A collaborative study on the suitability of commercial, assayed plasmas for one-stage factor VIII assays

A collaborative study was carried out to determine the suitability of commercial assayed plasmas for one-stage factor VIII assays. Activated partial thromboplastin times were performed on successive days on serially diluted paired freeze-dried reference plasmas (one sample buffered with HEPES), a freeze-dried test plasma and local laboratory standards-commercial assayed plasmas and in-house frozen pooled plasmas calibrated against commercial plasmas. The factor VIII activity levels of the study reference plasmas relative to average normal plasma were arrived at cumulatively by assaying them against a large number of individual fresh normal plasmas. Estimates of the activity of the test plasma relative to the claimed activity of local standards showed significant discrepancies from the expected value. However the estimates were more precise when the activity was assessed relative to the activity of the local standards derived by calibration against the reference plasmas. The study also showed that a HEPES buffered reference plasma may not contribute to inter-laboratory variability of one-stage factor VIII assays.