What really is blood compatibility?

The criteria for nonthrombogenicity are classically defined as long clottin g times and minimal platelet deposition. The inability to point to unequivo cal progress in the development of truly nonthrombogenic materials, highlig hts the inadequacy if not actually invalidity of these criteria. Our approa ch is to define nonthrombogenicity in terms of: (1) a thrombin production r ate constant, k(p) < 10(-4) cm s(-1); (2) low platelet consumption and low degree of platelet activation (e.g., microparticle formation), (3) perhaps some platelet spreading; and (4) low complement and leukocyte activation. O nly when the target becomes clear, will it be possible to identify clear st rategies for producing the materials we need.