Factor VIII activity is a blood test that measures the level of a protein called factor VIII, which plays a crucial role in blood clotting. This test is often used to diagnose hemophilia A, a bleeding disorder caused by a deficiency in factor VIII. It can also be used to monitor the effectiveness of treatment for hemophilia A or to evaluate other bleeding disorders. Normal factor VIII activity levels range from 50% to 150%. Levels below 50% may indicate hemophilia A, while levels above 150% may suggest other underlying conditions.

CORRECTION OF A PROLONGED APTT WITH MIXING GENERALLY IMPLIES A QUALITATIVE OR QUANTITATIVE ABNORMALITY OF ONE OR MORE CLOTTING FACTORS IN THE PATIENT PLASMA. IN CONTRAST, FAILURE OF APTT TO CORRECT COMPLETELY ON MIXING SUGGESTS THE PRESENCE OF INHIBITOR IN THE PATIENT PLASMA EG. LUPUS ANTICOAGULANTS & INHIBITORS TO SPECIFIC CLOTTING FACTORS.

DEFICIENCY OF FACTOR IX CAUSES PROLONGATION OF APTT THAT CORRECTS COMPLETELY ON MIXING. CONGENITAL FACTOR IX DEFICIENCY IS CALLED HEMOPHILIA B / CHRISTMAS DISEASE WHICH IS INHERITED IN X-LINKED RECESSIVE MANNER. IT AFFECTS 1 PER 25000 TO 30000 LIVE MALE BIRTHS.