Standardization of activated protein C resistance testing: effect of residual platelets in frozen plasmas assessed by commercial and home‐made methods

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Abstract
Summary.  We investigated 42 plasmas prepared at different centrifugation speeds with three activated protein C (APC) resistance methods. The APC ratio for fresh platelet‐poor plasma declined significantly after freezing and thawing. This effect was more evident with the original method (average reduction 11·3%) than with either the home‐made (3·8%) or the modified method (3·2%). No significant decrease in the APC ratio was observed after freezing and thawing of platelet‐free plasmas from the same patients. When frozen platelet‐poor plasma was centrifuged at high speed after thawing and before testing, there was no significant decrease in the APC ratio, in comparison with fresh platelet‐poor plasma using the home‐made and modified methods.