Some variables which influence the activated partial thromboplastin time assay.

‘ITHE partial thromboplastin time assay (PTT) as developed by Langdell et al. (1) has proved to be a very usef ul test for studying the coagulatioll mechanism silice it C 11 reveal tile al)SellCe of any factor except Factor VII and platelets. lnder rigorously cOlitrolled COll(litiOlIS, the reproducibility of tile test is satisfactory. However, any change ill tile surface of the svl’inge, test tube, etc., with which the blood comes in contact will affect tile results. To eliminate variation ill results caused by these surface effects, there has been a considerable number of attempts in the past few years to perform the test under conditions which would assure maximum activation of tile system and therefore eliminate the surface effect. Waa1er (.2) undertook all extensive investigation of the problem ill order to overcome difficulties which he encountered when using the “cephalin time” system. He describes a method for measurement of a plasma activity developed on contact which he terms the “activation product.” The importance of surface in various modifications of the PTT has also been emphasized by other workers (3-5). 111 his book, Nossel (6) discusses contact factors and tile effect of foreign surfaces. The purpose of this study was to investigate the varial)les that can also exist in activation tests and to descril)e a reliable, reproducible system for carryillg out these assays.