Conventional chromogenic heparin assays are influenced by patient's endogenous plasma Antithrombin levels

To determine, in vitro, the influence of plasma AT concentrations on heparin levels as measured by commercially available chromogenic kits.

Purified AT was added to plasma that was immune depleted of AT at the following final concentrations: 0, 0.5, 1.0, 1.5, 2.0, 2.5 u/ml. Heparin was added to each of the AT concentrations at the following final concentrations: 0, 0.2, 0.4 u/ml. Heparin levels were then determined using 5 different Anti-Xa (n=4) or Anti-IIa (n=1) commercial kits. All kits provided purified AT to be added to the assay system.

When heparin was added to plasma, heparin concentrations measured were dependent on plasma concentrations of AT (p<0.001). When plasma concentrations of AT were less than 1.0 u/ml heparin concentrations were underestimated and when plasma concentrations of AT were greater than 1.0 u/ml actual heparin levels were over estimated. There was no significant difference in the findings between the various commercial Xa or IIa kits. In the absence of heparin, anticoagulant activity was detected when plasma AT concentrations were above 1.0 u/ml. There was a statistically significant correlation between plasma concentrations of AT and the amount of anticoagulant activity (p<0.001).

Conventional chromogenic heparin assays are influenced by endogenous plasma AT levels.