A Simplified Procedure for Thromboplastin Calibration -The Usefulness of Lyophilized Plasmas Assessed in a Collaborative Study

The International Sensitivity Index (ISI) of thromboplastins is determined by calibration using fresh plasmas from 60 patients stabilized on oral anticoagulants and 20 healthy subjects. This procedure is demanding, particularly for those who have no easy access to patients. The alternative use of a smaller number of lyophilized plasmas has already been considered, but one important issue, the number of repeated measurements to be carried out, has never been addressed. Two commercial rabbit thromboplastins, A and B, were calibrated in 3 laboratories against CRM 149R. On each of 10 working days, prothrombin times were measured for a different set of 8 fresh plasmas and for the same set of 8 lyophilized plasmas. The ISI values for both thromboplastins were estimated by orthogonal regression on fresh and lyophilized plasmas. The between- and within-laboratory CV values of the estimated ISI were taken as measures of precision of the calibration. In addition, ISI and CV were calculated daily on cumulative results obtained with lyophilized plasmas from day 1 to day 10. The ISI values for both thromboplastins calculated with lyophilized plasmas were not significantly different from those with fresh plasmas (mean of 3 laboratories: 1.42 vs 1.48 for A and 1.22 vs 1.20 for B). The between-laboratory precision of the calibration with lyophilized plasmas was not considerably different from that with fresh plasmas (CV for 3 labs: 5.2% vs 6.8% for A and 0.9% vs 2.2% for B). The ISI estimated with lyophilized plasmas on results of day 1 were not different from those of days 2 through 10. Good within-laboratory precision of the calibration (CV around 2%) was already achieved on day 3. In conclusion, this study shows that lyophilized plasmas pooled from normals and patients on oral anticoagulants can be used as substitutes for individual fresh plasmas to simplify the existing procedure for thromboplastin calibration.

Results of a Multicenter Study Assessing the Status of Standardization of a Recombinant Thromboplastin for the Control of Oral Anticoagulant Therapy

An international collaborative study involving 20 laboratories was undertaken to evaluate the status of standardization of a commerical recombinant thromboplastin. The results show that the reagent fulfills the WHO requirements for thromboplastin calibration against two International Reference Preparations (IRP) from different species, because there was a linear relationship of log-transformed prothrombin times as measured by the recombinant reagent and the two IRPs in normal individuals and patients stabilized on oral anticoagulants. The regression lines drawn through the patient data points passed through the normal data points in the majority of instances. The average ISI values for three different batches were close to unity. On the average, the ISI value calculated against BCT/253 (human, plain) was slightly smaller than that against RBT/79 (rabbit, plain). The between laboratory precision of calibration (CV = 5-6%) did not change whether BCT/253 or RBT/79 was used, whereas it was substantially improved when the calibrations of batches 2 and 3 were performed against batch 1, arbitrarily taken as standard (CV ≤ 1.7%). Statistical analysis to test for differences between slopes revealed no significant between-batch differences in 16 of 20 laboratories. However slight differences for the third batch (2.8% less than the average ISI value of batches 1 and 2) leave room for further improvement in the standardization of the reagent.

Interlaboratory variability in the measurement of direct oral anticoagulants: results from the external quality assessment scheme

Essentials Tests for direct oral anticoagulants (DOACs) are not widely applied. These tests are perceived to be difficult to run and subjected to large between-lab variation. We carried out proficiency testing surveys for DOAC testing in Italy. Interlab variability was small and similar to that of the international normalised ratio.

SUMMARY

Background Tests for direct oral anticoagulants (DOACs) are not widely available. The perception that they are difficult to perform and are subject to large between-laboratory variation makes their implementation difficult. Aims We carried out proficiency-testing surveys for DOACs within the activity of the external quality-assessment scheme of the Italian Federation of Thrombosis Centers. Design Participants were provided with coded freeze-dried plasmas without or with graded concentrations of the three main DOACs, and asked to measure prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time and DOAC concentrations with dedicated tests. The results were centralized for statistical analysis. Results and conclusions All participants (n = 235) reported results for PT and APTT, and approximately one-third reported results for DOAC concentration. PT and APTT showed variable responsiveness to DOACs: PT was more responsive to rivaroxaban than to dabigatran or apixaban. APTT was more responsive to dabigatran than to rivaroxaban or apixaban. The thrombin time ratio (test/normal) was close to unity for plasmas without dabigatran, and was high (i.e. 7.6-fold or 15.4-fold longer than the plasma free from the drug) for plasmas containing dabigatran at low (i.e. 42 ng mL-1 ) or high (i.e. 182 ng mL-1 ) concentration. Dedicated tests were responsive to the respective drugs, and their interlaboratory variability was relatively small (overall coefficients of variation of 8.7%, 8.4% or 10.3% for dabigatran, rivaroxaban and apixaban, respectively) and was comparable to that observed within the same survey for the International Normalized Ratio (i.e. 11.4%). In conclusion, tests for DOAC measurement performed reasonably well in a national quality-control scheme. Regulatory authorities should urgently issue recommendations on their use, and clinical laboratories should make them available.

International collaborative study for the calibration of proposed International Standards for thromboplastin, rabbit, plain, and for thromboplastin, recombinant, human, plain

Essentials Two candidate International Standards for thromboplastin (coded RBT/16 and rTF/16) are proposed. International Sensitivity Index (ISI) of proposed standards was assessed in a 20-centre study. The mean ISI for RBT/16 was 1.21 with a between-centre coefficient of variation of 4.6%. The mean ISI for rTF/16 was 1.11 with a between-centre coefficient of variation of 5.7%.

SUMMARY

Background The availability of International Standards for thromboplastin is essential for the calibration of routine reagents and hence the calculation of the International Normalized Ratio (INR). Stocks of the current Fourth International Standards are running low. Candidate replacement materials have been prepared. This article describes the calibration of the proposed Fifth International Standards for thromboplastin, rabbit, plain (coded RBT/16) and for thromboplastin, recombinant, human, plain (coded rTF/16). Methods An international collaborative study was carried out for the assignment of International Sensitivity Indexes (ISIs) to the candidate materials, according to the World Health Organization (WHO) guidelines for thromboplastins and plasma used to control oral anticoagulant therapy with vitamin K antagonists. Results Results were obtained from 20 laboratories. In several cases, deviations from the ISI calibration model were observed, but the average INR deviation attributabled to the model was not greater than 10%. Only valid ISI assessments were used to calculate the mean ISI for each candidate. The mean ISI for RBT/16 was 1.21 (between-laboratory coefficient of variation [CV]: 4.6%), and the mean ISI for rTF/16 was 1.11 (between-laboratory CV: 5.7%). Conclusions The between-laboratory variation of the ISI for candidate material RBT/16 was similar to that of the Fourth International Standard (RBT/05), and the between-laboratory variation of the ISI for candidate material rTF/16 was slightly higher than that of the Fourth International Standard (rTF/09). The candidate materials have been accepted by WHO as the Fifth International Standards for thromboplastin, rabbit plain, and thromboplastin, recombinant, human, plain.

A Comparison of Two Sodium Citrate Concentrations in Two Evacuated Blood Collection Systems for Prothrombin Time and ISI Determination

The prothrombin time is usually measured in citrated plasma. The W.H.O. recommended concentration of sodium citrate for blood collection for laboratory control of oral anticoagulant therapy is 0.109 M. Some evacuated blood collection systems include 0.105 M sodium citrate. The purpose of the present study was to establish the difference in ISI calibration between 0.109 and 0.105 M citrate, using 7 types of thromboplastin and various types of instrumentation. The two citrate concentrations were provided in both evacuated siliconised glass tubes and in evacuated polyethylene terephtalate (PET) tubes. The ISI difference between the two citrate concentrations was 5.4% for one system but not greater than 3% for all other systems when blood samples were collected with either siliconized glass or PET tubes. Most of the ISI differences between the two citrate concentrations were not significant at the 5% level. It is concluded that the ISI differences between 0.105 M and 0.109 M citrate are not of practical importance. In contrast, ISI differences between siliconised glass and PET tubes, using either 0.105 or 0.109 M citrate, were significant (p <0.05) for most thromboplastin systems and amounted to 7%. ISI interchange between these glass and PET tubes could induce INR differences amounting to 14%, which could affect clinical dosage of oral anticoagulants.

Variability of cut-off values for the detection of lupus anticoagulants: results of an international multicenter multiplatform study

Essentials Between-lab variations of cut-off values in lupus anticoagulant detection are unknown. Cut-off values were calculated in 11 labs each testing plasma from 120 donors with 3 platforms. Major variation was observed even within the same platform. Cut-off values determined in different labs are not interchangeable.

SUMMARY

Background Cut-off values for interpretation of lupus anticoagulant (LA) detection are poorly investigated. Aims (i) To assess whether results from healthy donors were normally distributed and (ii) the between-laboratories differences in cut-off values for screening, mixing and LA confirmation when calculated as 99th or 95th centiles, and (iii) to assess their impact on the detection rate for LA. Methods Each of 11 laboratories using one of the three widely used commercial platforms for LA detection was asked to collect plasmas from 120 healthy donors and to perform screening, mixing and LA confirmation with two methods (activated partial thromboplastin time [APTT] and dilute Russell viper venom [dRVV]). A common set of LA-positive or LA-negative freeze-dried plasmas was used to assess the LA detection rate. Results were centralized (Milano) for statistical analysis. Results and conclusions (i) Clotting times or ratios for healthy subjects were not normally distributed in the majority of cases. The take-home message is that cut-off values should be determined preferably by the non-parametric method based on centiles. (ii) There were relatively large inter-laboratory cut-off variations even within the same platform and the variability was marginally attenuated when results were expressed as ratios (test-to-normal pooled plasma). The take-home message is that cut-off values should be determined locally. (iii) There were differences between cut-off values calculated as 99th or 95th centiles that translate into a different LA detection rate (the lower the centile the greater the detection rate). The take-home message is that cut-off values determined as the 95th centile allow a better LA detection rate.

The thrombin generation assay distinguishes inhibitor from non-inhibitor patients with severe haemophilia A

INTRODUCTION

Patients with haemophilia A (HA) have impaired thrombin generation (TG) capacity and TG assay (TGA) values are linearly related to plasma factor VIII (FVIII) levels.

AIM

This study carried out in patients with unmeasurable FVIII (<1 IU dL(-1) ) was aimed at unravelling any difference in TG capacity in patients with or without inhibitors.

METHODS

Blood samples were collected from patients in a non-bleeding state, after a 5-day wash-out period from last treatment.

RESULTS

TGA was performed in 102 patients with severe HA (15% with high-responding inhibitors; 51% with null F8 mutations, that as expected were more prevalent in inhibitor than in non-inhibitor patients). TG capacity was significantly lower in inhibitor than non-inhibitor patients and in those with null mutations than in those with non-null mutations. When the TG capacity was evaluated only in patients with null mutations with and without inhibitors it was lower in the presence of inhibitors.

CONCLUSIONS

This study shows a greater TG impairment in inhibitor patients irrespective of FVIII levels, inhibitor titre and F8 mutation type, suggesting a role for the TGA in unravelling functional interferences of anti-FVIII inhibitors on coagulation system activation.

Standardization of lupus anticoagulant. The Lupus Anticoagulant Sensitivity Index (LASI)

Results for lupus anticoagulant (LA) are expressed as ratio of patient-to-normal clotting times (LA-ratio) according to the equation LA-ratio = (Patient(Clotting time)/Normal(Clotting time)). However, numerical results vary according to the method used for testing, thus making difficult the between-method and between-laboratory comparison of results. The hypothesis that the standardization model currently employed for the international normalized ratio for patients on warfarin is valid also for LA standardization has been taken into consideration. The model calls for the determination of a LA-sensitivity index (LASI) for each commercial method for LA detection against a common standard method. The LASI is then used to convert the LA-ratio into a scale called standardized LA-ratio (SLA-ratio) according to the equation SLA-ratio = (LA-ratio)(LASI). The model proved effective in minimizing the between-method variability of results for LA detection. If implemented it could be a valuable tool to improve the comparability of results obtained in different laboratories, to quantify the LA potency and thus pave the way to the organization of collaborative clinical trials aimed at assessing whether the potency of LA is a risk factor for clinical events.

Severe hemophilia with mild bleeding phenotype: molecular characterization and global coagulation profile

BACKGROUND

Patients with severe hemophilia may show very varied bleeding tendencies, and the reasons for this heterogeneous clinical expression are unclear. The factor VIII/FIX genotype is the main determinant of the residual factor activity; however, different bleeding phenotypes have also been reported in patients sharing the same mutation. Such global coagulation tests as thrombin generation assays are tools with which to investigate different coagulation profiles among severe hemophiliacs.

OBJECTIVES, PATIENTS AND METHODS

This case-control study was aimed at comprehensively evaluating the role of genotype and endogenous thrombin potential (ETP) as predictors of the clinical phenotype in severe hemophiliacs with an extremely mild bleeding tendency (cases, n = 22), in comparison with those showing a typical bleeding tendency (controls, n = 50).

RESULTS

Cases were more frequently affected by hemophilia B than by hemophilia A, and showed a lower incidence of severe FVIII/FIX gene defects (referred to as null mutations), higher FVIII and FIX antigen levels and higher ETP values in platelet-rich plasma than controls (P < 0.05). By multivariate logistic regression, only non-null mutations were confirmed as an independent predictor of a mild clinical phenotype.

CONCLUSIONS

These results indicate that non-null mutations represent the main determinant of the bleeding tendency, and that ETP measurement in platelet-rich plasma is able to identify severe hemophiliacs with a mild clinical phenotype.

Thromboplastin standards

The Prothrombin Time (PT) test is used for monitoring of treatment with Vitamin K-antagonists (VKA). The result of the PT test should be expressed as the International Normalized Ratio (INR). Calculation of INR is based on the availability of International Standards (IS) for thromboplastin and a calibration model. Calibration of a new PT test system is performed with the appropriate IS and fresh plasma samples of healthy (normal) volunteers and patients treated with VKA. The calibration model is based on the assumption of a linear relationship between the log(PT)'s obtained with the new PT system and the reference IS for both normal and patients' samples. Patients' samples for calibration should be selected by rejecting samples beyond the 1.5-4.5 INR range. Outliers should be rejected defined as points with a perpendicular distance greater than three residual standard deviations from the line of relationship. Selection of patients' samples and rejection of outliers result in a reduction of the between-laboratory variation of calibration. In addition to monitoring of VKA, the PT is used for management of patients with chronic liver disease. Likewise, INR(liver) should be based on calibration with an IS using samples from patients with chronic liver disease.

Second international collaborative study evaluating performance characteristics of methods measuring the von Willebrand factor cleaving protease (ADAMTS-13)

BACKGROUND

Over the last 4 years ADAMTS-13 measurement underwent dramatic progress with newer and simpler methods.

AIMS

Blind evaluation of newer methods for their performance characteristics.

DESIGN

The literature was searched for new methods and the authors invited to join the evaluation. Participants were provided with a set of 60 coded frozen plasmas that were prepared centrally by dilutions of one ADAMTS-13-deficient plasma (arbitrarily set at 0%) into one normal-pooled plasma (set at 100%). There were six different test plasmas ranging from 100% to 0%. Each plasma was tested 'blind' 10 times by each method and results expressed as percentage vs. the local and the common standard provided by the organizer.

RESULTS

There were eight functional and three antigen assays. Linearity of observed-vs.-expected ADAMTS-13 levels assessed as r2 ranged from 0.931 to 0.998. Between-run reproducibility expressed as the (mean) CV for repeated measurements was below 10% for three methods, 10-15% for five methods and up to 20% for the remaining three. F-values (analysis of variance) calculated to assess the capacity to distinguish between ADAMTS-13 levels (the higher the F-value, the better the capacity) ranged from 3965 to 137. Between-method variability (CV) amounted to 24.8% when calculated vs. the local and to 20.5% when calculated vs. the common standard. Comparative analysis showed that functional assays employing modified von Willebrand factor peptides as substrate for ADAMTS-13 offer the best performance characteristics.

CONCLUSIONS

New assays for ADAMTS-13 have the potential to make the investigation/management of patients with thrombotic microangiopathies much easier than in the past.

High thrombin generation measured in the presence of thrombomodulin is associated with an increased risk of recurrent venous thromboembolism

BACKGROUND

The assessment of the risk of recurrent venous thromboembolism (VTE) is important to determine the optimal duration of secondary prophylaxis. The risk can be estimated by measuring individual parameters reflecting hypercoagulability. Because of the large numbers of such putative parameters, the assessment in individual patients is complex. Application of global assays reflecting the pro-/anti-coagulant balance in vivo would be desirable.

OBJECTIVES

To investigate the relationship between recurrent VTE and thrombin generation (TG).

PATIENTS-METHODS

Two hundred and fifty-four patients were followed-up after a first episode of unprovoked, objectively documented VTE for a period of 2.7 years after discontinuation of treatment with vitamin K antagonists. TG was measured 1 month after discontinuation of treatment as endogenous thrombin potential (ETP), peak thrombin and lag-time in the presence or absence of thrombomodulin. The study outcome was objectively documented symptomatic recurrent VTE.

RESULTS

Patients with ETP or peak (measured in the presence of thrombomodulin) of >960 nm(*)min or >193 nm had hazard ratios (HR) (95% CI) for recurrent VTE of 3.41 (1.34-8.68) or 4.57 (1.70-12.2) as compared with those with an ETP <563 nm(*)min or peak <115 nm. Patients with lag-time <14.5 min had HR of 3.19 (1.29-7.89) as compared with those with lag-time >20.8 min. HR for ETP, peak or lag-time measured in the absence of thrombomodulin were smaller than those measured in the presence of thrombomodulin.

CONCLUSIONS

The measurement of TG helps to identify patients at higher risk of VTE recurrence. The increased risk may be better appreciated if the test is performed in the presence of thrombomodulin.

The international normalized ratio to prioritize patients for liver transplantation: problems and possible solutions

The prothrombin time (PT) test once designed by Dr Quick to investigate patients with obstructive jaundice was later adapted and standardized by means of the international normalized ratio (INR) to monitor patients on treatment with vitamin K antagonists (VKA). After more than 70 years from its introduction it is now time to think about its standardization for those very patients for whom it was intended at the beginning of its history. Two studies carried out independently and published recently in the same issue of a specialized journal do exploit the very same idea on how to accomplish this standardization. Both of them confirm previous anecdotal observations that the INR as devised for patients on VKA (INR(vka)) is not valid to harmonize PT results for patients with chronic liver disease. This fact, that at first sight may appear academic, has important consequences because the PT INR is used to construct the model for end-stage liver disease (MELD) scores, which is widely used to prioritize patients for liver transplantation. The two studies further demonstrate that an alternative calibration model, modified from that recommended by the World Health Organization for patients on VKA, may be feasible also for patients with chronic liver disease. This alternative calibration model, which calls for the substitution of plasmas from patients on VKA with those from patients with chronic liver disease, may be highly beneficial to harmonize the calculation of the MELD score, with important implications for the prioritization of patients for liver transplantation.

The international normalized ratio to prioritize patients for liver transplantation: problems and possible solutions

The prothrombin time (PT) test once designed by Dr Quick to investigate patients with obstructive jaundice was later adapted and standardized by means of the international normalized ratio (INR) to monitor patients on treatment with vitamin K antagonists (VKA). After more than 70 years from its introduction it is now time to think about its standardization for those very patients for whom it was intended at the beginning of its history. Two studies carried out independently and published recently in the same issue of a specialized journal do exploit the very same idea on how to accomplish this standardization. Both of them confirm previous anecdotal observations that the INR as devised for patients on VKA (INR(vka)) is not valid to harmonize PT results for patients with chronic liver disease. This fact, that at first sight may appear academic, has important consequences because the PT INR is used to construct the model for end-stage liver disease (MELD) scores, which is widely used to prioritize patients for liver transplantation. The two studies further demonstrate that an alternative calibration model, modified from that recommended by the World Health Organization for patients on VKA, may be feasible also for patients with chronic liver disease. This alternative calibration model, which calls for the substitution of plasmas from patients on VKA with those from patients with chronic liver disease, may be highly beneficial to harmonize the calculation of the MELD score, with important implications for the prioritization of patients for liver transplantation.

International collaborative study for the calibration of a proposed international standard for thromboplastin, rabbit, plain

BACKGROUND

A preparation of rabbit brain thromboplastin, provisionally coded 04/162, is proposed as a candidate for the World Health Organization (WHO) International Standard (IS) for thromboplastin (rabbit, plain), meant to replace the IS coded RBT/90 (rabbit, plain), stocks of which are now exhausted.

RESULTS

The preparation was calibrated in an international collaborative study involving 21 laboratories from 13 countries and the calibration was performed against the existing WHO-IS (i.e. rTF/95 and OBT/79) and other Certified Reference Materials from the Institute for Reference Materials and Measurements of the European Commission (i.e. CRM149 S) and from the European Action on Anticoagulation (i.e. EUTHR-01). An additional candidate rabbit brain thromboplastin coded as 04/106 was also included in the study. On the basis of predefined criteria (the within- and between-laboratory precision of the calibration and the conformity to the calibration model), 04/162 was the preferred candidate.

CONCLUSIONS

The assigned International Sensitivity Index value was 1.15 and the inter-laboratory SD and coefficient of variation were 0.057% and 4.9%, respectively.

Measurement of von Willebrand factor cleaving protease (ADAMTS-13): results of an international collaborative study involving 11 methods testing the same set of coded plasmas

BACKGROUND

ADAMTS-13 is a von Willebrand factor (VFW)-cleaving protease. Its congenital or acquired deficiency is associated with thrombotic thrombocytopenic purpura (TTP) and more rarely with the hemolytic uremic syndrome. We report on a survey evaluating 11 methods for ADAMTS-13 measurement performed in different labs.

DESIGN

Two plasmas, one normal and one from a patient with familial TTP, were mixed at the co-ordinating center to prepare 6 plasmas with 0%, 10%, 20%, 40%, 80% and 100% ADAMTS-13 levels. Each plasma was aliquoted and assembled into sets of 60 (coded from 1 to 60), each containing 10 copies of the original 6 plasmas. Plasmas were frozen and shipped in dry ice to 10 labs with a common frozen reference plasma. Laboratories were asked to measure ADAMTS-13 with their methods. Results were sent to the coordinating center for statistical analysis.

RESULTS

Of the 10 methods performed under static conditions 9 were quantitative and one was semiquantitative. One method performed under flow conditions evaluated the extent of cleavage of endothelial cell-derived ultralarge VWF string-like structures and expressed results as deficient, normal, or borderline. Linearity (expected-vs-observed levels), assessed as the squared correlation coefficient, ranged from 0.98 to 0.39. Reproducibility, expressed as the coefficient of variation for repeated measurements, ranged from < 10% to 83%. The majority of methods were able to discriminate between different ADAMTS-13 levels. The majority were able to detect the plasma with 0% level and some of them to discriminate between 0% and 10%. Overall the best performance was observed for three methods measuring cleaved VWF by ristocetin cofactor, collagen binding, and immunoblotting of degraded multimers of VWF substrate, respectively. The poor interlaboratory agreement of results was hardly affected by the use of the common standard. The method performed under flow conditions identified the plasmas with 0%, 10%, 20% and 40% activity as deficient in 7, 5, 1 and 3 of the 10 replicate measurements. The plasmas with 80% and 100% were identified as normal in all of the 10 replicate measurements.

CONCLUSIONS

The survey shows varied performance, but supports an optimistic view about the reliability of current methods for ADAMTS-13.

Laboratory control of oral anticoagulant treatment by the INR system in patients with the antiphospholipid syndrome and lupus anticoagulant. Results of a collaborative study involving nine commercial thromboplastins

Because of the variable responsiveness of thromboplastins to lupus anticoagulants (LA), concerns have been raised about the validity of the prothrombin time-International Normalized Ratio (PT-INR) in monitoring oral anticoagulant treatment in patients with the antiphospholipid syndrome (APS) and LA. To date, few studies have been performed, numbers of patients investigated are relatively small and results are conflicting. We report on a multicentre study organized to investigate further this clinically relevant issue. Each of nine thrombosis centres was asked to collect plasma samples from patients with APS who were on oral anticoagulants (cases) and patients without APS who were on oral anticoagulants (controls). Nine thromboplastins (three human recombinant, one from human placenta and five from rabbit brain) were calibrated at the co-ordinating centre according to World Health Organization guidelines. Measurements of the INR and factor X amidolytic activity for all frozen plasmas were performed centrally. The numbers of patients investigated were 58 cases and 57 controls. Between-reagent variability of the INR was higher in cases [coefficient of variation (CV) = 12.4%] than in controls (CV = 6.7%), but this was because of one of the thromboplastins only (Thromborel R, human recombinant), which measured considerably higher INR values than the others in cases but not in controls. In conclusion, our data indicate that LA interference on the PT-INR measured with the majority of commercial thromboplastins is not enough to cause concern if insensitive thromboplastins, properly calibrated to assign them an instrument-specific International Sensitivity Index are used. New thromboplastins, especially those made of relipidated tissue factor, should be checked for their responsiveness to LA before they are used to monitor oral anticoagulant treatment in patients with APS.

Multicenter study of homocysteine measurement--performance characteristics of different methods, influence of standards on interlaboratory agreement of results

After the demonstration that moderate hyperhomocysteinemia is associated with thrombosis, many hematological labs are becoming interested in total homocysteine (tHcy) measurement. This prompted us to organize a collaborative study to investigate the performance of methods used in this setting and to assess the between-lab comparability of results. Two pairs of pooled plasma (A1-A2 and B1-B2) were prepared at the coordinating Center. tHcy levels were normal in A1-A2 and moderately high in B1-B2. Within each pair tHcy levels were similar but not identical. Aliquots were taken from each pool to prepare sets of 100 samples (coded from 1 to 100). Each set consisted of 25 replicates for each pool. Samples were frozen and shipped in dry ice to 16 labs with a common frozen aqueous standard. Labs were asked to measure (in blind) tHcy with their methods and standards. Results were sent to the coordinating Center both as raw readings and as tHcy levels. The following methods were used: High Pressure Liquid Chromatography (HPLC) in 12 labs (home-made in 10 and commercial in 2); Enzyme Immuno Assays (EIA) in 2; Fluorescence Polarization Immunoassay (FPIA) in 2 and Capillary Electrophoresis (CE) in one. Results for paired pools (A1-A2 and B1-B2) were analyzed by the Student t test to assess for the ability to discriminate between similar but not identical tHcy levels. Results for each pool were used to assess within-lab reproducibility and between-lab comparability. Within-lab reproducibility expressed as median CV ranged from 12.6 to 13.9% (home-made HPLC); from 9.2 to 11.4% (commercial HPLC); from 21.8 to 24.2% (EIA); from 2.7 to 3.3% (FPIA) and from 11.2 to 22.0% (CE). All labs, except one using CE and 2 using home-made HPLC, were able to discriminate between similar tHcy levels in the normal range (pools A1-A2). Ten labs (4 using home-made HPLC, 2 commercial HPLC, 2 FPIA, one EIA and one CE) were able to discriminate between similar moderately high tHcy levels (pools B1-B2). Between-lab comparability expressed as CV was 14.0% 13.9%, 15.6% and 14.5% for pools A1, A2, B1, and B2. These values were considerably lower (CV values < 5.2%) when a common plasma standard was used for calculation of tHcy levels, while the use of a common aqueous standard failed to achieve the necessary harmonization. In conclusion, performance characteristics of the FPIA method compare favorably with the well-established HPLC methods. It is simpler and more suitable to be used by general hematological labs. Between-lab comparability of results is still a problem. The establishment of an international plasma standard would be of help to harmonize tHcy measurement across laboratories.

A comparison of two sodium citrate concentrations in two evacuated blood collection systems for prothrombin time and ISI determination

The prothrombin time is usually measured in citrated plasma. The W.H.O. recommended concentration of sodium citrate for blood collection for laboratory control of oral anticoagulant therapy is 0.109 M. Some evacuated blood collection systems include 0.105 M sodium citrate. The purpose of the present study was to establish the difference in ISI calibration between 0.109 and 0.105 M citrate, using 7 types of thromboplastin and various types of instrumentation. The two citrate concentrations were provided in both evacuated siliconised glass tubes and in evacuated polyethylene terephtalate (PET) tubes. The ISI difference between the two citrate concentrations was 5.4% for one system but not greater than 3% for all other systems when blood samples were collected with either siliconized glass or PET tubes. Most of the ISI differences between the two citrate concentrations were not significant at the 5% level. It is concluded that the ISI differences between 0.105 M and 0.109 M citrate are not of practical importance. In contrast, ISI differences between siliconised glass and PET tubes, using either 0.105 or 0.109 M citrate, were significant (p <0.05) for most thromboplastin systems and amounted to 7%. ISI interchange between these glass and PET tubes could induce INR differences amounting to 14%, which could affect clinical dosage of oral anticoagulants.

Alpha(2)-macroglobulin levels are high in adult patients with congenital antithrombin deficiency

Antithrombin is responsible for about 80% of the progressive inhibitory activity of thrombin in human plasma. The role of other protease inhibitors known to inhibit thrombin is not completely clarified. However, their contribution may become relevant when antithrombin is low. We elected to investigate adult patients with congenital antithrombin deficiency to assess the concentration of other naturally occurring thrombin inhibitors such as alpha(2)-macroglobulin, alpha(1)-antitrypsin, heparin cofactor II, and C(1)-inhibitor. The study included 59 patients with congenital antithrombin deficiency with and without a previous history of thrombosis, together with an equal number of control subjects matched for age and sex. Statistically significant differences (patients vs. controls) were observed only for alpha(2)-macroglobulin (i.e., 120 vs. 102%, p<0.01). Further analysis of antithrombin-deficient carriers with and without a past history of thrombosis showed that alpha(2)-macroglobulin levels were higher than the 90th percentile of control distribution more often in asymptomatic than symptomatic men (odds ratio=0.04; confidence interval=0.003-0.60), but not in women (odds ratio=2.14; confidence interval=0.35-13.1). In conclusion, results from this cross sectional study showed that alpha(2)-macroglobulin levels were high in patients with congenital antithrombin deficiency. Furthermore, the high levels were found more often in asymptomatic than symptomatic men. Whether this increase provides protection against thrombosis should be evaluated in a prospective study.

Calibration of local systems with lyophilized calibrant plasmas improves the interlaboratory variability of the INR in the Italian external quality assessment scheme

Calibration with lyophilized calibrant plasmas certified in terms of PT with International Reference Preparations for thromboplastin has been proposed to minimize the effect of coagulometers on the INR. Aim of this study was to test the ability of local calibration with lyophilized calibrant plasmas, combined with a modified statistical approach, to improve the interlaboratory variability of the INR measured on two test plasmas (one coumarin and one artificially-depleted) by participants in the External Quality Assessment Scheme (EQAS). Sets of lyophilized calibrant and test plasmas were sent to the participants in the EQAS, who were asked to determine PT with their own reagent/ instrument combination (local system). Results were returned as PT together with information on the type of local system, the stated International Sensitivity Index (ISI) and the geometric mean of PTs determined by testing with the local system fresh plasmas from 20 healthy subjects. Ninety-two participants using 9 and 11 brands of reagents and instruments returned results. The CV of the INR determined with the stated ISI for the coumarin (Mean INR = 4.39) and artificially-depleted (Mean INR = 4.23) test plasmas were 11.2% and 10.3% and were reduced on the average by 34% and 54%, respectively, when the INR was calculated with the local ISI. In conclusions, results from this field study involving laboratories and testing systems representative of the real situation in oral anticoagulant monitoring in our country, indicate that local calibration by artificially-depleted plasmas, combined with the proposed statistical approach, is suitable to improve the interlaboratory agreement on the INR.

The G20210A mutation of the prothrombin gene in patients with previous first episodes of deep-vein thrombosis: prevalence and association with factor V G1691A, methylenetetrahydrofolate reductase C677T and plasma prothrombin levels

A common G to A transition at nucleotide 20210 of the prothrombin gene is associated with an increased risk for deep-vein thrombosis (DVT) and high plasma levels of prothrombin. We calculated the prevalences of prothrombin G20210A, factor V G1691A (also associated with high risk for DVT) and homozygous methylenetetrahydrofolate reductase (MTHFR) C677T (associated with increased susceptibility to develop hyperhomocysteinemia) in 118 patients with a first episode of DVT and in 416 healthy controls. 15.9% of the patients and 2.3% of the controls had prothrombin G20210A (odds ratio [OR]: 8.7, 95% C.I.: 3.8-21.4); 21.1% of the patients and 3.2% of the controls had factor V G1691A (OR 7.8, 3.9-17.1); 20.5% of the patients and 21% of the controls had homozygous MTHFR C677T (OR: 1.0, 0.7-1.2). Exclusion of patients with other hereditary risk factors for DVT did not substantially modify the results. Mutant factor V and prothrombin coexisted in three patients but in no control. The concomitant presence of the MTHFR mutation did not increase the thrombotic risk associated with prothrombin G20210A. 63.2% of individuals with prothrombin G20210A had plasma levels of prothrombin in the upper quartile of distribution. After adjustment for age and sex, subjects with prothrombin levels in the upper quartile carried a slightly higher risk for thrombosis than those with lower prothrombin concentrations (OR: 1.9, 1.1-3.2). In conclusion, we found that prothrombin G20210A is relatively common in Italy and is associated with high prothrombin levels and an 8.7-fold increase in the risk for DVT. Such risk is independent of the coexistence of other known inherited risk factors for thrombosis and increases in patients with associated mutant factor V. Whether it is due to the associated increase in plasma prothrombin levels remains to be established.

Assessment of the influence of citrate concentration on the International Normalized Ratio (INR) determined with twelve reagent-instrument combinations

Citrate concentration is one of the variables that can affect coagulation tests. However, few studies have so far been performed to assess the magnitude of this effect on coagulation tests in general and PT in particular. The aim of this study was to assess the extent of influence of citrate concentration on the PT test with results expressed as INR. Twelve reagent-instrument combinations (systems) were calibrated vs. the Reference Preparation BCT/441 using plasmas collected in either 105 mM or 129 mM citrate from normals and anticoagulated patients (OAT). PTs for plasmas collected in 129 mM citrate were longer than those collected in 105 mM both for normals and patients on OAT, but the ratios (patient-to-normal clotting times) for the two citrate concentrations were significantly different in many instances, implying that the International Sensitivity Index (ISI) is also different. ISIs for calibrations with plasmas collected in 105 mM were greater (up to 10%) than those with plasmas collected in 129 mM citrate. When PT ratios were transformed into INR using crossover ISIs (i.e., plasmas collected in 105 mM and ISI determined with plasmas collected in 129 mM citrate, or vice versa) we found that an INR of 4.5 could be up to 20% apart from the value that would have been obtained if the appropriate ISI was used. Moreover, if the ISI determined with the manual technique was used to convert PTs obtained with a particular instrument into INR, the effect of citrate concentration was even greater (INR difference up to 64%). Should these observations be valid for other systems, they might provide additional explanations for the frequent reports which document discrepancies in the INR determined with different systems to which incorrect ISI might have been applied. World-wide consensus on a single citrate concentration to collect patients' as well as lyophilized plasmas to be used in External Quality Assessment Schemes and for local system calibration is therefore urgently needed.

Standardization of the APC resistance test. Effects of normalization of results by means of pooled normal plasma

Results for APC resistance tests are expressed as the ratio of the clotting time with and without APC (APC ratio). Normalization by dividing the patient's APC ratio by that of the pooled normal plasma (PNP) (n-APC ratio) was proposed to minimize between-lot reagent variability. To evaluate the merits of different expressions of the results, sets of 80 coded frozen plasmas from carriers (n = 30), non-carriers (n = 30) of the FV:Q506 mutation and 10 copies each of two control plasmas were sent to 7 expert laboratories which were asked to assess APC resistance by their methods. Results were expressed as APC ratio and n-APC ratio by the local PNP and 2 common PNP (A and B). These contained plasmas from the same (n = 20) non-carriers. In PNP A, plasma from one non-carrier was replaced with that from one heterozygous carrier. The merits of different expression of results were judged by (i) within-laboratory reproducibility: (ii) discrimination of carriers from non-carriers: (iii) between-method comparability of results. The influence of FV:Q506 plasma in the preparation of PNP was also assessed. Reproducibility, generally good even with results expressed as APC ratio (median CV 4.6% and 3.0% for normal and abnormal control), was not changed by normalization. Discrimination did not change whatever the method of result expression. Between-method comparability of results was scarcely affected by normalization with the local PNP, whereas it was considerably improved after normalization with the common PNP, but only for the non-carriers. APC ratios for PNP A by all methods were significantly lower than those for PNP B. Thus, the presence of mutant FV in a proportion as low as 2.5% may reduce the APC ratio of the PNP.

International collaborative study for the calibration of a proposed reference preparation for thromboplastin, human recombinant, plain. On behalf of the Subcommittee on Control of Anticoagulation

Stocks of the International Reference Preparation (IRP) for thromboplastin, human, plain, coded BCT/253 and held by the World Health Organization (WHO) are nearly exhausted and must be replaced. For practical reasons the choice of the replacement candidate was restricted to two available human recombinant preparations which were coded as X/95 and Y/95 and calibrated in an international collaborative study involving 19 laboratories from Europe, Australia, Canada and Argentina. To minimize the differences between routes of calibration, the two candidates were calibrated against the existing WHO-IRP from human, rabbit and bovine origin and the final ISI was the resultant average value. On the basis of predefined criteria (i.e., within- and between-laboratory precision of the calibration and the conformity to the calibration model), X/95 was the preferred candidate. The assigned ISI (SE of the mean) value is 0.940 (0.0060) and the interlaboratory coefficient of variation 4.7%.

Determination of the International Sensitivity Index of a new near-patient testing device to monitor oral anticoagulant therapy--overview of the assessment of conformity to the calibration model

A key issue for the reliable use of new devices for the laboratory control of oral anticoagulant therapy with the INR is their conformity to the calibration model. In the past, their adequacy has mostly been assessed empirically without reference to the calibration model and the use of International Reference Preparations (IRP) for thromboplastin. In this study we reviewed the requirements to be fulfilled and applied them to the calibration of a new near-patient testing device (TAS, Cardiovascular Diagnostics) which uses thromboplastin-containing test cards for determination of the INR. On each of 10 working days citrated whole blood and plasma samples were obtained from 2 healthy subjects and 6 patients on oral anticoagulants. PT testing on whole blood and plasma was done with the TAS and parallel testing for plasma by the manual technique with the IRP CRM 149S. Conformity to the calibration model was judged satisfactory if the following requirements were met: (i) there was a linear relationship between paired log-PTs (TAS vs CRM 149S); (ii) the regression line drawn through patients data points, passed through those of normals; (iii) the precision of the calibration expressed as the CV of the slope was <3%. A good linear relationship was observed for calibration plots for plasma and whole blood (r = 0.98). Regression lines drawn through patients data points, passed through those of normals. The CVs of the slope were in both cases 2.2% and the ISIs were 0.965 and 1.000 for whole blood and plasma. In conclusion, our study shows that near-patient testing devices can be considered reliable tools to measure INR in patients on oral anticoagulants and provides guidelines for their evaluation.

Effect of tamoxifen on measurements of hemostasis in healthy women

BACKGROUND

Tamoxifen citrate is being evaluated for primary prevention of breast cancer, but this drug with estrogen-like properties may cause changes in the hemostatic system that would increase the risk of thrombosis.

METHODS

Women who had undergone hysterectomy were consecutively enrolled in the placebo-controlled, randomized, double-blind Breast Carcinoma Chemoprevention Tamoxifen Study, which was designed to evaluate the efficacy of oral tamoxifen citrate (20 mg/d). Our substudy of hemostasis and lipid measurements included the first 68 consecutive women assigned to tamoxifen (n = 31) or placebo (n = 37). Blood specimens were obtained before treatment and after 1,2,4, and 6 months of treatment. Measurements included blood cell counts, lipid levels, coagulation activation markers, clotting factors, and anticoagulant and fibrinolysis proteins.

RESULTS

Hematocrit and hemoglobin and platelet levels fell slightly but significantly in women treated with tamoxifen. No between-treatment differences were observed in any of the clotting factors. Naturally occurring anticoagulant proteins such as antithrombin and protein C fell slightly in women treated with tamoxifen. However, no significant changes were observed in any of the markers of activated coagulation or fibrinolysis (fibrinopeptide A, prothrombin fragment 1 + 2, thrombin-antithrombin complex, D-dimer). Total and low-density lipoprotein cholesterol levels fell significantly in women treated with tamoxifen.

CONCLUSIONS

Tamoxifen induced a modest decrease in anticoagulant proteins, but without biochemical signs of activation of coagulation and fibrinolysis. Tamoxifen improved the lipid profile and induced changes in blood cell counts, which should determine an improvement in blood rheologic factors. These preliminary findings seem to justify continuation of the double-blind study in healthy women, but only direct comparison of thromboembolic complications in the 2 treatment groups will establish whether tamoxifen carries a risk of thrombosis.

A simplified procedure for thromboplastin calibration--the usefulness of lyophilized plasmas assessed in a collaborative study

The International Sensitivity Index (ISI) of thromboplastins is determined by calibration using fresh plasmas from 60 patients stabilized on oral anticoagulants and 20 healthy subjects. This procedure is demanding, particularly for those who have no easy access to patients. The alternative use of a smaller number of lyophilized plasmas has already been considered, but one important issue, the number of repeated measurements to be carried out, has never been addressed. Two commercial rabbit thromboplastins, A and B, were calibrated in 3 laboratories against CRM 149R. On each of 10 working days, prothrombin times were measured for a different set of 8 fresh plasmas and for the same set of 8 lyophilized plasmas. The ISI values for both thromboplastins were estimated by orthogonal regression on fresh and lyophilized plasmas. The between- and within-laboratory CV values of the estimated ISI were taken as measures of precision of the calibration. In addition, ISI and CV were calculated daily on cumulative results obtained with lyophilized plasmas from day 1 to day 10. The ISI values for both thromboplastins calculated with lyophilized plasmas were not significantly different from those with fresh plasmas (mean of 3 laboratories: 1.42 vs 1.48 for A and 1.22 vs 1.20 for B). The between-laboratory precision of the calibration with lyophilized plasmas was not considerably different from that with fresh plasmas (CV for 3 labs: 5.2% vs 6.8% for A and 0.9% vs 2.2% for B). The ISI estimated with lyophilized plasmas on results of day 1 were not different from those of days 2 through 10. Good within-laboratory precision of the calibration (CV around 2%) was already achieved on day 3. In conclusion, this study shows that lyophilized plasmas pooled from normals and patients on oral anticoagulants can be used as substitutes for individual fresh plasmas to simplify the existing procedure for thromboplastin calibration.

A partial factor V deficiency in anticoagulated lyophilized plasmas has been identified as a cause of the international normalized ratio discrepancy in the external quality assessment scheme

Lyophilized normal and anticoagulated plasmas are increasingly used in External Quality Assessment schemes (EQAS) to assess the quality of performance of laboratories engaged in the control of oral anticoagulant therapy by means of the International Normalized Ratio (INR) system and for the standardization of the prothrombin time (PT). The main feature of the INR system is that the value measured for an individual plasma should be independent of the reagent used. This holds true only if the variable factors influencing the responsiveness of the reagents are the vitamin K-dependent clotting factors, the other factors being constant and within normal limits. We provide evidence that even a partial factor V (FV) deficiency (FV activity < 60 U/dl) in anticoagulated plasmas can be responsible for discrepancies between INR values as measured for a single lyophilized or fresh plasma by different reagents and that the discrepancy is proportional to and magnified by the International Sensitivity Index (ISI) of the reagent used. The greater the difference between ISI values, the wider the gap between INR values. Since FV is one of the most labile plasma clotting factors, its activity is likely to be lost during freeze-drying. Hence, we recommend a thorough control of the FV activity in commercial and home-made lyophilized plasmas before their use in EQAS and in the standardization of the PT.

Results of a collaborative study for fibrinogen measurement. Evidence that the use of a common calibrator improves interlaboratory agreement

One hundred and sixty-five laboratories were asked in two different surveys to perform duplicate fibrinogen measurements on the same lyophilized plasmas by their own methods and reagents and two different methods of calibration. The between-laboratory variability, expressed as coefficient of variation (CV), and the extent of between-method and between-reagent differences were taken as a measure of agreement between measurements. It was found that the overall between-laboratory range of CV values was 9-16% and that it could be improved if a common calibrator was used. The improvement was more pronounced for the thrombin time-derived (Clauss) method (average CV reduction 43%) than for the prothrombin time (PT)-derived method (average CV reduction 12%). A satisfactory agreement was found between the PT-derived and the Clauss method for the normal-PT plasma, whereas the PT-derived was slightly higher than the Clauss method for the abnormal-PT plasmas, irrespective of calibration. There were statistically significant between-reagent differences within the Clauss method, which were reduced if the common calibrator was used. In conclusion, the results shows that a system of common calibration for fibrinogen determination improves interlaboratory agreement by reducing the CV and minimizing between-reagent differences, particularly for the Clauss method. The authors recommend manufacturers of reagents to link their calibrators to the International Standard for fibrinogen.

Results of a multicenter study assessing the status of standardization of a recombinant thromboplastin for the control of oral anticoagulant therapy

An international collaborative study involving 20 laboratories was undertaken to evaluate the status of standardization of a commercial recombinant thromboplastin. The results show that the reagent fulfills the WHO requirements for thromboplastin calibration against two International Reference Preparations (IRP) from different species, because there was a linear relationship of log-transformed prothrombin times as measured by the recombinant reagent and the two IRPs in normal individuals and patients stabilized on oral anticoagulants. The regression lines drawn through the patient data points passed through the normal data points in the majority of instances. The average ISI values for three different batches were close to unity. On the average, the ISI value calculated against BCT/253 (human, plain) was slightly smaller than that against RBT/79 (rabbit, plain). The between laboratory precision of calibration (CV = 5-6%) did not change whether BCT/253 or RBT/79 was used, whereas it was substantially improved when the calibrations of batches 2 and 3 were performed against batch 1, arbitrarily taken as standard (CV < or = 1.7%). Statistical analysis to test for differences between slopes revealed no significant between-batch differences in 16 of 20 laboratories. However slight differences for the third batch (2.8% less than the average ISI value of batches 1 and 2) leave room for further improvement in the standardization of the reagent.