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Collagen (soluble bovine tendon type I) coated onto microtiter plates binds von Willebrand antigen (vW:Ag) in a dose-dependent manner. An ELISA test was set up with both antibody and collagen coated microtiter plates. Test specimens assayed were: 1) normal plasmas, 2) type I vW plasmas, 3) type IIa vW plasmas, and 4) factor VIII concentrates (KoateR, Cutter; Conco-VIII, Green Cross). Normal and type I vW plasmas exhibited comparable values for vW:Ag in binding studies to both collagen and antibody-coated plates. Type IIa vW plasmas demonstrated decreased (less than 1/2) collagen to antibody-binding ratios. Ristocetin cofactor (VIII:RCO) levels in type IIa vW plasmas correlated with quantified collagen-binding levels. Factor VIII concentrates show variable results when comparing collagen and antibody-binding levels. A comparison of vW:Ag ELISA (antibody) with VIII:RCO shows ratios of 2:1 (KoatR) or 20:1 (Conco-VIII). Collagen-binding ELISA levels in concentrates show parallel decreases, reflecting presumed binding to collagen of only the high M.W. multimers. The vW:Ag collagen binding ELISA represents a possible replacement assay for the laborious and imprecise VIII:RCO method of measurement of in vitro vWf functional activity.

Incubation of purified factor VIII/von Willebrand factor complex or plasma with certain phospholipid preparation induced a two to threefold increase in apparent factor VIII activity, while a slight decrease of factor VIII coagulant antigen was observed. Between six or eightfold activity increases occurred when 10-3 units/ml of thrombin was present during the incubation. When the phospholipid was added first and the thrombin added 30 minutes later a twofold increase in apparent factor VIII activity occurred during the first part and this activity was then increased threefold further after the addition of thrombin. In separate experiments using gel filtration on Sepharose 2B, association of factor VIII activity to phospholipid vesicles could be demonstrated. This phospholipid bound factor VIII could be activated by thrombin. The relationship between these observations and the formation of factor X activating complex is discussed.