Current Tasks

The chairman welcomed the participants, numbering over 300, and outlined the program which was divided into three sessions and based on the tasks as agreed upon at the last subcommittee meeting:

Heparin

1. Development of reference plasmas
2. Development of recommendations on monitoring of LMW heparin

Thrombin Inhibitors

Review of collaborative study data on Ecarin time

PT Standardization

1. Establishment of new WHO human thromboplastin IRP
2. Work toward recommendations on calibrated plasmas
3. Review of revised WHO requirements on thromboplastins

1. Unfractionated Heparin

   Dr. Gray gave a progress report on the development of reference plasmas with defined activities of unfractionated heparin. The effects of high fibrinogen and FVIII concentrations on APTT and anti-Xa assays were described, since such high levels are often found in patients. It is intended that pilot batches, consisting of pooled patients' plasmas, ex vivo plasmas from normal volunteers, and modified in vitro spiked plasmas, will be prepared in the near future and subjected to an inter-laboratory study during the coming year.

   Dr. Gray also announced that the WHO standard for unfractionated heparin would require replacement during the next year, and WHO is organizing a joint meeting together with the United States, European, and Japanese pharmacopoeias to devise suitable materials and methods.

2. LMW Heparin

   Dr. Giles described the activities of the working party which had been formed last year to deal with the issue of monitoring LMW heparin. The Working Party consisted of Dr. Giles (Chairman), Drs. Samama, Andrew, Boneu, Hemker, Ofosu, and Nesheim and has divided its activities into three areas: A) monitoring in children, B) monitoring in adults, and C) methodology.

   1. Monitoring in Children

      Dr. Andrew emphasized that laboratory monitoring of LMW heparin in children should be considered separately from that in adults because of the following:

      1. Different pharmacokinetics
      2. Greater risk of bleeding with underlying primary disease
      3. Possible development of renal compromise and/or acquired coagulopathy
      4. Long-term treatment
      5. Possible dosage errors with home treatment

      Because of these issues, it was considered by the working group that monitoring LMW heparin in children is important both for prophylactic and therapeutic use. The appropriate timing and frequency, and the therapeutic ranges for children have not been determined with certainty, and, as with adult monitoring, the methodology is still a matter for discussion, though in practice the anti-Xa method has mostly been used.

   2. B Monitoring in Adults

      Dr. Samama reviewed the clinical relevance of laboratory monitoring of LMW heparin in adults. Platelet monitoring is important for both prophylaxis and treatment because of the risk of thrombocytopenia, which although lower than with unfractionated heparin, is still present.

      Measurement of LMW heparin levels in prophylaxis is neither necessary nor useful except in special cases, such as very low or very high body weight, renal insufficiency, or the occurrence of a bleeding or thrombotic episode.

      In treatment of established DVT or pulmonary embolism, several published studies found no relationship between anti-Xa levels and clinical outcome, but other studies did find a positive relationship between anti-Xa and either efficacy or bleeding. At present, there is insufficient data for a definite conclusion on the clinical utility of monitoring in these indications and further clinical studies are needed.

      Until now, anti-Xa assays have been mostly used, though for practical reasons rather than their demonstrated superiority.

   3. Methodology

      Dr. Nesheim described the heterogeneity of LMW heparins with regard to their molecular weight binding to antithrombin and activities against FIIa and FXa. He concluded that it was necessary to measure both types of activities to characterize any sample containing LMW heparin.

      In discussion, Dr. Hemker agreed with Dr. Nesheim and emphasized the difference between methods to measure LMW heparin concentrations (anti-Xa and anti-IIa) and methods to measure their effect on the coagulation system such as the thrombin potential which was still under development. Dr. Harenberg commented on the utility of the Heptest for both LMW heparin and unfractionated heparin monitoring.

      Dr. Giles concluded by indicating that, despite some uncertainties, the Working Group should be able to establish practically useful recommendations in this area which, subject to agreement by the Subcommittee and the SSC, could be published. It was agreed that the working group would prepare written recommendations during the coming year with a view to presentation and final adoption by the Subcommittee at next year's meeting.

THROMBIN INHIBITORS

Dr. Rübsamen presented the interim results of a collaborative study on the ecarin clotting time (ECT) for the detection of PEG hirudin. Twelve laboratories participated and eight had returned data. The results were somewhat disappointing in that despite the use of the same ecarin reagent in all laboratories, there was a wide variation in the slopes of the dose response lines, and, in two laboratories, the samples gave very long clotting times or were incoagulable. It was considered that freezing and thawing of the samples may have contributed to their variability and that another study with lyophilized samples might give better results.

Dr. Fareed reviewed the use of the ECT in clinical settings. It was found that the ECT was not affected by concomitant use of oral anticoagulants, or by heparin up to 1 iu/ml. There was a poor correlation with APTT, and this was considered to be due to the more specific nature of the ECT. For the peptide inhibitor Argatroban the HPLC method did not correlate well with the ECT--this was found to be due to a metabolite with reduced but still detectable anticoagulant activity.

Dr. Fareed concluded by emphasizing that, although the ECT was useful for monitoring a variety of thrombin inhibitors, each agent required its own standard.

In discussion, it was agreed that, whilst the ECT is clearly a useful test, further work to reduce inter-laboratory variability is necessary before it can be recommended as the method of choice.

PT STANDARDIZATION

Collaborative Studies:

Dr. Tripodi presented the final report on the establishment of the human recombinant thromboplastin, to be labeled rTF/95, as the new WHO International Reference Preparation for human thromboplastin, to replace BCT/253. Following endorsement by the Subcommittee last year, a report was submitted to WHO, and the material has now been officially established. A manuscript has been prepared by Dr. Tripodi and this has been approved by the Subcommittee as an SSC publication in Thrombosis and Haemostasis.

Dr. Hubbard gave a final report on the collaborative study on proposed European reference plasmas for INR. As described at the last meeting, there were significant differences in INR results with the three IRPs; the INRs with BCT/253 and RBT/90 were similar, but those for OBT/79 were lower. The majority of participants preferred to combine INRs of BCT/253 and RBT/90, keeping the data on OBT/79 available separately, and this proposal has now been incorporated into a final report which has been sent to the European Commission. It is hoped that the reference plasmas will be officially established by the end of 1997.

International Perspectives: Two presentations were given describing the practice and problems of control of oral anticoagulants in different countries.

Dr. Uhetsuka (Japan) presented on behalf of Dr. Sakuragawa. Seventy percent of Japanese hospitals use the thrombotest for anticoagulant control. Results are converted into INRs by means of an equation. Good correlation is claimed between INR and thrombotest. The usual therapeutic range is 2.5-3.5. Bleeding tends to occur when the derived INR>3.0. The bleeding risk may be accentuated by frequent concomitant usage of antiplatelet drugs.

Dr. Kabaeva (Russia): Russia has its own thromboplastins. These are derived from human brain tissue and have an ISI close to 1.0. Monitoring of oral anticoagulant control is performed by determination of the prothrombin index. Phenindione is the most frequently used oral anticoagulant agent. Warfarin is unknown. Only 1:15,000 people receive OAT. This compares with an estimate figure of 1:200 in Europe and the United States. Russia would welcome assistance from the Subcommittee in this area.

Dr. van der Meer reported different methods to determine therapeutic quality control of OAT. He recommended the method developed by colleagues in Leiden. He also indicated that this method could be adopted to define appropriate therapeutic ranges in different patient populations. Dr. van der Meer described the establishment of a new European group whose main activity would relate to clinical aspects of oral anticoagulant control. Those interested should contact Dr. van der Meer.

There was strong support for the chairman's expressed view that future meetings should include more clinical aspects of OAT.

European Concerted Action on Anticoagulation

Dr. Poller presented results from the field study of European Concerted Action on Anticoagulation. Effects of coagulometers were presented in some detail. The study emphasized the need for an additional step using certified lyophilized plasmas to provide local ISIs.

Dr. Houghton presented a comparison of calibration exercises comparing orthogonal regression and linear regression analyses. The results suggested that the simpler linear regression method gave a reasonable approximation to orthogonal regression and was worthy of further study.

WHO Guidelines (information in this report's Appendix).

Dr. van den Besselaar gave a synopsis of the important changes and additions to the revised WHO Requirements on Thromboplastin, which had been discussed at a recent WHO consultation meeting. The revised document prepared by the drafting group of Dr. van den Besselaar, Dr. Tripodi and Dr. Poller, has now been circulated to the members of the Subcommittee for comments. Any additional comments should be sent in writing to either the Chairman or Dr. Padilla at WHO by the end of June.

Calibrated Plasmas

Due to time constraints, there was no time for the final presentations and discussions in this area. The chairman proposed that a working group be formed with the task of drafting recommendations on the preparation and use of calibrated plasmas. This was agreed and it was suggested that this should be a major task for the Subcommittee in the next two years. The working group would consist initially of Drs. Barrowcliffe, Poller, Houbouyan, Taberner, van den Besselaar, and Johnston. Others interested in contributing should get in touch with the chairman.

ADDITIONAL TOPICS

Three additional topics had been suggested that could not be incorporated in the current programme. There were i) citrate concentration for PT, ii) review of duration of oral anticoagulant therapy, iii) review of adverse events in oral anticoagulant treatment.

Considering the earlier discussions on the inclusion of more clinically oriented topics, the chairman proposed to ask the SSC if the remit of the Subcommittee could be broadened and possibly the name changed to "Subcommittee on Anticoagulants." A longer time for the next meeting could be requested to allow for additional topics.

FUTURE TASKS

The following future tasks were agreed upon:

Unfractionated Heparin: Reports on reference plasmas and WHO standard to be presented in 1998.

LMW Heparin: Recommendations on monitoring to be drafted by the working group, for prospective approval at 1998 meeting.

Oral Anticoagulants:

WHO Requirements-- Comments to WHO by end of June 1997.

Calibrated Plasmas-- Preliminary report by working group at 1998 meeting.

Clinical Aspects-- New tasks on reviews of duration of therapy and adverse events to be initiated.

Appendix: WHO Guidelines:

A revision of the WHO guidelines was initiated in April 1997. A draft document was written after a consultation at WHO headquarters in May 1997. Definitions are given for tissue factor, thromboplastin, prothrombin time, prothrombin time system, mean normal prothrombin time (MNPT), prothrombin time ratio, international sensitivity index (ISI) and international normalized ratio (INR). It is recommended that the manual ISI of a thromboplastin should be in the range 0.9-1.7. Four types of PT system calibration are distinguished as follows: 1) calibration of International Reference Preparations, 2) calibration of secondary reference materials, 3) calibration of manufacturer's preparations against the corresponding secondary (in-house) standard, 4) local PT calibration. The principle of like-to-like calibration is maintained. Secondary human reference thromboplastins should be calibrated against rTF/95, secondary rabbit against RBT/90, and secondary bovine or combined thromboplastins against OBT/79. It is recommended that calibration of secondary reference thromboplastins should be carried out by at least two laboratories. Calibrations of type (1) and (2) should be performed with fresh normal and fresh coumarin samples. Calibration type (3) may be carried out with frozen lyophilized plasmas if it is shown to provide the same results as with fresh samples.

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