The Collagen Binding Assay (vWF:CBA) is a test procedure that measures the binding of vWF to collagen. Collagen binding of vWF is associated with the functionally more important HMW forms of vWF. Therefore, vWF:CBA may correlate more closely with vWF function and bleeding problems than vWF antigen assay (vWF: Ag) which measures total vWF multimers. When used in conjunction with the vWF: Ag assay, vWF:CBA can be used to differentiate between vWD type 1 and type 2 (A and B subtypes) by calculating a ratio of the two assay results (ratio = vWF:CBA/vWFAg).