Platelet Antiheparin Activity

S ummary . A method for assaying the heparin‐neutralizing activity of platelets washed by albumin density gradient separation (ADGS) is described. Residual heparin, not neutralized by platelets, is measured by its capacity to potentiate the inactivation of factor Xa by antifactor Xa. The assay is sensitive to concentrations of heparin as low as 5 × 10 −4 units (approximately 5 ng) per ml. One major advantage of the assay is that it specifically detects the heparin‐neutralizing and not the clot‐promoting effect of platelets. The calibration curve relating the logarithm of clotting time to heparin concentration is linear. Platelets washed by ADGS (original method) spontaneously released 35% of the total antiheparin activity, where as only 2.75% of the total antiheparin activity was spontaneously released from platelets washed by a modified method of ADGS. Normal platelets washed by ADGS (modified method) and solubilized with Triton X‐100 neutralized 19.08 units of heparin per 10 10 platelets (mean of 19 samples). Two patients with platelet adenine nucleotide ‘storage pool deficiency’ had 16.3% and 21.4% of normal total antiheparin activity.