The prophylaxis triangle. Prophylaxis treatment regimen has 3 main determinants: (1) the given resources/concentrate availability to target a specific trough level and/or intervals of substitutions, which both reflect the costs; (2) the bleeding trigger, which comprises physical activity, presence and degree of arthropathy, and presence of chronic synovitis; and (3) the number of bleeds, especially joint bleeds, that are regarded as acceptable. These 3 determinants form a triangle. If 1 determinant is changed, the other 2 will adjust. With unlimited resources, zero bleeds and normal physical activity may be targeted; with few resources, only 2 low-dose substitutions per week may be given, thus accepting a certain number of bleeds and limited physical activity.