There were two main sessions, the first dealing with PT standardization and the second with control of heparin therapy.

PT Standardization

Reference Thromboplastins
1. Dr. Tripodi - The stocks of the WHO IRP for human thromboplastin (BCT/253) are almost exhausted. A multicenter study for replacement of BCT/253 is planned. The study will include two candidate replacement materials (human recombinant thromboplastin) and the three current WHO IRPs (BCT/253, RBT/90, and OBT/79). In addition, a human placenta thromboplastin will be included to assess the comparison with the human recombinant materials. The results of the study will be discussed by members of the working party and later by the SSC subcommittee on Control of Anticoagulation. One of the candidates will be chosen as the new IRP from human origin.

Reference Plasmas
2.(i) Dr. Van der Besselaar - There are several practical problems with the current INR/1S1 system. Few laboratories can perform the manual technique required for the IRPs. Instruments have an unpredictable influence on the ISI of thromboplastins. In some cases, there are minor statistical deviations from the WHO model for thromboplastin calibration. The ISI of several reagents is influenced by the citrate concentration used to anticoagulate the blood. This problem can be solved easily by adopting only one citrate concentration for all coagulation tests. It is strongly recommended to use 0.11 mol/l, because this concentration has been used in all multi-center study of lyophilized plasmas. ISI differences were observed between fresh (native) and lyophilized plasmas.
(ii) Dr. Hubbard - In a project on European Reference Plasmas, a comparison was made between lyophilized coumarin and artificially depleted plasmas for local calibration. INR equivalents of the lyophilized coumarin plasmas determined with different thromboplastins were relatively close to each other. Greater differences were observed with artificial plasmas.
(iii) Dr. Carroll - An international multi-center study was performed to assess the value of lyophilized plasma calibrants in correcting coagulometer effects on INRs (local ISI determination). Differences between artificially depleted and coumarin plasmas were not of clinical importance.
(iv) Dr. D'Angelo - A comparison was made between PTs of the VDGH lyophilized normal reference plasma and fresh normal plasmas with a number of different thromboplastins. Although there were statistical differences, the lyophilized plasma PT was close to MNPT (differences <3%) with almost all thromboplastins except for recombinant thromboplastins (differences about 10%). The lyophilized normal reference plasma can be used with a correction factor for determination of the PT-ratio.
3.(i) Dr. Van der Besselaar (on behalf of Dr. Poller) - The European Concerted Action on Anticoagulation (ECAA) was described briefly. The aims of ECAA involving participants in EC and EFTA states will be to improve reliability of INRs using a new simplified system of ISI determination performed locally based on lyophilized calibrated plasmas, and to standardize clinical dosage by computerized programs of INR based oral anticoagulant dosage to improve clinical efficacy.
(ii) Dr. Wenzel - In Germany, a program has started to monitor the indications for oral anticoagulant treatment and the clinical efficacy of the treatment. Summary and recommendations for future work The following actions were agreed upon: i) WHO human thromboplastin reference - final report next year. ii) Reference plasmas - the subcommittee should work towards establishing recommendations on the preparation, calibration, and use of reference plasmas. iii) Revision of the WHO guidelines on thromboplastin calibration - this would be considered in detail at the next meeting. iv) A clinical study on the contribution of INR to the safety and efficiency of oral anticoagulant treatment was suggested by Dr. Wenzel - detailed proposals will be elaborated for the next meeting.

Heparin

1. The chairman gave an overview of the problems of standardization of heparin assays and suggested that establishment of heparin reference plasmas might help with standardization, not only for APTT but also anti-Xa and other methods. APTT
2.(i) Dr. Exner gave the final report of a collaborative study on the Proctor and Rapaport method. A combined reagent with purified phospholipids performed well and might be considered as a reference reagent for the INR system if adopted.
(ii) Dr. Denson considered the INR system unworkable in practice and recommended the measurement of heparin in units by anti-Xa assays.
(iii) Dr. Samama gave the results of a French multicenter study in which the therapeutic range of a new highly sensitive reagent was established by comparison with the existing reagent and also with heparin concentration measured by anti-Xa. Dr. Callahan described how one manufacturer approaches standardization of reagents on a lot to lot basis, and commented on the usefulness of plasmas with known heparin concentrations. Dr. van der Besselaar commented that if an INR scheme was adopted the effects of different instruments would be the same as for the PT, therefore, heparin reference plasmas would be required. In general, there was much support for the concept of heparin reference plasmas to calibrate the various reagents. Anti-Xa
3. Dr. Houbouyan presented the results of large French multicenter surveys (>1000 labs) of anti-Xa assays in the period 1989-1991. Although inter-laboratory variability was low, the cv's between laboratories were quite high from 30-50% dispersion was greater for unfractionated heparin than for LMW heparin. A more recent survey with a limited number of labs indicated considerably lower inter-laboratory variability. Thrombin Generation
4. Dr. Hemker described alternative tests based on thrombin generation and emphasized the difference between measurement of the actual amounts of heparin in plasma, and its effect on the patient's clotting system. The amount of active heparin in plasma could now be assessed in molar quantities from the known specific activities of the molecules above and below the critical value of 5400. A different test, the endogenous thrombin potential (ETP), was proposed for measurement of the overall anticoagulant effect of heparin and was also sensitive to the effect of oral anticoagulants. Summary and Recommendations for Future Work
i) It was agreed that standardization not only of APTT but also other methods should continue to be addressed.
ii) The possibilities of establishment of heparin reference plasmas could be explored further between now and the next meeting.
iii) Thrombin inhibitors, which were not discussed at the current meeting, were an important new area and would be included in the agenda for the next meeting.