The control of heparin therapy by the activated partial thromboplastin time: results of collaborative studies

Collaborative studies to measure the effect of heparin on the activated partial thromboplastin time (APTT) have been conducted by the National (UK) Reference Laboratory for Anticoagulant Reagents and Control (NRLARC) in Great Britain and overseas and by the College of American Pathologists (CAP) in the United States. The value of multicentre trials to assess the various APTT methods, as opposed to single centre investigations, is discussed. Results from the NRLARC studies indicate that APTT methods from commercial manufacturers are relatively insensitive to heparin at low levels, compared with the standardised APTT reagent and technique produced by the NRLARC. The latter shows good sensitivity over a wide range of heparin concentrations. Variables encountered with the different commercial APTT methods are outlined. An APTT reagent should show good sensitivity at low levels and a linear response to a wide range of heparin concentrations. The in vivo response of the NRLARC standardised method in detecting circulating heparin during a clinical study of low-dose heparin prophylaxis during surgery has also been evaluated.