A new medium improves sampling for determination of platelet factor 4 in human plasma

The determination of alpha-granule specific platelet proteins in human plasma is a valuable indicator of in-vivo release and platelet activation provided that an artificial in-vitro release during sampling can be avoided. A new sampling medium (M) has been compared with the commercially available ones for the sampling and the subsequent determination of platelet factor 4 and beta-thromboglobulin. M allows blood withdrawal at room temperature (22 degrees C) (3.49 +/- 0.7 ng/ml) without any difference to 4 degrees C (3.47 ng/ml) up to a 30 minutes sedimentation period. However, after 40 minutes there is a significant (p less than 0.01) increase, amounting at 22 degrees C (13.30 ng/ml) and at 4 degrees C 6.68 ng/ml. The addition of prostacyclin (PGI2) in-vitro as well as blood withdrawal from volunteers infused with PGI2 at a rate of 5 ng/kg/min did not alter the values of M-preserved samples, whereas in the commercially available ones, PGI2 caused a decrease. A comparison of radioimmunoassay and enzyme immunoassay exhibited no significant difference in the absolute values. Beta-thromboglobulin was assayed as control. The findings indicate M being optimal for the sampling of platelet factor 4, but not for other alpha-granule release products. Thus, there is again evidence, that the alpha-granule products are not released to an equal extent upon a certain stimulus. Furthermore, it seems likely, that various substances inhibit their liberation from the platelet to a differing extent.