetes ketoacidosis is accumulation of predominantly 3‐hydroxybutyrate due to insulin deficiency.2, 3 It is a potentially fatal diabetes complication, and the patient may need acute hospital care. The diagnosis of diabetes ketoacidosis includes measurement of glucose, acid‐base status, and ketones.4, 5 Traditionally, ketones have been identified by acetoacetate on urine dipstick. However, several recent guidelines including the American Diabetes Association (ADA) guidelines and Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus suggest that urine dipstick should be replaced by blood 3‐hydroxybutyrate for earlier and more accurate detection of ketones.5, 7, 8 Joint British Diabetes Society and International Society for Pediatric and Adolescent Diabetes (ISPAD) Clinical Practice Consensus Guidelines have suggested that the biochemical criteria for the diagnosis of diabetic ketoacidosis should be blood 3‐hydroxybutyrate ≥3 mmol/L.4, 9, 10 For monitoring the resolution of ketoacidosis, the target should be reduction of blood 3‐hydroxybutyrate by 0.5 mmol/L/h.9
However, robust methods measuring plasma 3‐hydroxybutyrate are until now not frequently available at hospital laboratories.3 In the absence of laboratory routine methods, point‐of‐care testing (POCT) devices are frequently used in the clinic but there are not many published evaluations on current instruments. This study evaluates two POCT instruments for blood 3‐hydroxybutyrate measurement for hospital use, Nova StatStrip, and FreeStyle Precision Pro, using an enzymatic end point spectrophotometric laboratory method as reference.
2. MATERIALS AND METHODS
Fresh venous blood samples from two healthy volunteers with low values of 3‐hydroxybutyrate were collected in heparinized tubes and pooled. Twelve samples were prepared by adding different volumes of 3‐hydroxybutyrate stock solution in aliquots of the pooled whole blood. The use of human samples was approved by the ethics committee, and the volunteers have signed an informed consent.
3‐Hydroxybutyrate concentrations in whole blood were tested in triplicates on a StatStrip Hospital Connectivity Glucose/Ketone meter (Nova Biomedical, Waltham, MA, USA) and on two FreeStyle Precision Pro meters (Abbott, Chicago, IL, USA). The measuring range was 0.1‐7 mmol/L and imprecision 4.0 CV% at 2.6 mmol/L and 5.2 CV% at 3.1 mmol/L for StatStrip.11 The measuring range was 0.1‐8 mmol/L and imprecision 3.8 CV% at 2.4 mmol/L and 3.1 CV% at 6.3 mmol/L for FreeStyle according to the manufacturer. After POCT analyses, the samples were stored on ice until centrifugation. All samples were centrifuged within 90 minutes, and the plasma was stored frozen until analysis. An enzymatic end point spectrophotometric method at Centre for Inherited Metabolic Diseases, Karolinska University Hospital was used as the reference method. Plasma 3‐hydroxybutyrate concentrations were analyzed in duplicates with the reference method.
3‐Hydroxybutyrate concentrations from the POCT instruments (mean value of triplicates) and the reference method were evaluated with linear regression analysis. Bias between the reference method and evaluated POCT instrument were shown in an altered Bland‐Altman plot. Imprecision was calculated using analysis of variance.
3. RESULTS
Blood 3‐hydroxybutyrate on StatStrip was linear with the reference method up to concentrations of approximately 4 mmol/L, see Figure 1. Higher values showed a negative bias, and the mean deviation from the reference method was −0.9 mmol/L, see Figure 2. Five of 6 values above measuring range of 7 mmol/L were, unexpectedly, measured and showed falsely too low concentrations. Only 1 of 6 values above 7 mmol/L was correctly measured as >7 mmol/L.
Figure 1.
The linear correlation between whole blood 3‐hydroxybutyrate on FreeStyle Precision Pro (light blue and dark blue) and Nova StatStrip (red), respectively, and plasma 3‐hydroxybutyrate measured with the reference method. Each sample is analyzed in triplicates on the POCT meters, and the linear regression and correlation coefficient are based on the mean value of the triplicates
Figure 2.
Difference between the POC instrument and reference method at different concentrations of 3‐hydroxybutyrate. Results from FreeStyle are shown in light and dark blue and StatStrip in red. Blue dashed lines are mean bias for Freestyle and red dashed line marks mean bias for StatStrip
Blood 3‐hydroxybutyrate on FreeStyle was linear with the reference method up to concentrations of approximately 6 mmol/L, see Figure 1. Higher values showed a minor negative bias but the mean deviation from the reference method was only −0.2 mmol/L for one instrument and +0.1 mmol/L for the other instrument, see Figure 2. Samples with 3‐hydroxybutyrate concentrations of 8.4 mmol/L (just above the measuring range of 8 mmol/L) were measured and showed approximately 1 mmol/L too low values.
The within‐run coefficient of variation (CV), based on 3 replicates from a single StatStrip instrument, for the doped samples analyzed, varied from 3% to 43% depending on 3‐hydroxybutyrate concentration, see Table 1. The within‐run CV, based on 3 replicates, on 2 FreeStyle devices each, varied from 0% to 9% depending on concentration. Notably, the samples 11 and 12 were outside the measuring range of the POCT methods and no values were expected there. The laboratory reference method had a median method CV of less than 1% calculated from the duplicate analysis. The imprecision was additionally calculated using control material and analyzed 5 times during 4 different days. The total CV was 20.8% at 0.6 mmol/L with StatStrip, and 12.7% at 0.8 mmol/L and 3.4% at 2.4 mmol/L, with FreeStyle.
Table 1.
Within‐run coefficient of variation (CV %) of the reference method and the POCT methods, respectively
| Spiked samples | 3‐Hydroxybutyrate (mmol/L) by laboratory reference method | Laboratory reference method; CVa | Nova StatStrip; interval in mmol/L, CVb | FreeStyle Precision Pro; interval in mmol/L, CVc |
|---|---|---|---|---|
| 1 | 0.049 | 1% | 0.1‐0.2, 43% | 0.1‐0.1, 0% |
| 2 | 0.75 | 0% | 0.8‐1.3, 24% | 0.7‐0.8, 7% |
| 3 | 1.5 | 0% | 1.3‐1.6, 12% | 1.3‐1.5, 6% |
| 4 | 2.2 | 0% | 1.8‐1.9, 3% | 1.7‐2.2, 9% |
| 5 | 2.8 | 0% | 2.2‐2.6, 9% | 2.8‐3.1, 5% |
| 6 | 3.4 | 0% | 3.2‐3.9, 12% | 3.4‐3.9, 5% |
| 7 | 4.2 | 0% | 3.2‐3.5, 5% | 3.7‐4.5, 7% |
| 8 | 4.5 | 0% | 3.7‐4.0, 4% | 4.7‐5.6, 6% |
| 9 | 5.4 | 1% | 4.1‐4.3, 3% | 5.5‐6.1, 3% |
| 10 | 6.9 | 1% | 4.4‐5.4, 11% | 6.2‐6,8, 3% |
| 11 | 8.4 | 0% | 5.9‐6.9, 8% | 7.0‐7.6, 3% |
| 12 | 11 | 0% | 6.3‐>7, – | >8,– |
Based on 2 replicates.
Based on 3 replicates on a single Nova StatStrip instrument.
Based on 3 replicates on 2 FreeStyle instruments.
4. DISCUSSION
The manufacturers claim linearity for 3‐hydroxybutyrate measurements up to 7 (StatStrip) or 8 mmol/L (FreeStyle). Our results indicate, however, that the tested methods are only linear up to approximately 4 mmol/L for StatStrip and 6 mmol/L for FreeStyle. At higher concentrations, the instruments give falsely too low results, especially the StatStrip meter. Previous studies have indicated that StatStrip and the Optium FreeStyle meter11 and the Precision Xceed Pro meter12 are linear up to 6 mmol/L but that values above 3 mmol/L should not be reported because of high inter‐individual CV %.12 Yu et al showed that only values up to 3 mmol/L could be correctly measured by the Precision Xceed Pro meter.13 The within‐run CV % between 3‐6 mmol/L in the present study was, on the contrary, reasonably low for both meters; 3%‐12% for StatStrip and 3%‐7% for the FreeStyle.
Despite the clinical guidelines implicating 3‐hydroxybutyrate as an important diagnostic and management tool in diabetes ketoacidosis in both adults and children, 3‐hydroxybutyrate is not frequently available at hospital laboratories. There are enzyme‐based assays for detection of serum or plasma 3‐hydroxybutyrate available for laboratory platforms but the use is so far limited.3 In the absence of plasma 3‐hydroxybutyrate laboratory tests, POCT devices to measure capillary blood 3‐hydroxybutyrate are frequently used. POCT has obvious advantages such as rapid test results bedside and capillary sampling which is convenient for the patient. However, based on the results of this study and other studies the limitations with POCT meters should be acknowledged. The risk of obtaining too low results in the higher range of 3‐hydroxybutyrate levels is clinically significant and starts at 4 mmol/L for StatStrip and 6 mmol/L for FreeStyle according to our study results. A 3‐hydroxybutyrate method in the laboratory allows dilution of samples outside of the linear range of the POCT meters which may be necessary in the monitoring of the resolution of ketoacidosis. Furthermore, the target values for 3‐hydroxybutyrate for diagnosing and managing diabetes ketoacidosis are precise which require 3‐hydroxybutyrate meters with high precision and low bias. A laboratory plasma 3‐hydroxybutyrate assay could facilitate the validation of POCT devices and quality controls over time to ensure the quality of the meters.
Based on the results of this study, we suggest that Nova StatStrip measures blood ketones correctly up to 4 mmol/L and FreeStyle Precision Pro up to about 6 mmol/L. Above those concentrations, there is a risk that the meters underestimate the ketone concentration and a plasma or serum laboratory method should preferably be used. Furthermore, a broader use of external quality control programs for 3‐hydroxybutyrate and development of quality routines locally is necessary for a safe use of blood ketone analysis in clinical practice.
Helmersson‐Karlqvist J, Höög Hammarström K, Palmberg K, Backman‐Johansson C. Evaluation of Nova StatStrip and FreeStyle Precision Pro blood ketone tests using 3‐hydroxybutyrate doped samples. J Clin Lab Anal. 2019;33:e22851 10.1002/jcla.22851
REFERENCES
- 1. Laffel L. Ketone bodies: a review of physiology, pathophysiology and application of monitoring to diabetes. Diabetes Metab Res Rev. 1999;15:412‐426. [DOI] [PubMed] [Google Scholar]
- 2. Lebovitz HE. Diabetic ketoacidosis. Lancet. 1995;345:767‐772. [DOI] [PubMed] [Google Scholar]
- 3. Misra S, Oliver NS. Utility of ketone measurement in the prevention, diagnosis and management of diabetic ketoacidosis. Diabet Med. 2015;32:14‐23. [DOI] [PubMed] [Google Scholar]
- 4. Kitabchi AE, Umpierrez GE, Miles JM, Fisher JN. Hyperglycemic crises in adult patients with diabetes. Diabetes Care. 2009;32:1335‐1343. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5. Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Diabetes Care. 2011;34:e61‐99. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6. Savage MW, Dhatariya KK, Kilvert A, et al. Hamersley MS for the Joint British Diabetes Societies Joint British Diabetes Societies guideline for the management of diabetic ketoacidosis. Diabet Med. 2011;28:508‐515. [DOI] [PubMed] [Google Scholar]
- 7. Dhatariya K. Blood Ketones: measurement, interpretation, limitations, and utility in the management of diabetic ketoacidosis. Rev Diabet Stud. 2016;13:217‐225. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8. Klocker A, Phelan H, Twigg SM, Craig ME. Blood beta‐hydroxybutyrate vs. urine acetoacetate testing for the prevention and management of ketoacidosis in Type 1 diabetes: a systematic review. Diabet Med. 2013;30:818‐824. [DOI] [PubMed] [Google Scholar]
- 9. Joint British Diabetes Societies Inpatient Care Group . The Management of Diabetic Ketoacidosis in Adults [homepage on the internet]. September 2013. [Cited 2019 Jan 09]. https://abcd.care/joint-british-diabetes-societies-jbds-inpatient-care-group
- 10. Wolfsdorf JI, Glaser N, Agus M, et al. ISPAD Clinical Practice Consensus Guidelines 2018: Diabetic ketoacidosis and the hyperglycemic hyperosmolar state. Pediatric Diabetes. 2018;19:155‐177. [DOI] [PubMed] [Google Scholar]
- 11. Ceriotti F, Kaczmarek E, Guerra E, et al. Comparative performance assessment of point‐of‐care testing devices for measuring glucose and ketones at the patient bedside. J. Diabetes Sci. Technol. 2015;9:268‐277. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12. Janssen MJ, Hendrickx BH, Habets‐van der Poel CD, van denBergh JP, Haagen AA, Bakker JA. Accuracy of the Precision(R) point‐of‐care ketone test examined by liquid chromatography tandem‐mass spectrometry (LC‐MS/MS) in the same fingerstick sample. Clin Chem Lab Med. 2010;48:1781‐1784. [DOI] [PubMed] [Google Scholar]
- 13. Yu HY, Agus M, Kellogg MD. Clinical utility of Abbott Precision Xceed Pro(R) ketone meter in diabetic patients. Pediatr Diabetes. 2011;12:649‐655. [DOI] [PubMed] [Google Scholar]