Standardization of coagulation tests.

ABSTRACT

A lot of attempts have been made to standardise both activated partial thromboplastin time (APTT) and prothrombin time (PT). Only the standardization of PT has been successfully implemented while the standardization of APTT is still underway. The PT test is a common method for monitoring oral anticoagulant therapy. Owing to the variable response of the thromboplastins and the different ways of reporting, PT results obtained from patients treated with oral anticoagulants have not been interchangeable between laboratories. In 1977, the World Health Organization (WHO) designed a batch of human brain thromboplastin as the first international reference preparation (IRP) for thromboplastin and a calibration system was proposed in 1982, based on the assumption that a linear relationship exists between the logarithm of the PT obtained with the IRP and test thromboplastins. This calibration model is used to standardize the reporting of the PT by converting the PT ratio observed with the local thromboplastin into an International Normalized Ratio (INR). The INR system is being adopted by an increasing number of hospitals in many countries. With the increasing use of the INR system, a number of problems have been identified with the INR system. The most serious one is that the ISI of a thromboplastin depends on the coagulometer used. Besides, a number of investigators have noted that the ISI value provided by the manufacturer for each new batch of thromboplastin reagent may be incorrect and the use of inappropriate control plasma can lead to erroneous INR calculations. Four solutions have been proposed to solve the problems of the INR system as follows (a) the local system calibration with lyophilized plasma calibrants with assigned manual PT determined in terms of the relevant IRP for thromboplastin; (b) the use of a mean normal prothrombin time (MNPT) obtained with the coagulometer to derive the prothrombin ratio (c) PT standardization by means of the procedure using plasma calibrants and (d) selection of sensitive thromboplastin with low ISI values. The INR system has been adopted in Thailand since 1984. There are 3 steps in the implementation as follows (a) preparation of National Reference Thromboplastin; (b) selection of high sensitive thromboplastin; and (c) optimal therapeutic range for Thai patients. The anticoagulant effect of heparin is usually monitored by the APTT, a test that is sensitive to the inhibitory effects of heparin on thrombin, factor Xa. and factor IXa. However, the type of clot detection system, the contact activator; and the phospholipid composition of the reagent affect the APTT response. In 1995. ISTH/ICSH proposed a calibration model for APTT standardization. As the problem showed a great similarity to PT standardization, the same model of calibration was applied but no international reference preparation for the APTT is yet available. In 1998. van den Besselaar et al proposed a lyophilized APTT reagent comprising synthetic phospholipids and colloidal silica as a candidate IRP for the APTT.