The effect of increased contact activation time on the activated partial thromboplastin time.

With the kaolin-cephalin activated partial thromboplastin time technic, the plasmas of persons who have Fletcher factor deficiency have shown considerable shortening of clotting times when contact activation has been lengthened from 3 (PTT-3) to 10 minutes (PTT-10). The authors demonstrate that in plasma of most normal individuals, and in coagulopathies of other sorts, only slight shortening usually occurs. Abnormal shortening occurs in plasmas of a few otherwise normal people, the "slow activators," and patients receiving coumarin drugs, who have greatly prolonged prothrombin times. Longer activation may produce greatly prolonged PTT's in plasmas containing heparin in relatively high concentrations. The authors discuss the significance of these findings.