aPTT in Heparinized Plasma: Influence of Reagent, Acute-Phase Reaction, and Interval Between Sampling and Testing

Plasma samples from 10 healthy persons and 10 patients with acute-phase reaction were heparinized in vitro to obtain 0.00-0.70 U/ml. The activated partial thromboplastin time (aPTT) was then determined, using an optical method (Automated Coagulation Laboratory) and four reagents (actin, Cephotest, Platelin, and Throm bosil). The heparin sensitivity showed variation between individuals and was lower in acute-phase plasma than in normal plasma. There was also a marked difference in heparin sensitivity among the different reagents; actin was the least sensitive reagent, while Platelin was the most sensitive reagent in normal plasma and Thrombosil the most heparin-sensitive reagent in acute-phase plasma. Delay in testing prolonged the aPTT values in both acute- phase and normal heparinized plasma. With actin and Ce photest, a delay of 90 min at 22°C resulted in 30-50% prolongation of the aPTT. A delay of 150 min caused a prolongation of 75-110% with actin. Cephotest, Platelin, and Thrombosil were less prolonged. Ex vivo samples from heparinized plasma showed similar degrees of pro longation. Storage at 4°C resulted in less prolongation. Assuming a therapeutic range of 0.35-0.70 U/ml of hep arin, the therapeutic aPTT ratio ranges in heparinized, acute-phase plasma were 1.5-3.0 for actin and 2.5-4.5 for Cephotest, Platelin, and Thrombosil. These results un derscore certain limitations in monitoring heparin therapy with the aPTT system. Unless the assay is performed within 30 min after sampling, unduly prolonged aPTT val ues will be recorded. This may lead to underdosing of the patient.