Monitoring heparin therapy by the activated partial thromboplastin time--the effect of pre-analytical conditions.

Blood and plasma specimens from patients receiving heparin were collected and stored under various conditions. The effect of these conditions on the activated partial thromboplastin time (APTT) was assessed. Four APTT reagents were used. Blood samples centrifuged at 600 X g gave slightly shorter APTTs than samples centrifuged at 940 X g and 2200 X g. Storage of uncentrifuged citrated-blood at room temperature resulted in 15-29% shortening of the APTT, depending on the reagent used. Storage of the same blood samples at 4 degrees C resulted in 6-19% lengthening of the APTT. The presence of HEPES-buffer in citrated-blood shortened the APTT of heparinized patient specimens, but not of normal specimens. When blood was collected in a mixture of citric acid, theophylline, adenosine and dipyridamole (CTAD-mixture), storage at room temperature induced 0-11% decrease of the APTT, depending on the reagent used. Storage of CTAD-blood at 4 degrees C resulted in 7-19% lengthening of the APTT. Shortening of the APTT could be explained by release of platelet factor 4 (PF4). Release of PF4 could be inhibited by CTAD-mixture. These data suggest that storage of CTAD-blood at room temperature is the best pre-analytical condition for reliable monitoring of heparin therapy by the APTT.