Influence of temperature and time before centrifugation of specimens for routine coagulation testing

Analyte stability in whole blood using experimental and datamining approaches

The analytical stability of laboratory tests relies mostly on internal and external quality control procedures. Summarized patient data has in several studies been shown to be a good supplement for monitoring analytical stability. In our present investigation, we evaluate a datamining method for retrospective evaluation and assessment of analyte stability in whole blood. Results from the laboratory information system were used as the basis for the datamining approach. Blood tests were requested by the general practitioners and drawing of the blood sample was either at the general practitioner's or at the hospital outpatient clinics. We were able to split data into groups based on sample collection place and time to analysis. The datamining approach was compared to experiments where samples were incubated at a single temperature as well as an experiment where the temperatures were changed during incubation. To demonstrate the method, we selected three laboratory tests considered representative: potassium, phosphate, and lactate dehydrogenase. The datamining approach showed results similar to the reference experiment. Furthermore, our results show that the analytes phosphate and potassium were not stable after short storage at a lower temperature.

The stability of 'add-on' coagulation assays in refrigerated citrated plasma stored on a packed cellular fraction

INTRODUCTION

Haematology laboratories are increasingly faced with requests for add-on coagulation testing. This study explores extending the specimen storage proposals by examining coagulation parameters on refrigerated citrated plasma retained on a cellular fraction over a 24-hour period.

METHODS

Sodium citrate (Sarstedt^® S-Monovette 3.2%) specimens from 206 patients in University Hospital Limerick, Ireland were refrigerated immediately post-analysis and re-analysed in the centrifuged primary container at 4, 8 and 24-hour intervals using the Diagnostica Stago coagulometer and reagent combination. Coagulation assays examined for statistically and clinically significant differences included PT, APTT, D-Dimer, fibrinogen and Protein C.

RESULTS

PT, APTT and Protein C values displayed statistical significance from 4 hours. Fibrinogen differences were statistically significant from 8 hours. D-Dimer differences were not statistically significant at any interval over the 24-hour period. The refrigerated storage limit for PT and APTT results was determined to be 4 hours. D-Dimer was the only test parameter to report a mean percentage variance >10%. However, result changes at the threshold region of 0.5 µg/mL FEU were found to be within assay precision limits and desirable bias up to 8 hours. Maximum mean differences for Protein C (-1.3%) and fibrinogen (2.3%) were within assay precision limits and desirable biases up to 24 hours.

CONCLUSION

PT and APTT results are stable in refrigerated citrated plasma maintained on a cellular fraction up to 4 hours post-phlebotomy. D-Dimers results are reliable up to 8 hours, while fibrinogen and Protein C results are stable for at least 24 hours.

Is citrate theophylline adenosine dipyridamole (CTAD) better than citrate to survey unfractionated heparin treatment? Has delayed centrifugation a real impact on this survey?

Unfractionated heparin (UFH) is the main anticoagulant used in intensive care unit. The anticoagulant effect is monitored by activated partial thrombin time (aPTT) and anti-Xa activity (anti-Xa) measurement. However, delayed centrifugation induces platelet factor 4 (PF4) release and anti-Xa decrease. Several studies have concluded that aPTT and anti-Xa measurement should be performed within 2 h in citrated anticoagulant but may be delayed longer in Citrate Theophylline Adenosine and Dypiridamol (CTAD) anticoagulant. The objective of this study was to compare the stability of both aPTT and anti-Xa in citrate and CTAD samples, and to determine the effect of delayed centrifugation on both aPTT, anti-Xa results, and PF4 release in citrate samples only. aPTT and anti-Xa were measured in citrate and CTAD anticoagulant samples from 93 patients. Delayed centrifugation was performed in citrate samples from 31 additional patients, with hourly aPTT and anti-Xa measurement from 1 to 6 h. In 14 of these last patients, PF4 release was also evaluated with Human CXCL4/PF4 Quantikine ELISA Kit. We observed a significant correlation between citrate and CTAD anticoagulant for aPTT (r² = 0.94) and anti-Xa (r² = 0.95). With Bland-Altman correlation, a minor bias was observed for anti-Xa (- 0.025 ± 0.041). Delayed centrifugation in citrated anticoagulant showed an excellent concordance from 1 to 4 h for aPTT (- 4.0 ± 5.3 s) and anti-Xa (1.10^-9 ± 0.058 UI/ml) measurements. Moreover, PF4 release was not different between 1 h (31.5 ± 14.7 ng/ml) and 4 h (33.8 ± 11.8 ng/ml). We have demonstrated that anti-Xa measurement for unfractionated heparin should be done 4 h in citrated plasma and that CTAD was not better than citrate. However, these initial findings require confirmation using other aPTT and calibrated anti-Xa assays.

Impact of different preanalytical conditions on results of lupus anticoagulant tests

INTRODUCTION

The currently recommended preanalytical conditions for lupus anticoagulant (LA) analysis require analyzing samples in fresh or freshly frozen platelet-poor plasma. The aim of this study was to evaluate whether alternative and less cumbersome preanalytical procedures for LA testing give significantly different results compared to recommended conditions.

MATERIALS AND METHODS

Citrated blood samples were drawn from 29 study participants, 15 with negative and 14 with positive LA results. The samples were processed according to the ISTH guideline for LA testing and compared to several alternative preanalytical conditions. Measurements were performed using the dilute Russell's viper venom time (DRVVT) and silica clotting time (SCT), both screen and confirm, on a STA-R Evolution analyzer. Stability criteria were based upon biological variation.

RESULTS

All DRVVT tests (normalized screen, confirm, and screen/confirm ratio) met the stability criteria for all the preanalytical conditions. The SCT tests (normalized screen, confirm, and screen/confirm ratio) met the stability criteria only when treated according to the ISTH guideline, except for SCT normalized screen/confirm ratio which also met the stability criteria for double-centrifuged aliquoted plasma stored in room temperature for 24 hours and then analyzed "fresh" or after being frozen. One warfarin-treated patient was reclassified from positive to negative for DRVVT after the preanalytical modifications, while 2 of 29 participants became falsely positive for 2 of 8 conditions for SCT.

CONCLUSIONS

The DRVVT assays met the criteria for stability for all preanalytical conditions tested, while the SCT assays should be interpreted with caution if the preanalytical guidelines from ISTH are not followed.

D-dimer: Preanalytical, analytical, postanalytical variables, and clinical applications

D-dimer is a soluble fibrin degradation product deriving from the plasmin-mediated degradation of cross-linked fibrin. D-dimer can hence be considered a biomarker of activation of coagulation and fibrinolysis, and it is routinely used for ruling out venous thromboembolism (VTE). D-dimer is increasingly used to assess the risk of VTE recurrence and to help define the optimal duration of anticoagulation treatment in patients with VTE, for diagnosing disseminated intravascular coagulation, and for screening medical patients at increased risk of VTE. This review is aimed at (1) revising the definition of D-dimer; (2) discussing preanalytical variables affecting the measurement of D-dimer; (3) reviewing and comparing assay performance and some postanalytical variables (e.g. different units and age-adjusted cutoffs); and (4) discussing the use of D-dimer measurement across different clinical settings.

Pre-analytical practices for routine coagulation tests in European laboratories. A collaborative study from the European Organisation for External Quality Assurance Providers in Laboratory Medicine (EQALM)

Background

Correct handling and storage of blood samples for coagulation tests are important to assure correct diagnosis and monitoring. The aim of this study was to assess the pre-analytical practices for routine coagulation testing in European laboratories.

Methods

In 2013–2014, European laboratories were invited to fill in a questionnaire addressing pre-analytical requirements regarding tube fill volume, citrate concentration, sample stability, centrifugation and storage conditions for routine coagulation testing (activated partial thromboplastin time [APTT], prothrombin time in seconds [PT-sec] and as international normalised ratio [PT-INR] and fibrinogen).

Results

A total of 662 laboratories from 28 different countries responded. The recommended 3.2% (105–109 mmol/L) citrate tubes are used by 74% of the laboratories. Tube fill volumes ≥90% were required by 73%–76% of the laboratories, depending upon the coagulation test and tube size. The variation in centrifugation force and duration was large (median 2500 g [10- and 90-percentiles 1500 and 4000] and 10 min [5 and 15], respectively). Large variations were also seen in the accepted storage time for different tests and sample materials, for example, for citrated blood at room temperature the accepted storage time ranged from 0.5–72 h and 0.5–189 h for PT-INR and fibrinogen, respectively. If the storage time or the tube fill requirements are not fulfilled, 72% and 84% of the respondents, respectively, would reject the samples.

Conclusions

There was a large variation in pre-analytical practices for routine coagulation testing in European laboratories, especially for centrifugation conditions and storage time requirements.

Effect of Centrifuge Temperature on Routine Coagulation Tests

BACKGROUND

This study investigated the effects of cooled and standard centrifuges on the results of coagulation tests to examine the effects of centrifugation temperature.

METHODS

Equal-volume blood samples from each patient were collected at the same time intervals and subjected to standard (25°C) and cooled centrifugation (2-4°C). Subsequently, the prothrombin time (PT), international normalized ratio (INR), activated partial thromboplastin time (aPTT), fibrinogen, and D-dimer values were determined in runs with the same lot numbers in the same coagulation device using the Dia-PT R (PT and INR), Dia-PTT-liquid (aPTT), Dia-FIB (fibrinogen), and Dia-D-dimer kits, respectively.

RESULTS

The study enrolled 771 participants. The PT was significantly (p < 0.018) higher in participants on anticoagulant therapy. The respective median values of the test parameters determined using the standard and cooled centrifuges were as follows: PT 10.30 versus 10.50 s; PT (INR) 1.04 versus 1.09 s; APTT 28.90 versus 29.40 s; fibrinogen 321.5 versus 322.1 mg/dL; and D-dimer 179.5 versus 168.7 µg FEU/mL. There were significant differences (p < 0.001) in the parameters between the values obtained with the standard and cooled centrifuges.

CONCLUSIONS

Centrifuge temperature can have a significant effect on the results of coagulation tests. However, broad and specific disease-based studies are needed.

Is it acceptable to use coagulation plasma samples stored at room temperature and 4°C for 24 hours for additional prothrombin time, activated partial thromboplastin time, fibrinogen, antithrombin, and D-dimer testing?

INTRODUCTION

Coagulation laboratories are faced on daily basis with requests for additional testing in already analyzed fresh plasma samples. This prompted us to examine whether plasma samples stored at room temperature (RT), and 4°C for 24 hours can be accepted for additional prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen (Fbg), antithrombin (AT), and D-dimer testing.

METHODS

We measured PT, aPTT, Fbg in 50 and AT in 30 plasma samples with normal and pathological values, within 4 hours of blood collection (baseline results) and after 24-hours storage at RT (primary tubes), and 4°C (aliquots). D-dimer stability was investigated in 20 samples stored in primary tubes at 4°C.

RESULTS

No statistically significant difference between baseline results and results in samples stored at RT and 4°C was observed for PT (P=.938), aPTT (P=.186), Fbg (P=.962), AT (P=.713), and D-dimers (P=.169). The highest median percentage changes were found for aPTT, being more pronounced for samples stored at 4°C (13.0%) than at RT (8.7%).

CONCLUSION

Plasma samples stored both at RT and 4°C for 24 hours are acceptable for additional PT, Fbg, and AT testing. Plasma samples stored 24 hours in primary tubes at 4°C are suitable for D-dimer testing.

Preanalytical Issues in Hemostasis and Thrombosis Testing

Hemostasis testing is critical to many hemorrhagic and thrombotic disorders, wherein laboratory diagnostics can provide critical information for diagnosis, prognostication, and therapeutic monitoring. Due to this crucial role in modern medicine, hemostasis tests should be carried out at their highest degree of quality, thus encompassing standardization and monitoring of all phases of the testing process. It is now clearly established that the preanalytical phase is the most critical and vulnerable part of the total testing process, since up to 70% of diagnostic errors are due to highly manual activities encompassing patient preparation and collection of biological samples, as well as handling, transportation, preparation and storage of blood specimens. Due to the peculiar sample matrix required for hemostasis testing (i.e., plasma anticoagulated with buffered sodium citrate), additional critical issues may impair the reliability of these tests. Therefore, this article aims to provide an updated overview of the most important preanalytical variables that may ultimately impair the quality of hemostasis and thrombosis testing.

Pre-analytical issues in the haemostasis laboratory: guidance for the clinical laboratories

Ensuring quality has become a daily requirement in laboratories. In haemostasis, even more than in other disciplines of biology, quality is determined by a pre-analytical step that encompasses all procedures, starting with the formulation of the medical question, and includes patient preparation, sample collection, handling, transportation, processing, and storage until time of analysis. This step, based on a variety of manual activities, is the most vulnerable part of the total testing process and is a major component of the reliability and validity of results in haemostasis and constitutes the most important source of erroneous or un-interpretable results. Pre-analytical errors may occur throughout the testing process and arise from unsuitable, inappropriate or wrongly handled procedures. Problems may arise during the collection of blood specimens such as misidentification of the sample, use of inadequate devices or needles, incorrect order of draw, prolonged tourniquet placing, unsuccessful attempts to locate the vein, incorrect use of additive tubes, collection of unsuitable samples for quality or quantity, inappropriate mixing of a sample, etc. Some factors can alter the result of a sample constituent after collection during transportation, preparation and storage. Laboratory errors can often have serious adverse consequences. Lack of standardized procedures for sample collection accounts for most of the errors encountered within the total testing process. They can also have clinical consequences as well as a significant impact on patient care, especially those related to specialized tests as these are often considered as "diagnostic". Controlling pre-analytical variables is critical since this has a direct influence on the quality of results and on their clinical reliability. The accurate standardization of the pre-analytical phase is of pivotal importance for achieving reliable results of coagulation tests and should reduce the side effects of the influence factors. This review is a summary of the most important recommendations regarding the importance of pre-analytical factors for coagulation testing and should be a tool to increase awareness about the importance of pre-analytical factors for coagulation testing.

Effect of Freezing Plasma at –20°C for 2 Weeks on Prothrombin Time, Activated Partial Thromboplastin Time, Dilute Russell Viper Venom Time, Activated Protein C Resistance, and d-Dimer Levels

To assess the impact of preanalytical variables of time and temperature on prothrombin time (PT), activated partial thromboplastin time (aPTT), dilute Russell viper venom time (DRVVT), activated protein C resistance (APCR), and d-dimer, samples from 23 healthy individuals and 18 patients having coagulopathy with known abnormal PT and aPTT were collected. Plasma from each individual was separately pooled and aliquoted; the first 2 aliquots were stored at room temperature then analyzed at 2 hours (baseline) and 4 hours postcollection. The remaining aliquots were stored at −20°C and thawed for analysis at 48 hours, 1, and 2 weeks. In both healthy participants and participants with coagulopathy, PT, aPTT, APCR, DRVVT, and D-dimer had no significant changes at 4 and 48 hours, and 1 and 2 weeks postcollection compared to baseline, or the changes were less than 10%. The results indicate PT, aPTT, DRVVT, APCR, and d-dimer can be stored for 2 weeks at −20°C without compromising clinical interpretation in both healthy individuals and individuals with coagulopathy. Increasing storage time will facilitate sample processing from off-site clinics.

Effects of storage time and temperature on coagulation tests and factors in fresh plasma

Coagulation tests and factors measurements have been widely applied in clinical practice. Pre-analytical conditions are very important in laboratory assessment.Here,we aim to determine the effects of storage time and temperature on activated partial thromboplastin time (APTT), fibrinogen (Fbg), prothrombin time (PT), the international normalized ratio (INR), thrombin time (TT), factor VIII activity (FVIII:C), and factor IX activity (FIX:C) in fresh plasma. Seventy-two blood samples were tested after storage for 0 (baseline), 2, 4, 6, 8, 12, and 24 h at 25°C (room temperature) and 4°C (refrigeration) in two centers. The mean percentage change of greater than 10% and the numbers of samples with greater than 10% percentage changes more than 25% were used to determine clinically relevant difference. We demonstrated that samples for Fbg, PT/INR, and TT could be safely stored for ≤24 h; FVIII:C for ≤2 h; FIX:C for ≤4 h both at 4°C and 25°C; and APTT for ≤12 h at 4°C and ≤8 h at 25°C.

Preanalytic variables of thrombin generation: towards a standard procedure and validation of the method

BACKGROUND

Thrombin generation assays are sensitive methods for assessment of the overall clotting potential of plasma, but, despite their common use in thrombosis research, standardization of preanalytic conditions is lacking. In order to set up a standardized protocol, we analyzed different preanalytic variables and validated the calibrated automated thrombogram method.

METHODS AND RESULTS

Thrombin generation was assessed with 0, 1 and 5 pm tissue factor (TF). Variations in thrombin generation were mostly attributable to the type of collection tube, mainly because of variations in contact activation. The collection tube also determined the influence of other preanalytic variables on thrombin generation, e.g. the need for a discard tube, the storage of whole blood, and the centrifugation method. Regarding the collection system, blood drawn through intravenous catheters or butterfly needles showed significantly more hemolysis than blood obtained with venipuncture using conventional needles. The results showed that a discard tube is still needed for thrombin generation measurements. After blood collection, whole blood is best centrifuged immediately, to prevent activation or degradation of coagulation proteins, and a second centrifugation step at 10,000 × g is recommended. After thawing, plasma is best analyzed immediately, as storage resulted in thrombin generation results outside the 10% range of the reference sample. On the basis of these results, we set up an in-house standardized protocol, which was used for validation, resulting in coefficients of variations of < 15% for all derived parameters with both the 1 and 5 pm TF triggers.

CONCLUSION

Thrombin generation was greatly influenced by preanalytic conditions, demonstrating the need for an international standardized protocol.

Impact of the phlebotomy training based on CLSI/NCCLS H03-a6 - procedures for the collection of diagnostic blood specimens by venipuncture

Introduction:

The activities involving phlebotomy, a critical task for obtaining diagnostic blood samples, are poorly studied as regards the major sources of errors and the procedures related to laboratory quality control. The aim of this study was to verify the compliance with CLSI documents of clinical laboratories from South America and to assess whether teaching phlebotomists to follow the exact procedure for blood collection by venipuncture from CLSI/NCCLS H03-A6 - Procedures for the Collection of Diagnostic Blood Specimens by Venipuncture might improve the quality of the process.

Materials and methods:

A survey was sent by mail to 3674 laboratories from South America to verify the use of CLSI documents. Thirty skilled phlebotomists were trained with the CLSI H03-A6 document to perform venipuncture procedures for a period of 20 consecutive working days. The overall performances of the phlebotomists were further compared before and after the training program.

Results:

2622 from 2781 laboratories that did answer our survey used CLSI documents to standardize their procedures and process. The phlebotomists’ training for 20 days before our evaluation completely eliminated non-conformity procedures for: i) incorrect friction of the forearm, during the cleaning of the venipuncture site to ease vein location; ii) incorrect sequence of vacuum tubes collection; and iii) inadequate mixing of the blood in primary vacuum tubes containing anticoagulants or clot activators. Unfortunately the CLSI H03-A6 document does not caution against both unsuitable tourniquet application time (i.e., for more than one minute) and inappropriate request to clench the fist repeatedly. These inadequate procedures were observed for all phlebotomists.

Conclusion:

We showed that strict observance of the CLSI H03-A6 document can remarkably improve quality, although the various steps for collecting diagnostic blood specimens are not a gold standard, since they may still permit errors. Tourniquet application time and forearm clench should be verified by all quality laboratory managers in the services. Moreover, the procedure for collecting blood specimens should be revised to eliminate this source of laboratory variability and safeguard the quality.

Biological variation of INR in stable patients on long-term anticoagulation with warfarin

Within-individual biological variation of INR (CV(B)) was assessed in 245 selected stable warfarin-treated patients monitored by three thrombosis centers. Selection criteria were: treatment period of six months or longer before the observation period; at least six consecutive INRs within the therapeutic range of 2.0 - 3.0; interval between consecutive INR measurements of two weeks or longer; no change in warfarin dose; no changes in the patient's circumstances which may influence the INR, such as intercurrent diseases, invasive procedures, starting or stopping drugs interacting with warfarin. The minimum, maximum and mean within-individual coefficient of variation CV(B) of the INR measurements in the 245 selected patients were 0.4%, 14.5%, and 9.0%, respectively Analytical performance goals for the INR measurement (imprecision) could be derived from the mean CV(B). For a therapeutic range of 2.0 - 3.0 with warfarin, the desirable and optimum imprecision of INR determination is <4.5% CV and <2.25% CV, respectively. The biological variation and analytical performance goals have been derived using classic laboratory methods but should be applicable to point-of-care testing as well.

Sample collection and handling considerations for peptidomic studies in whole saliva; implications for biomarker discovery

BACKGROUND

Proteomic studies in saliva have demonstrated its potential as a diagnostic biofluid, however the salivary peptidome is less studied. Here we study the effects of several sample collection and handling factors on salivary peptide abundance levels.

METHODS

Salivary peptides were isolated using an ultrafiltration device and analyzed by tandem mass spectrometry. A panel of 41 peptides common after various treatments were quantified and normalized. We evaluated the effects of freezing rate of the samples, nutritional status of the donors (fed vs. fasted), and room-temperature sample degradation on peptide abundance levels. Repeatability of our sample processing method and our instrumental analysis method were investigated.

RESULTS

Increased sample freezing rate produced higher levels of peptides. Donor nutritional status had no influence on the levels of measured peptides. No significant difference was detected in donors' saliva following 5, 10 and 15 min of room-temperature degradation. Sample processing and instrumental variability were relatively small, with median CVs of 9.6 and 6.6.

CONCLUSIONS

Peptide abundance levels in saliva are rather forgiving towards variations in sample handling and donor nutritional status. Differences in freezing methods may affect peptide abundance, so consistency in freezing samples is preferred. Our results are valuable for standardizing sample collection and handling methods for peptidomic-based biomarker studies in saliva.

Centrifugation protocols: tests to determine optimal lithium heparin and citrate plasma sample quality

Background

Currently, no clear guidelines exist for the most appropriate tests to determine sample quality from centrifugation protocols for plasma sample types with both lithium heparin in gel barrier tubes for biochemistry testing and citrate tubes for coagulation testing.

Methods

Blood was collected from 14 participants in four lithium heparin and one serum tube with gel barrier. The plasma tubes were centrifuged at four different centrifuge settings and analysed for potassium (K+), lactate dehydrogenase (LD), glucose and phosphorus (Pi) at zero time, poststorage at six hours at 21 °C and six days at 2–8°C. At the same time, three citrate tubes were collected and centrifuged at three different centrifuge settings and analysed immediately for prothrombin time/international normalized ratio, activated partial thromboplastin time, derived fibrinogen and surface-activated clotting time (SACT).

Results

The biochemistry analytes indicate plasma is less stable than serum. Plasma sample quality is higher with longer centrifugation time, and much higher g force. Blood cells present in the plasma lyse with time or are damaged when transferred in the reaction vessels, causing an increase in the K+, LD and Pi above outlined limits. The cells remain active and consume glucose even in cold storage. The SACT is the only coagulation parameter that was affected by platelets >10 × 109/L in the citrate plasma.

Conclusion

In addition to the platelet count, a limited but sensitive number of assays (K+, LD, glucose and Pi for biochemistry, and SACT for coagulation) can be used to determine appropriate centrifuge settings to consistently obtain the highest quality lithium heparin and citrate plasma samples. The findings will aid laboratories to balance the need to provide the most accurate results in the best turnaround time.

Changes of platelet function and blood coagulation during short-term storage of CPDA-1-stabilised ovine blood

The objective of this study was to detect the influence of short-term storage on the haemostatic function in whole citrated ovine blood at different storage temperatures. Ovine blood was collected in a commercial transfer bag system containing CPDA-1 and stored on a wobbler at room (20-25 °C; n=5) or refrigerator temperature (4 °C; n=5). The following analyses were performed initially and after 1, 2, 3, 4, 5, 6, 8, 12, 24, 48 and 72 h of storage: platelet count and (spontaneous) aggregates, agonist-induced platelet aggregation with two methods (impedance aggregometry, turbidimetric method), prothrombin time, activated partial thromboplastin time, thrombin time, fibrinogen concentration and resonance thrombography. Platelet count remained stable at room temperature, whereas a significant decrease was detected after 48 h storage at 4 °C. The latter was associated with the formation of a high percentage of platelet aggregates (50-60%) after 5h storage. Decrease in platelet aggregation was significantly more pronounced when blood was stored at 4°C. The plasmatic coagulation tests were stable within the observation period. Results indicate that platelet count and aggregability of CPDA-1-stabilised ovine blood is better preserved at room temperature and provides adequate haemostatic function for ex vivo experiments for one working day. Functional loss and high percentage of platelets within aggregates which were observed in ovine blood stored at refrigerator temperature have to be considered in blood transfusion in sheep.

Discrepancies in International Normalized Ratio Results between Instruments: A Model to Split the Variation into Subcomponents

BACKGROUND

Observed differences between results obtained from comparison of instruments used to measure international normalized ratio (INR) have been higher than expected from the imprecision of the instruments. In this study the variation of these differences was divided into subcomponents, and each of the subcomponents was estimated.

METHODS

Blood samples were collected at 4 different patient visits from each of 36 outpatients who were receiving warfarin treatment and were included in the study. INR was determined on 1 laboratory instrument (STA Compact®) and 3 point-of-care instruments (Simple Simon®PT, CoaguChek®XS, and INRatio™). All 4 INR instruments were compared in pairs. Linear regression was used to correct for systematic deviations. The remaining variation of the differences was subdivided into between-subject, within-subject, and analytical variation in an ANOVA nested design.

RESULTS

The mean difference between instruments varied between 1.0% and 14.3%. Between-subject variation of the differences (expressed as CV) varied between 3.3% and 7.4%, whereas within-subject variation of the differences was approximately 5% for all 6 comparisons. The analytical imprecision of the differences varied between 3.8% and 8.6%.

CONCLUSIONS

The differences in INR between instruments were subdivided into calibration differences, between- and within-subject variation, and analytical imprecision. The magnitude of each subcomponent was estimated. Within results for individual patients the difference in INR between 2 instruments varied over time. The reasons for the between- and within-subject variations of the differences can probably be ascribed to different patient-specific effects in the patient plasma. To minimize this variation in a monitoring situation, each site and patient should use results from only 1 type of instrument.