Evaluation of Five Methods for the Determination of Heparin
Heparin was added in vitro to plasma and the heparin content assayed by five clotting methods. The accuracy was lower in pathological than in normal plasma. At a heparin concentration of 0.5 U/ml plasma the coefficient of variation in eleven pathological plasmas was: Polybrene titration 3 per cent, method of Denson and Bonnar 15 per cent, method of Yin et al. 20 per cent, calcium thrombin time 40 per cent. With the APTT method the results ranged from 0.05 U/ml to above 0.6 U/ml (no clotting in five pathological plasmas).At a heparin concentration of 0.05 U/ml plasma, only the calcium thrombin time and the factor Xa methods were sensitive, but the coefficient of variation ranged from 61 to 78 per cent. The influence of the concentration of fibrinogen, antithrombin III and α1-acid glucoprotein was studied. It is concluded that of the methods tested, only polybrene titration gives accurate results in pathological plasmas. This method is, however, laborious and insensitive to heparin concentrations below 0.1 U/ml.