The stability of factor VIII in heparinized plasma.

We studied the stability for 24 hr of factor VIII:C and factor VIII C:Ag in plasma collected in citrate, EDTA or heparin, and confirmed the previously reported two-phase decay of factor VIII:C in plasma when calcium ions have been chelated. We observed that plasma factor VIII:C is remarkably stable at 22 degrees C (+/- 2 degrees) when normal calcium ion concentrations are maintained. The loss of activity of factor VIII:C between one and 24 hr after blood collection was on average only 0.5% per hr for heparinized plasma. There was also an apparent loss of factor VIII C:Ag in plasma where calcium ions had been removed, compared with factor VIII C:Ag in heparinized plasma. However, a comparison of one-site and two-site assays suggested that calcium chelating agents may lead to factor VIII C:Ag levels being under-estimated when one-site fluid-phase assays are employed. To confirm the action of calcium ions in maintaining factor VIII:C stability, we carried out a series of experiments where calcium chloride was added four hr after blood collection to plasma anticoagulated by a mixture of citrate plus heparin; we observed total recovery of factor VIII:C activity within four hr. The stability of factor VIII:C, even at room temperature, in the presence of physiological calcium ion concentrations has implications for manufacturers of factor VIII concentrates and cryoprecipitates.