State-of-the-Art Review : Activated Partial Thromboplastin Time in Heparinized Plasma: Influence of Reagent, Acute-Phase Reaction, and Interval Between Sampling and Testing

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Abstract
Plasma samples from 10 healthy persons and 10 patients with acute-phase reaction were heparinized in vitro to obtain up to 0.70 U/ml. Activated partial throm boplastin time (APTT) was then determined using an op tical method (Automated Coagulation Laboratory or ACL Machine) and four different reagents (Actin, Ceph otest, Platelin, and Thrombosil). Heparin sensitivity var ied between individuals, and it was lower in acute-phase plasma than in healthy plasma. There was also a marked difference in heparin sensitivity among the different re agents; Actin was the least sensitive reagent, while Plate lin was the most sensitive reagent in healthy plasma and Thrombosil the most sensitive reagent in acute-phase plasma. Delay in testing prolonged the APTT values both in acute-phase and healthy heparinized plasma. With Ac tin and Cephotest, a delay of 90 min at 22°C resulted in 30-50% prolongation of APTT. A delay of 150 min caused a prolongation of 75-110% with Actin. Cephotest, Plate lin, 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 ratios 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 values will be recorded, which may lead to giving the patient an inappropriately small dose of heparin.