Table 1. The Absence of Effect of Heparin Added In Vitro on the Thrombin-lnduced Conversion of Fibrinogen to Fibrin

Recent interest in the treatment of disseminated intravascular coagulation with heparin (1, 2) has caused some clinicians and laboratory workers to express doubt concerning the accuracy of fibrinogen methods based on thrombin-induced fibrin clot formation and subsequent measurement of the fibrin by biuret methods. It was reasoned that inaccurately low results would be produced owing to the presence of heparia in the plasma. This concern most certainly is influenced by the apparent discrepancies in the literature surrounding the effect of heparin on fibrin formation (3-7). Although there appears to be uniform agreement that heparin slows the conversion of prothrombin to thrombin (4), the effect of heparin on the thrombin-induced conversion of fibrinogen to fibrin is somewhat uncertain. Thus, when heparia is present in plasma, clot formation induced by endogenous thrombin after recalcification of plasma would be expected to fail, and does (5, 6), while clot formation as a result of an excess of exogenous thrombin added to the system may fail (4) or may succeed (1,5). We have tested the effect of heparin in vitro at concentrations far greater than those used in therapy of this disease process, and find no interference with our fibrinogen method, which involves exogenous thrombin-induced fibrin formation and analysis by a biuret method. Briefly, the fibrinogen procedure, modified from Ware et a!. (8) and Henry (9), is as follows: Add 1 ml of plasma to 6 ml of NaCI solution (9 g/ liter) and 3 ml of 0.066M phosphate buffer, pH 6.4 (±0.1). Add 0.2 ml of “Topical Thrombin” (1000 units/ml, Parke-Davis), mix, and wind the clot on a wooden applicator stick. The clotting process is complete within 10 mm. Remove the stick from the tube and place in a NaCI solution (9 g/liter) for 5 mm. Scrape the clot down to the tip of the stick and blot the clot dry on a paper towel. Place the clot into a clean tube and add 5.0 ml of the albumin-biuret reagent of Kingsley (10). Heat in a water bath at 50#{176}-56#{176}C until the clot completely dissolves. Ap-