Standardization of the APTT test. Current status

The partial thromboplastin time (PTT) which is used as an overall measure of the intrinsic clotting system, is the commonest coagulation test employed in routine laboratories apart from the prothrombin time. The main functions of the test are: - 1. to screen intrinsic coagulation defects; 2. to control heparin administration; 3. in the control of oral anticoagulant treatment. Many different preparations of phospholipid of human, animal and vegetable origin are used as PTT reagents. In addition, different activators, incubation times, concentrations of calcium chloride and test dilutions of plasma and phospholipid are recommended for routine laboratory use. The need to standardise both reagents and technique has been emphasised in many reports. To meet the need for a reference preparation for the PTT test, a large batch of phospholipid material of human brain origin, designated 71/25, was prepared in the National (UK) Reference Laboratory in lyophilised form in 1971 and proposed as a provisional international reference material. This primary master preparation has been used to calibrate subsequent batches of secondary reference preparations distributed to hospitals in the UK and overseas. Results of a number of collaborative studies have demonstrated the greater reliability of the standardised PTT extract compared with commercial extracts used at the same centres in the three aspects of PTT testing. The conclusion to be drawn from these extensive studies of the laboratory and clinical application of the provisional international reference preparation (71/25) is that we may now be in a position to define an acceptable international standard for the APTT.