Different Sensitivities of Various Thromboplastins to Two Blood Collection Systems for Monitoring Oral Anticoagulant Therapy

International Council for Standardisation in Haematology (ICSH) recommendations for collection of blood samples for coagulation testing

This guidance document has been prepared on behalf of the International Council for Standardisation in Haematology (ICSH). The aim of the document is to provide guidance and recommendations for collection of blood samples for coagulation tests in clinical laboratories throughout the world. The following processes will be covered: ordering tests, sample collection tube and anticoagulant, patient preparation, sample collection device, venous stasis before sample collection, order of draw when different sample types need to be collected, sample labelling, blood-to-anticoagulant ratio (tube filling) and influence of haematocrit. The following areas are excluded from this document, but are included in an associated ICSH document addressing processing of samples for coagulation tests in clinical laboratories: sample transport and primary tube sample stability; centrifugation; interfering substances including haemolysis, icterus and lipaemia; secondary aliquots-transport and storage; and preanalytical variables for platelet function testing. The recommendations are based on published data in peer-reviewed literature and expert opinion.

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.

The effect of different hormonal contraceptives on plasma levels of free protein S and free TFPI

Use of combined oral contraceptives is associated with a three- to six-fold increased risk of venous thrombosis. Hormonal contraceptives induce acquired resistance to activated protein C (APC), which predicts the risk of venous thrombosis. The biological basis of the acquired APC resistance is unknown. Free protein S (PS) and free tissue factor pathway inhibitor (TFPI) are the two main determinants of APC. Our objective was to assess the effect of both hormonal and non-hormonal contraceptives with different routes of administration on free TFPI and free PS levels. We conducted an observational study in 243 users of different contraceptives and measured APC sensitivity ratios (nAPCsr), free TFPI and free PS levels. Users of contraceptives with the highest risk of venous thrombosis as reported in recent literature, had the lowest free TFPI and free PS levels, and vice versa, women who used contraceptives with the lowest risk of venous thrombosis had the highest free TFPI and free PS levels. An association was observed between levels of free TFPI and nAPCsr, and between free PS and nAPCsr. The effect of oral contraceptives on TFPI and PS is a possible explanation for the increased risk of venous thrombosis associated with oral contraceptives.

Comparison between siliconized evacuated glass and plastic blood collection tubes for prothrombin time and activated partial thromboplastin time assay in normal patients, patients on oral anticoagulant therapy and patients with unfractioned heparin therapy

INTRODUCTION

The study was designed to evaluate whether there was a statistically significant effect between evacuated glass tubes and plastic tubes on prothrombin time (PT) and activated partial thromboplastin time (aPTT).

METHODS

Blood samples were drawn into four different tubes from three patient populations-apparently healthy patients, patients on oral anticoagulant therapy with vitamin K antagonists (OAT-vka) and patients being treated with unfractioned heparin (UFH). Testing was performed on an automated coagulation analyzer, and statistical analysis was achieved using a test of variance (anova).

RESULTS

For normal patients, there were no statistically significant differences for the aPTT test; however, there were statistically significant differences for the PT test. For patients on OAT-vka, statistically significant differences were clearly observed between the four tube types for the PT test. For patients treated with UFH, there were no statistically significant differences for the aPTT test.

CONCLUSION

The data showed a statistically significant difference between glass and plastic tubes in the normal population only for the PT test, with consequent repercussions for patients on OAT. This means that appropriate care and validation should take place whenever there is a change in tube type.

Monitoring treatments with unfractionated heparin: CTAD must be used instead of citrate as the anticoagulant solution when using partial-draw collection tubes. Results of a multicenter evaluation

BACKGROUND

Sampling small volumes of blood may be necessary, particularly in pediatric patients, or in case of difficult or recurrent venipunctures.

METHODS

Routine hemostasis test results evaluated in partial- and full-draw evacuated polymer tubes obtained in 4 centers were compared.

RESULTS

No relevant discrepancy (Bland-Altman) was found between test results measured in partial- and full-draw tubes obtained from untreated patients and from patients on vitamin K-antagonist or low molecular weight heparin. In patients on unfractionated heparin (UFH), significantly lower anti-FXa activity [median=0.29IU/mL (range:0.04-1.15) vs. 0.39 (0.05-1.25), n=89, p<0.0001] and shorter aPTT were measured in partial-draw tubes. This discrepancy was likely to be related to the release of higher amounts of PF4 after increased platelet activation in partial-draw tubes. As CTAD is known to counteract platelet activation, we then collected blood into partial-draw CTAD tube and full-draw citrate tube. Both in patients on UFH and in untreated patients, no relevant difference could be demonstrated for all studied parameters (Bland-Altman), including aPTT and anti-FXa activity, even if analytical comparison showed significantly higher anti-FXa activity in partial-draw CTAD than in full-draw citrated tubes with a mean bias of 0.02 IU/mL, identical throughout the measuring range.

CONCLUSIONS

These results suggest that samples collected into partial-draw citrate tubes allow accurate routine coagulation testingin all patients but those requiring UFH assessment,in which their use led to a significant underestimation ofanticoagulation. In such cases, partial-draw tubes containing CTAD could be validly used to monitor heparin therapy as well as to perform routine coagulation testing.

A new plastic collection tube made of polyethylene terephtalate is suitable for monitoring traditional anticoagulant therapy (oral anticoagulant, unfractionated heparin, and low molecular weight heparin)

To improve the safety of blood collection, plastic tubes have been developed but various interactions with the coagulation system and/or antithrombotic drugs were reported with the first generation of such tubes. The aim of this multicentre study was to compare hemostasis test results measured in evacuated plastic tubes made of polyethylene terephtalate (VenoSafe, Terumo Europe) and in siliconized glass tubes containing the same citrate concentration (0.129 M). In addition, the impact of aging of the plastic tube was investigated by collecting blood samples in tubes at 8 months and at 1 month before expiry. Blood was drawn in 3 centres from untreated patients (n=269), patients on oral anticoagulant treatment (OAT, n=221), and patients treated with either unfractionated heparin (UFH, n=73) or a low molecular weight derivative (LMWH, n=48). Prothrombin time (PT) or INR, activated partial thromboplastin time (APTT) and anti-FXa activity were locally performed, when applicable. In untreated patients and in patients on OAT, PT and APTT values were found statistically shorter (p<0.05) when evaluated in plastic tubes than in glass tubes, except when PT was evaluated using a human thromboplastin. Surprisingly, significantly longer APTT and higher anti-FXa activities were obtained when blood from patients on UFH was drawn in plastic than in glass tubes. However, none of the differences had any clinical relevance (Bland-Altman analysis). In patients on anticoagulant treatment, there was no effect of aging of the plastic tubes. These results suggest that the plastic tube VenoSafe is suitable for coagulation testing both in untreated subjects and more interestingly in patients on traditional anticoagulant therapy during the whole shelf life indicated by the manufacturer.

Multicenter evaluation of a bilayer polymer blood collection tube for coagulation testing: effect on routine hemostasis test results and on plasma levels of coagulation activation markers

We compared the results of different hemostasis tests obtained in an evacuated bilayer polymer tubes (Vacuette, Greiner Bio-One) and in a siliconized glass tubes containing the same citrate concentrations (0.109 M and 0.129 M). For that purpose, blood was collected in five centers from 60 untreated patients and from patients on oral anticoagulant (n = 168), unfractionated heparin (n = 111) or a low molecular weight derivative (n = 108). Test results obtained in polymer tubes were not significantly different from those in glass tubes, except for INR when a high ISI thromboplastin was used (p < 0.0001 for tubes containing 0.129 M sodium citrate) and for APTT (p < 0.05 for both citrate concentrations). However, these differences had no clinical relevance (Bland-Altman analysis). In addition, no effect of aging of the polymer tubes on the test results could be demonstrated. The plasma levels of F1+2 and TAT, measured in a subset of 30 untreated patients, were significantly lower when blood was collected in polymer than in glass tubes, for both citrate concentrations. These results suggest that samples collected into the Vacuette polymer tubes allow accurate routine hemostasis testing both in untreated patients and in patients on traditional anticoagulant treatment during the whole shelf-life indicated by the manufacturer.

Circulating activated protein C is reduced in young survivors of myocardial infarction and inversely correlates with the severity of coronary lesions

BACKGROUND

Cardiovascular risk factors for myocardial infarction (MI) are less frequent in younger than in older MI survivors. Therefore, the thrombotic component of MI may play a more important role at a young age. As activated protein C (APC) provides systemic anticoagulant and anti-inflammatory protection, a low plasma APC level may be an arterial thrombotic risk factor.

AIM

To determine whether there is an association between reduced APC levels and early MI and severe coronary lesions.

METHODS

APC was measured in 231 young MI survivors and 231 controls.

RESULTS

Low APC levels were significantly associated with MI. Compared with the fourth quartile, the odds ratio (OR) for APC values in the first quartile was 3.7 [95% confidence interval (CI) = 2.1-6.4], and 3.2 (1.5-7.0) after adjustment for cardiovascular risk factors. Moreover, each decrease of 0.43 ng mL(-1) (1 SD) in APC increased the OR 1.7 times (1.4-2.2), and 1.5 times (1.2-1.9) after adjustment for cardiovascular risk factors. Low APC levels were also associated with the number of coronary arteries affected and with the severity of coronary lesions (P < 0.001).

CONCLUSIONS

There is a significant association between low circulating APC levels and both early MI and the extent and severity of coronary atherosclerosis, which might be related to the anticoagulant and anti-inflammatory properties of APC.

High plasma levels of factor VIII: An important risk factor for isolated pulmonary embolism

OBJECTIVE

The aim of the study was to investigate whether factor V Leiden and prothrombin G20210A mutations, elevated levels of factor VIII and factor IX are associated with pulmonary embolism (PE).

METHODS

Sixty-four patients with objectively documented PE and 64 control subjects were included in this study. The authors divided the 64 subjects with PE into those with PE and deep vein thrombosis (combined form of venous thromboembolism, n = 26) and those with PE without deep vein thrombosis (isolated PE n = 38).

RESULTS

There was no significant difference between the PE groups and the control subjects with regard to the presence of factor V Leiden and prothrombin mutations and elevated levels of factor IX. Using the 90th percentile measured in control subjects (P(90) = 168 U/dL) as a cut-off point for factor VIII levels, the authors found an 11-fold increased risk for both isolated PE patients and patients with a combined form of venous thromboembolism who have factor VIII levels >168 U/dL compared with individuals having factor VIII levels below this cut-off point. The risk was not affected by adjustments for other possible risk factors.

CONCLUSIONS

Elevated plasma factor VIII levels were found to be a significant, independent risk factor for PE.

The John Charnley Award: heritable thrombophilia and development of thromboembolic disease after total hip arthroplasty

We retrospectively assessed whether heritable thrombophilia-hypofibrinolysis was more common in patients developing venous thromboembolism after total hip replacement than among control patients who did not develop venous thromboembolism, as an approach to better identify causes of venous thromboembolism after total hip arthroplasty. Twenty patients with proximal deep venous thrombosis after THA and 23 patients with symptomatic pulmonary embolism were compared with 43 control patients who did not have postoperative venous thromboembolism. Five of 42 patients with venous thromboembolism (12%) and 0 of 43 control patients (0%) had antithrombin III deficiency (< 75%). Nine of 42 patients with venous thromboembolism (21%) and 2 of 43 control patients (4.7%) had protein C deficiency (< 70%). Ten of 43 patients with venous thromboembolism (9 heterozygous, 1 homozygous; 23%) and 1 of 43 control patients (heterozygous; 2%) had the prothrombin gene mutation. Patients who had venous thromboembolism after total hip arthroplasty were more likely than matched control patients to have heritable thrombophilia with antithrombin III or protein C deficiency, or homo-heterozygosity for the prothrombin gene mutation. Screening for these three tests of heritable thrombophilia before total hip arthroplasty should improve the identification of patients with a reduced risk of venous thromboembolism who may need only mild thromboprophylaxis, and of those patients with heritable thrombophilia in whom prophylaxis should be more aggressive.

LEVEL OF EVIDENCE

Prognostic study, Level II-1 (lesser-quality RCT). See the Guidelines for Authors for a complete description of levels of evidence.

Elevated Clotting Factor Levels and Venous Thrombosis

Increased plasma levels of fibrinogen, factor VIII (FVIII), factor IX (FIX), factor XI (FXI) and prothrombin all were reported to be independent risk factors of venous thromboembolism (VTE). However there is only limited information on the molecular basis of these plasma phenotypes. In addition some of these plasma phenotypes may form clusters, which may point to defects in common pathways affecting biosynthesis or clearance. Uncertainties as to what should be considered the cut-off for an elevated plasma level and in some cases (FIX, FXI) the lack of published confirmatory and/or prospective studies, have delayed the incorporation of these parameters in thrombophilia screening procedures.

Third- and second-generation oral contraceptives are associated with similar risk estimates for venous thromboembolism

A scientific discussion between 1995 and 1999 addressed the question whether second- and third-generation oral contraceptives (OCs) were associated with different risks of venous thromboembolism (VTE). Results from three epidemiological studies became available in the course of 1995, in which such differences were observed. Although it was unclear at that time whether these observations reflected causality or were induced by bias and/or confounding, some regulatory bodies in Europe restricted the indication for use of third-generation oral contraceptives. Immediate media attention generated a pill scare in those, but also other, countries. Indications for the influence of bias were observed in the initial studies of 1995 and further substantiated in subsequent utilization and prescribing surveys. The most important bias seemed to be related to differences in age and duration of use between third- and second-generation OC users. A number of new studies as well as new analyses in two of the 1995 databases included measures to limit the effect of the identified biases/confounders. These studies observed similar risks of venous thromboembolism with second- and third-generation oral contraceptives. Two other recently published studies did not or could not include the same level of control for confounding and reported similar results as the 1995 studies, thus reconfirming the relevance of the identified confounders. Population data show that the massive switch in the UK from third-generation OCs to second-generation OCs in 1995 has not resulted in a reduction of the incidence of VTE in OC users after 1995, illustrating that the risk of VTE is not determined by the type of low-dose pill used. In addition, data from trend analyses, spontaneous reporting and studies addressing hemostatic mechanisms in pill-users also do not support a potential difference in risk of VTE between users of second- and third-generation OCs.