Collection If multiple tubes are being drawn, draw coagulation studies last. If only coagulation studies are being drawn, 1-2 mL, discard, and collect coagulation studies. This procedure avoids contamination of specimen with tissue thromboplastin. Patients with hematocrits >50% or <35% must have the anticoagulant adjusted. Call the Coagulation Laboratory for the adjusted tubes. Must be delivered to the laboratory within 4 hours of collection. If testing is delayed, centrifuge specimen, remove, and freeze plasma (-30°C).
Storage Instructions Place specimen on ice immediately. Transport specimen to the laboratory as soon as possible.
Causes for Rejection Specimen clotted, diluted with I.V. fluid, hemolyzed, contaminated with heparin; tubes underfilled; specimen not received on ice; specimen does not meet labeling requirements; specimen not delivered to the laboratory within 1 hour
Use Determine the inhibitor titer directed against the activity of factor VIII. The titer is useful if medical emergency requires infusion of factor VIII products. The titer also is useful in following the recovery from inhibitor induction.
Methodology Normal plasma is mixed with an equal volume of patient's plasma or with buffer and incubated for 1 hour at 37°C. The factor VIII coagulant assay using the APTT principle and deficient substrate plasma is performed. The inhibition of 50% of the factor VIII activity in normal plasma is defined as one Bethesda inhibitor unit.
