Factor Vlll replacement therapy for patients with hemophilia A Is conventionally monitored using a plasma-based factor VII1:C assay (a modified activated partial thromboplastln time [ A P m test). The plasma factor Vlll assay requires the preparation of plasma from cltrated whole blood and measurement of the clotting times of mixtures of patient plasma, factor VIIIdeflclent substrate, and APTT reagent. Results are not routinely available In less than 1.5 hr, reducing the clinical value of the laboratory data regarding the ability to immedlately adjust patient therapy. Results from the whole blood factor Vlll assay, performed on a portable coagulation analyzer and using test tubes prefllled with the necessary A P l l and factor Villdeflclent reagents, are available within 5-7 min. This Immediate determination of the factor VII1:C level from cltrated whole blood provides the opportunity to greatly reduce turnaround time and Improve the efficacy of factor Vlll replacement therapy. Based on clotting time, factor VII1:C activity is read from a standard curve. A clinical evaluation of this whole blood test was performed In two hemophilia centers. A high degree of correlation was seen (r = 0.813, n = 220) between the whole blood values obtained and conventional laboratory results. This level of correlation was superior to that obtained when comparing two different plasma-based systems (r = 0.753, n = 23). Factor VII1:C activity levels measured using the whole blood assay system were similar, irrespective of the test operator (laboratory technologist, nurse clinician, or patient). This study indicates that the whole blood factor Vlll assay provides results comparable to those of conventional plasma-based assays, but in a more rapid and efficient manner. It provides an opportunity to reduce unnecessary patient consumption of replacement preparations, hence reducing the cost of hemophilia A maintenance and prophylaxis regimens, and to reduce overall patient exposure to human blood products.