SUMMARY The in vivo recovery of factor VIII has been estimated on 84 occasions in 53 severely affected adolescent haemophiliacs. There was wide individual variation in recovery, which was not affected by differences in the administered dose. Recovery increased steadily with increasing surface area, and it was only over a surface area of 1-7 m2 that a recovery of 2% of factor VIII per unit per kg became the norm. It is suggested that the only safe assumption to make below that surface area is an in vivo recovery of 1[5 %.The treatment of haemophilic haemarthroses with inadequate doses of factor VIII will predispose the joints to chronic synovitis, progressive arthropathy, and ultimately to crippling deformity.' There are, however, potentially harmful side effects associated with adequate and excessive exposure to factor VIIIcontaining materials,2 which also strain human and financial resources.3 Clearly, the use of the correct dose of factor VIII in varying situations is of great importance.Recent studies have identified minimum dosage schedules for bleeds of varying severity in specific sites,4 and in joints with or without previous and/or present damage. Patients and methodsThe subjects studied were 53 severely affected adolescent haemophiliacs (factor VIIIC < 1 %) resident at the Lord Mayor Treloar College. Blood samples were taken immediately before and between 20 and 30 minutes after infusions of factor VIII-containing material. Samples were immediately dispensed into plastic tubes containing 3 8 % sodium citrate (9 vols blood to 1 vol anticoagulant). The plasma was separated within two hours by centrifuging at 2000 g at 4°C and tested immediately.Factor VIIIC was tested by a two stage method modified from that of Denson." During the year in which the study took place, approximately 700 factor VIII assays were undertaken in our laboratory, 10 % of which were done in duplicate. Average variation between operators was never more than 10%.All weights and heights used were obtained within two months of the relevant estimation.

Summary A new activated factor IX product (Autoplex) has been used for the treatment of 18 haemarthroses in three haemophiliac patients with antibodies to factor VIII and a history of high anamnestic response to challenge with factor VIII. Results were evaluated by assessing pain, tenderness, range of movement and girth at 8 and 24 hours. A score of 2 was given for improvement at 8 hours and 1 for improvement at 24 hours. The result was expressed as a percentage of the possible score. Nine episodes treated with less than 35 units of Autoplex/kg had a mean score of 52% and nine episodes treated with more than 35 units/kg had a mean score of 73%. When only the more severe group of bleeds was considered, an even more marked dose related response emerged. When the ratio of the percentage fall in the PT to the percentage fall in the KCCT was plotted against the outcome, a significant correlation emerged (r=0.43; P= 0.02).