Evaluation of Assay of Factor VIII by Automated Partial Thromboplastin Time

An automated method for determination of factor VIII concentration by partial thromboplastin time has been devised and evaluated. Factor VIII concentrations determined by the automated method correlate well with those done by manual methods in the concentration range between 0.1 and 7.2 units per ml. The reproducibility of coagulation times done by the automated method was excellent, the 95% confidence limit being ± 2.4% with plasma and ± 3% with cryoprecipitate. The standard error progressively increases with higher plasma dilutions and higher concentrations of factor VIII. A dose-response relationship between coagulation time and factor VIII concentration was present in plasma dilutions between 0.7 to 40%. When calculating the factor VIII concentrations by a graphic double logarithmic plot of coagulation times against plasma dilutions, the 95% confidence range is 9.9% for plasma and to 15.7% for cryoprecipitate. The findings indicate that factor VIII concentrations in biologic materials can be reproducibly determined with no loss of accuracy by an automated partial thromboplastin time method with a significant increase in speed.