Studies of Phospholipid Reagents Used in Coagulation II: Factors Influencing Their Sensitivity to Heparin

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Abstract
This study demonstrates the many technical variables which exist in the use of APTT reagents to detect and measure heparin. With the same phospholipid reagent different activation regimes produced significant differences in heparin sensitivity. The concentration of phospholipid was also an important influence; decreasing concentration led to increasing sensitivity, and the highest sensitivity was given by omitting the phospholipid altogether. Other factors which influenced the sensitivity to heparin were buffering of the reagents and the presence of oxidation products in the phospholipid. The results emphasize the need for standardisation of at least some of these variables if the APTT is to be used to determine the therapeutic range of heparin.