Determination of nonfibrin protein in fibrin clots by use of I-131-labeled protein fractions.

The amount of nonfibrin proteins occluded within fibrin clots was investigated with specific radioiodinated human protein fractions. Vigorous washing of the clot followed by syneresis on absorbant paper removed all but small traces of albumin and alpha and gamma globulins. These fractions constituted approximately 6% of the total proteins in fibrin clots when fibrinogen concentrations were normal. The percentage of occluded protein increased as fibrinogen levels decreased. At fibrinogen concentrations of 48 mg./100 ml. plasma, albumin approached 20% of the total. Fibrinogen determinations which are based on the protein content, or dry weight, of the isolated clot are thus subject to systematic error which varies inversely with the fibrinogen concentration.