Sensitivity of commercial prothrombin time reagents to detect coagulation factor deficiencies in equine plasma

The sensitivity of commercial prothrombin time (PT) tests was assessed based on a dilution series of equine pooled plasma (EPP) (experiment 1) and on 40 equine plasma samples with reduced activity of coagulation factors II, V, VII and X (experiment 2). Two different PT reagents (reagent 1, human placental thromboplastin; reagent 2, recombinant human tissue factor) were used according to the manufacturers' instructions (standard test, PT([ST])) and compared to a modified test procedure (modified test, PT([MT])) using sample dilution and fibrinogen addition. In all samples, sensitivity was lower (P<0.01) when using PT([ST]) with reagent 2 (0.20) than when using either PT([ST]) with reagent 1 (0.65) or PT([MT]) with both reagents (reagent 1, 0.60-0.75, reagent 2, 0.58-0.70, depending on sample dilution). The highest sensitivity was found for PT([MT]) when using a 1:20 sample dilution. In those samples in which at least one coagulation factor activity was decreased (by 20%; n=18), the sensitivity of PT([ST]) with reagent 2 (0.33) was found to be inadequate, in contrast to all other test procedures (0.83-0.94). This low sensitivity corresponded to shorter time intervals between different coagulation activity levels prepared by EPP dilution. The results indicate that adequate sensitivity of PT measurements in equine plasma can be achieved using a standard test procedure as long as a suitable reagent is used.

Short-term venous stasis influences routine coagulation testing

Preanalytical variability is a common source of errors in coagulation testing, as clotting assays are particularly susceptible to poor standardization of the whole analytical process. To investigate the effect of a short-term venous stasis on routine coagulation testing, we measured activated partial thromboplastin time, prothrombin time, fibrinogen and D-dimer in plasma specimens collected either without venous stasis or following the application of a 60 mmHg constant, standardized external pressure by a sphygmomanometer, for 1 (1-min stasis) and 3 min (3-min stasis). When compared with blood specimens collected without stasis, the Pearson's correlation coefficients and the corresponding slopes of the Passing and Bablok regression line of samples collected following 1 and 3-min stasis were acceptable. However, statistically significant differences by paired Student's t-test could be observed for all parameters tests following 3-min stasis, and for all but the activated partial thromboplastin time after 1-min stasis. Significant difference between specimens collected after 1- and 3-min stasis was also achieved for prothrombin time (P < 0.01), fibrinogen (P < 0.01) and D-dimer (P < 0.05). The agreement between measurements was yet acceptable after 1-min stasis, but achieved clinical significance for prothrombin time, fibrinogen and D-dimer after 3-min stasis. Taken together, results of the present investigation confirm that the effects of venous stasis during venipuncture are clinically meaningful. As hematocrit values and activities of clotting factors VII, VIII and XII significantly increased, whereas that of activated factor VII remained unchanged, we hypothesize that a short-term venous stasis, as induced by up to 3-min tourniquet placing, might not be sufficient to produce additional procoagulant responses besides hemoconcentration.

Using an independent quality control software program, EZ Runs™, to monitor quality control procedures for a bench-top coagulation analyzer

BACKGROUND

Comprehensive quality control (QC) procedures are necessary to ensure accurate analytic method performance. Highly automated systems typically have inherent QC programs that facilitate performance and maintenance of QC procedures; however, for bench-top analyzers that lack internal systems, independent QC programs must be used.

OBJECTIVE

The goal of this study was to evaluate the adaptability of an independent QC program, EZ Runs (Westgard QC Inc, Madison, WI, USA), to the maintenance of QC procedures for a mechanical, bench-top coagulation unit and to compare the results with our current, manual, QC method in a qualitative way.

METHODS

A QC application file for activated partial thromboplastin time (aPTT) performed on a STart4 (Diagnostica Stago, Parsippany, NJ) was created in EZ Runs. Results were recorded and interpreted using this software package as well as the current, manual, QC method.

RESULTS

EZ Runs was adaptable to QC monitoring for the bench-top analyzer, and the program permitted identification of both random and systematic errors not detected by the manual QC system.

CONCLUSIONS

EZ Runs improved the performance and maintenance of QC procedures for this bench-top coagulation analyzer. The results indicated the need to improve staff training in assay performance and QC interpretation. In addition, use of the software program indicated that a multirule QC design was needed to monitor assay performance.

European Concerted Action on Anticoagulation. A multicentre calibration study of WHO international reference preparations for thromboplastin, rabbit (RBT/90) and human (rTF/95)

A 10 centre calibration was performed after six years to determine the international sensitivity index (ISI) of rTF/95 relative to RBT/90, and to assess any international normalised ratio (INR) bias compared with the original multicentre calibration. After exclusion of one outlying centre, the follow up calibration gave a mean ISI for rTF/95 of 0.99, which although a small difference, is significantly greater than the mean ISI of 0.94 obtained previously. The change in ISI for international reference preparation (IRP) rTF/95 relative to RBT/90 would lead to a slight bias in INR for human compared with rabbit thromboplastins. At a theoretical INR of 3.0, the INR bias is 6.0%, and this is below the accepted 10% level of clinical relevance. Ongoing stability monitoring of World Health Organisation thromboplastin IRP is advised.

Evaluation of the phospholipid-rich dilute Russell's viper venom assay to monitor oral anticoagulation in patients with lupus anticoagulant

The International Normalized Ratio (INR) is generally recommended to monitor anticoagulant therapy in patients treated with warfarin. However, there has been concern about the validity of the INR to monitor warfarin therapy in patients with lupus anticoagulant, particularly when there is prolongation of the baseline INR. An alternative approach is to use a chromogenic factor X assay that is not sensitive to lupus anticoagulant. However, this assay is expensive, not widely available, and does not have an established therapeutic range. We hypothesized that the phospholipid-rich dilute Russell viper venom time (prdRVVT), a simple, rapid and inexpensive assay, might be suitable to monitor warfarin therapy in this situation since Russell's viper venom directly activates coagulation factor X while the phospholipid in the reagent reduces or negates any effect of lupus anticoagulant on the assay. We measured the INR, chromogenic factor X, and prdRVVT in 50 patients stabilized on warfarin for at least 6 weeks, 12 of whom had lupus anticoagulant, and 37 patients not taking warfarin, 17 of whom had lupus anticoagulant. Factor X was negatively correlated with INR in anticoagulated patients both in the absence (r = -0.45, P = 0.01) and presence (r = -0.43, P = 0.17) of lupus anticoagulant. The prdRVVT was also strongly correlated with INR in anticoagulated patients without lupus anticoagulant (r = 0.60, P < 0.0001) but there was no correlation in the presence of lupus anticoagulant (r = -0.13, P = 0.68). Our results suggest that the prdRVVT is not suitable for monitoring warfarin therapy in patients with lupus anticoagulant.

[A discussion of standardization for prothrombin time in patients with advanced liver diseases]

OBJECTIVE

To determine which expression mode of prothrombin time (PT) might achieve PT standardization in patients with advanced liver diseases.

METHODS

PT was measured with six thromboplastins with different ISI values in 16 severe chronic hepatitis patients, 50 decompensated liver cirrhosis patients and 30 patients on oral anticoagulation therapy. The results were expressed in PT (second), PTA (%), PTR and INR.

RESULTS

In chronic hepatitis patients, the means of the six group's PTAs ranged from 24% to 34%, while their upper limits ranged from 47% to 61%. The means of the INRs ranged from 2.55 to 5.13, while their upper limits ranged from 4.65 to 12.77. Through one-way ANOVA of repeated measures, PPTA (0.489) was > PINR (0.120). In patients with liver cirrhosis, the means of the PTA in six groups ranged from 50% to 59%, while their upper limits ranged from 82% to 90%. The means of the INR ranged from 1.40 to 1.80, while their upper limits ranged from 1.97 to 3.69. Through one-way ANOVA of repeated measures, PPTA (0.102) was > PINR (0.01). In patients on oral coagulation therapy, the means of PTA ranged from 26% to 37%, while their upper limits ranged from 39% to 49%. The means of INR ranged from 2.35 to 2.66, while their upper limits ranged from 3.16 to 4.26. Through one-way ANOVA of repeated measures, PPTA (0.01) was less than PINR (0.102). The correlation between the results detected by Neoplastine and by other reagents were analyzed. They correlated well with each other when PTA was used as the expression mode of PT in patients with advanced liver disease. But in patients on oral anticoagulation therapy, when only the INR was used as the expression mode of PT, the correlation was well with each other.

CONCLUSION

The use of INR provides inadequate standardization. Only when the PT is expressed in PTA, then it may provide a standardization mode in patients with advanced liver diseases.

Significance of consecutive international normalized ratio (INR) outcomes using statistical control rules in long-term anticoagulated patients. Optimization of laboratory monitoring and interpretation of borderline measurements

Analysis of serial measurements is needed to elaborate observations in all fields of medical research. In the present study, retrospective data of anticoagulated patients were used to calculate a mean of observations and control limits, X+/-1sigma and X+/-2sigma (mean+/-standard deviation). During 18 months of coagulation monitoring, 45 patients without major oral anticoagulant therapy complications, with more international normalized ratio (INR) determinations in the therapeutic range and with a normal distribution of INR values according to the Kurtosis coefficient, were selected. The cumulative distribution functions allowed us to obtain critical limits of INR with a cumulative probability (p). Control limits, calculated for a therapeutic control chart, indicated through different control rules, 1S2 or 2S1, an alarm signal to analyze the cause of INR outside the therapeutic range. Our investigation suggested that for results at the level of the therapeutic control limits, we needed at least two consecutive INR results to detect a significant over- or under-anticoagulation. The therapeutic control chart method should be a useful means in clinical practice for evaluating the statistical significance of consecutive and borderline INR outcomes. Analytical improvements and control rules applied to laboratory monitoring may help optimize drug dose administration.

Poor agreement among prothrombin time international normalized ratio methods: comparison of seven commercial reagents

BACKGROUND

Prothrombin time (PT) has long been the most popular test for monitoring oral anticoagulation therapy. The International Normalized Ratio (INR) was introduced to overcome the problem of marked variation in PT results among laboratories and the various recommendations for patient care. According to this principle, all reagents should be calibrated to give identical results and the same patient care globally. This is necessary for monitoring of single patients and for application of the results of anticoagulation trials and guidelines to clinical practice.

METHODS

We took blood samples from 150 patients for whom oral anticoagulation had been prescribed. Plasmas were separated and PTs determined by use of seven commercial reagents and four calibrator sets. The differences in results were assessed by plotting, for each possible pair of methods, the differences in INR values for each sample against the mean INR value (Bland-Altman difference plots).

RESULTS

Mean results differed significantly (P <0.001) for 17 of 21 possible paired comparisons of methods. Only two pairs of methods produced very similar results when assessed for problems of substantial differences in INR values; a significant, systematic increase in the difference with INR; and a significant systematic increase in the variation in difference with increasing INR values.

CONCLUSIONS

The agreement among several (and perhaps most) commercial INR methods is poor. The failure of current calibration strategies may severely compromise both the monitoring of individual patients and the application of oral anticoagulation guidelines and trial results to clinical practice.

Agreement of a new whole-blood PT/INR test using capillary samples with plasma INR determinations

PURPOSE

The objective of the study was to compare in anticoagulated patients the international normalized ratio (INR) measured with a new capillary whole-blood device, the i-STAT Portable Clinical Analyser, with conventional plasma INR obtained from the central laboratory.

PATIENTS AND METHODS

Between-cartridge variability was first determined with two lyophilized controls with INR levels of 1.60 and 2.75 (n=10). Next, in 35 patients under different intensities of oral anticoagulation, capillary blood INR was measured with two i-STAT devices and was compared to central laboratory plasma INR (Innovin reagent and BCS analyser).

RESULTS

Between-cartridge coefficients of variation were 5% (95%, CI 3.4-9.1) and 3% (95%, CI 2.1-5.5) at INR levels of 1.60 and 2.75. Mean INR difference between the two i-STAT devices was 0.1, and the correlation coefficient was 0.98. Between i-STAT and central laboratory INR, the correlation coefficient was 0.95. Bias values were 0.04, 0.2, and -0.04 at INR levels of 2.0, 2.5, and 3.5, respectively.

CONCLUSION

The INR measured with the i-STAT Portable Clinical Analyser is precise and compares well with plasma INR performed in a central laboratory.

Analytical Performance of the New Coagulation Monitoring System INRatio? for the Determination of INR Compared with the Coagulation Monitor Coaguchek� S and an Established Laboratory Method

An evaluation of the INRatio Prothrombin Time Monitoring system for determination of INR was done in two centers with a total of 5 healthy subjects and 77 subjects on oral anticoagulation. The INRatio and the Coaguchek S were compared with an established laboratory method. The correlation coefficient of the comparison with the laboratory was r=0.954 for INRatio and r=0.937 for Coaguchek S. The mean relative deviation from the lab method calculated according to Hill was 6.87% for INRatio, which is rated "very goo", and 9.72% for Coaguchek S ("goo"). The imprecision in the normal range (INR=1.1) showed a coefficient of variation (CV) of 7.8% and a standard deviation (SD) of 0.09. In the therapeutic range (INR 3.9) the CV was 5.4%, the SD 0.21 and above therapeutic range (INR 5.3), the CV was 8.4% (SD 0.44), rated satisfactory. The concordances of the compared methods with the routine were 81% for INRatio and 79% for Coaguchek S, which is considered state-of-the-art. Most of the patients' perceptions of the INRatio were very positive. In the hands of professionals the INRatio demonstrated very good accuracy and precision and an excellent technical reliability. Further studies using INRatio for self testing by patients are warranted.

In-house calibration of the international sensitivity index or calibration curve for determination of the international normalized ratio

CONTEXT

The international normalized ratio (INR) has been used since 1983 to standardize prothrombin time results for patients on oral anticoagulants. However, significant interlaboratory variations have been noted. Attempts have been made to address these differences with the use of instrument-specific International Sensitivity Index (ISI) values and in-house calibration of ISI values.

OBJECTIVE

To assess the performance of laboratories using a calibration curve for INR testing.

DESIGN

Attempts to improve performance of the INR include the use of instrument-specific ISI values, model-specific ISI values, in-house calibration of ISI values, and more recently, the preparation of a calibration curve. Several studies have shown an improvement in performance using these procedures. In this study of licensed laboratories performing routine coagulation testing in the Province of Ontario, Canada, the determination of the INR by a calibration curve was compared with the laboratories' usual method of assessment. These methods were subsequently analyzed by comparing the results to instrument-specific ISI, model-specific ISI, and in-house calibrators. International normalized ratios derived by both methods were analyzed for accuracy and precision. The stability of a calibration curve was also investigated.

RESULTS

Performance of INR testing has improved with use of a calibration curve or in-house calibrators.

CONCLUSION

The results confirm that either in-house calibrators or the calibration curve improve performance of INR testing. The calibration curve may be easier to use and appears stable up to 4 months.

Prothrombin time international normalized ratio monitoring by self-testing

PURPOSE OF REVIEW

The increasing numbers of patients on oral anticoagulants may challenge the traditional organization of patient monitoring. The availability of portable coagulometers capable of measuring prothrombin time (PT) international normalized ratio (INR) in a drop of capillary blood facilitates decentralization of monitoring by self-testing. This article reviews the literature on use of portable coagulometers.

RECENT FINDINGS

Numerous studies have evaluated the reliability of portable coagulometers in testing the PT-INR. This has been assessed by statistically as well as clinically relevant criteria. Other studies have been devoted to developing calibration models fulfilling the criteria recommended by the World Health Organization (WHO) for the calibration of INR measuring systems. Finally, studies have assessed the value of schemes for patient training and for the long-term quality assurance of portable coagulometers.

SUMMARY

It can be concluded from the published studies that PT-INR self-testing may be considered as a suitable alternative to conventional laboratory testing. For the PT-INR to be reliable, manufacturers of portable coagulometers should calibrate their devices against international standards for thromboplastin with procedures similar to those recommended by WHO for conventional measuring systems. Training of patients and implementation of appropriate quality assessment schemes are also essential prerequisites for the success of PT-INR self-testing.

A critical evaluation of the prothrombin time for monitoring oral anticoagulant therapy

The Quick prothrombin time is the most common clotting test performed, principally for monitoring oral anticoagulant therapy. The International Normalized Ratio (INR) for comparing patient results from prothrombin time measurements and the International Standardized Index (ISI) for achieving greater consistency of results using different thromboplastins have made it possible to compare the results of vitamin K antagonist drug therapy that was impossible before the introduction of the INR and ISI. However, INR values obtained from the same patient plasma sample using different thromboplastins are significantly different. This is so even when the thromboplastins have nearly the same ISI values. We suggest that investigation of patient-specific differences can provide a means by which the INR discrepancies can be identified and understood and thus lead to better methods for monitoring oral anticoagulant therapy.

No deterioration after 13 years in a stability study of a rabbit brain, plain, thromboplastin, RBT 1010, in rubber-stoppered ampoules

RBT 1010, a rabbit brain thromboplastin, plain, was prepared at the Thrombosis Reference Centre, Withington Hospital, Manchester, in 1989. The batch has been stored at -20 degrees C, in rubber-stoppered ampoules, for 13 years. The material was unchanged after this time. This was confirmed in a stability study, in which the reagent was used to test the prothrombin times of a panel of plasmas stored in liquid nitrogen, one from a normal volunteer and two from stable anticoagulated patients. RBT 1010 was also tested in calibrations, according to World Health Organization recommendations, against the reference thromboplastin preparations CRM 149S and RBT 90.

Heterogeneity of Russell's viper venom affects the sensitivity of the dilute Russell's viper venom time to lupus anticoagulants

A number of studies have shown that commercial dilute Russell's viper venom time (DRVVT) reagents vary in their sensitivity for lupus anticoagulant (LA) detection. The differences in performance are considered to be predominantly due to antibody heterogeneity and a wide variation in phospholipid content, and also the techniques and clot detection methods employed. The present study compared the LA detection rates of five different Russell's viper venom (RVV) preparations using identical phospholipid reagents to assess the contribution of venom heterogeneity to LA detection by DRVVT. Initial analysis of 300 samples sent for thrombophilia screening identified 48 (16.0%) LAs by DRVVT and confirmatory tests with a single RVV reagent. Subsequent DRVVT analysis of all 300 samples using the other four venom reagents, with confirmatory tests on samples with elevated screen ratios, revealed a further 38 LAs in a variety of combinations of reagents. Only 15 of 86 (17.4%) of LAs were detected with all five RVV reagents. Significant biological variation of RVV exists due to differences between Russell's viper subspecies and a variety of environmental and physiological factors. The clear variations in diagnostic performance between alternative RVV preparations are most probably due to biological venom heterogeneity, and also differences in the manufacturing processes.

Clinical importance of positive test results for lupus anticoagulant and anticardiolipin antibodies

OBJECTIVES

To assess the performance of 4 clotting assays for lupus anticoagulant (LA) detection, to determine the prevalence of LA and anticardiolipin antibodies (aCL), and to correlate LA and aCL prevalence with systemic disease and thrombosis.

PATIENTS AND METHODS

We studied 664 consecutive patients at the Mayo Clinic in Rochester, Minn, who were referred for laboratory testing because of a clinical suspicion of LA or thrombophilia between June 25, 1990, and July 1, 1991.

RESULTS

Of 664 patients tested for LA, 584 also were tested for aCL. Of patients tested for both LA and aCL, 137 (235%) had positive results for one or both tests (13 [95%], LA-positive only; 76 [555%], aCL-positive only; and 48 [35.0%], positive for both). The dilute Russell viper venom time (DRVVT) was the most frequently positive LA assay (74% of the 61 patients with positive results for LA). Twenty-two patients (36.1% of the 61) had positive results for all 4 LA assays, whereas 21 (34.4% of the 61) had positive results for only 1 LA assay: activated partial thromboplastin time (3 patients [4.9%]), plasma clot time (5 patients [8.2%]), kaolin clot time (5 patients [8.2%]), or DRVVT (8 patients [13.1%]). Thromboembolism prevalence was not definitely associated with positive test results (LA only, aCL only, or LA plus aCL), nor was it strongly associated with aCL isotype or titer. Furthermore, thromboembolism prevalence was not increased when all LA assays were positive, although a history of deep venous thrombosis or pulmonary embolism was nonsignificantly associated with positive results for all 4 LA tests. The likelihood of having both LA- and aCL-positive test results was higher among patients with systemic lupus erythematosus (26 [19.0%] of 137 patients with positive results for one or both tests), but they had no more thrombotic events or fetal loss than other patients in our study group.

CONCLUSIONS

The DRVVT identified more patients with LA than the other LA tests, but more than 1 LA test was required to identify all patients with LA. Positive results were much more common for aCL than for LA. No single LA test or anticardiolipin isotype correlated with thrombosis or systemic disease in this population.

European Concerted Action on Anticoagulation. Quality Assessment of the CoaguChek Mini and TAS PT-NC Point-of-Care Whole-Blood Prothrombin Time Monitors

Background: International Normalized Ratios (INRs) for prothrombin time obtained with the CoaguChek Mini and TAS (RapidPointCoag) PT-NC systems are markedly different and also differ from the “true” INR. There is therefore a need for local quality assessment (QA) of the two systems.

Methods: A set of 60 lyophilized artificially depleted and 60 lyophilized coumarin plasmas were tested at 10 centers on both point-of-care testing monitors. Subsets of three and five plasmas were selected as QA plasmas and compared with the remaining 55 to assess the relative ability of the systems to characterize performance at the individual centers. The incidence of aberrant results (outliers; &gt;15% deviation from the true INR) was also recorded. The expected incidence with the QA plasmas was calculated and compared.

Results: On both systems, INR with the common sets of 55 lyophilized plasmas varied considerably between centers. With the TAS PT-NC, subsets of five and three European Concerted Action on Anticoagulation (ECAA) artificially depleted plasmas gave good correlation with the 55 plasmas, but the coumarin plasmas performed less well. With the CoaguChek Mini, correlation was good with sets of five artificially depleted QA plasmas and reasonable with three but was less satisfactory with the coumarin plasmas. Outliers were detected with both types of plasmas on both test systems but with variable success.

Conclusions: With the TAS PT-NC, three ECAA artificially depleted lyophilized plasmas provided reliable QA, but five lyophilized coumarin plasmas were required. With the CoaguChek Mini, five artificially depleted plasmas gave reliable QA but coumarin plasmas gave poorer results. ECAA QA plasmas provide a local system for checking INRs obtained with monitors of both types.

[International normalized ratio in the prothrombin test: clinical significance and use]

The safety and efficiency of therapy by peroral anticoagulants (PA) depend on a laboratory monitoring based on the prothrombin test (PT). The test is distinguished through its variability conditioned by different means of results' presentation as well as through the sensitivity of thromboplastin and a type of a device used in coagulation detection. WHO recommended, 1983, to standardize the thromboplastin preparations through adjusting their sensitivity (the so-called International Sensitivity Index--ISI) to blood coagulation defects induced by PA versus the primary international reference thromboplastin. Thromboplastin ISI as well as the mean normal prothrombin time (MNPT) of blood plasma are used to calculate the international normalized ratio (INR). The presentation of PT results as INR is justified exclusively for the PA-therapy stabilized patients. The INR system makes it possible to optimize the PA therapy only if the laboratory expert and clinician can clearly understand the PT standardization essence and observe the key WHO recommendations, i.e. definition of a coagulometer-specific ISI by manufacturing companies, estimation of MNPT by laboratories and use of the correct anticoagulant concentration.

Comparison between CoaguChek S- and Owren-type prothrombin time assay for monitoring anticoagulant therapy

INTRODUCTION

Anticoagulation therapy with warfarin is monitored by the prothrombin time (PT) assay. The PT is standardized using international normalized ratios (INRs). By keeping the INR within specific values, it is possible to reduce potential complications from the treatment. To facilitate the PT monitoring, point-of-care devices suitable for capillary whole blood measurements have been developed. The aims of this study were to compare the INR values obtained by such a device, CoaguChek S, with those obtained from the Owren-type PT assay and to evaluate the differences seen.

MATERIALS AND METHODS

In 351 consecutive warfarin-treated patients, INR was measured in capillary whole blood samples with CoaguChek S and was compared to venous plasma samples analyzed with the Owren PT method. Sixty-nine of these patients, including those deviating the most between the methods, were further evaluated according to levels of factor II (prothrombin), factor V, factor VII, factor X, fibrinogen, activated partial thromboplastin time (aPTT) and antiphospholipid antibodies.

RESULTS

The results from CoaguChek S produced a correlation coefficient of 0.81 to the Owren-type PT assay and a concordance of 85.2%. Factor V and fibrinogen correlated significantly (p<0.05) to the degree of deviation between the methods. The presence of antiphospholipid antibodies did not influence the degree of deviation between the two methods.

CONCLUSIONS

INR analysis of whole blood with CoaguChek S is comparable with INR measured in plasma with Owren chemistry. The activities of factor V and fibrinogen contribute to the deviation seen between the methods. Differences in sensitivity to antiphospholipid antibodies could not be demonstrated.

Quality assurance program for whole blood prothrombin time–international normalized ratio point-of-care monitors used for patient self-testing to control oral anticoagulation

Whole blood coagulation monitors are increasingly used for patient self-testing to control oral anticoagulation, but there are no comprehensive quality assurance (QA) programs to check their performance. We report on the experience with one of such programs applied in a field study where patients on prothrombin time (PT)-international normalized ratio (INR) self-testing were asked to bring their monitors to the anticoagulation clinic for checking. PT-INR testing was performed three times over 3 months with 14 patient's monitors and test strips on three recalcified QA plasmas by an experienced laboratory operator. Each patient was also asked to perform PT-INR self-testing (his/her own capillary blood) which was then compared to the laboratory PT-INR (plasma). Overall, the comparison between the observed and the consensus PT-INR on QA plasmas was acceptable with the majority of measurements lying within +/-15% or 20% of the consensus values. The comparison between the PT-INR self-testing and the laboratory method was also acceptable: overall, there was no statistical significant difference between the mean PT-INR values and the majority of paired measurements were less than 15% or 20% apart. In conclusion, our results show that the proposed QA scheme is feasible and may be implemented on a larger scale. Monitors should be recalled periodically to the clinic where they have been prescribed to the patient. During each visit, the clinic may check the monitors and patient self-testing performance as described. Such comprehensive QA system would make monitoring of oral anticoagulant treatment by self-testing safer and more effective.

Potential dosing errors using portable prothrombin time monitoring devices

Portable prothrombin time (PT) monitors facilitate the control of warfarin therapy. Few studies have compared the influence of using different monitors on dosage decisions. We determined the comparability of data generated by two portable PT monitors, Coaguchek S, (Roche Diagnostics Boehringer-Mannheim) and Hemochron Jr (International Technidyne Corporation Ltd.), with that of a reference laboratory. Simultaneous International Normalized Ratio (INR) measurements (portable monitor and laboratory) were performed in 193 consecutive patients receiving warfarin for at least 3 months. Agreement of measurements was assessed by both regression analysis and influence on dosage decisions in accordance with pre-defined criteria. The Coaguchek S versus laboratory INR regression line (n = 111; r2 = 0.88; P < 0.001) was close to the line of identity, while that of the Hemochron Jr (n = 82; r2 = 0.61; P < 0.001) was not. The overall proportion of dual INR measurements that fulfilled the clinical criteria of agreement was 90% for the Coaguchek S compared with 62% for the Hemochron Jr (P < 0.0001). For laboratory INRs 2.0-2.5, 2.6-4.0 and > 4.0, the proportions of portable measurements that satisfied the clinical criteria for the Coaguchek S versus the Hemochron Jr were 96 versus 63% (P < 0.001), 81 versus 45% (P < 0.04), and 67 versus 17% (P < 0.85), respectively. Warfarin dosing based solely on the portable devices would have resulted in unjustified dose increments in 22% of the patients with the Hemochron Jr device compared with 8% with the Coaguchek S monitor (chi2 = 4.43; P = 0.035). The Coaguchek S monitor provides measurements for INR values within the therapeutic range that agree well with the standard laboratory. The Hemochron Jr measurements result in different dosage adjustments even within the therapeutic range, but especially for INR values > 4.0. For both monitors, agreement of INR measurements with the standard decreases with increasing INR values.