The effects of heparin on the activated partial thromboplastin time of the College of American Pathologists Survey specimens. Responsiveness, precision, and sample effects.

Heparinized survey samples from the College of American Pathologists from 1981 through 1984 were reviewed to assess effects of the various reagents and instrumentation on the performance of the activated partial thromboplastin time (aPTT) in response to heparin. Responsiveness of the aPTT to the concentration of heparin as well as its sensitivity to the detection of heparin in the therapeutic range was most affected by choice of reagents. Precision was most affected by choice of instruments with photo-optical devices being roughly comparable. A previously described effect of calcium vs sodium salts of heparin was not supported by these data. Survey results were compared with results from fresh plasma samples from patients who had received heparin therapeutically and with response curves obtained from specimens heparinized ex vivo. The pattern of reagent dose responsiveness to the heparinized patient samples differed from that seen with survey samples and with the ex vivo heparinized fresh plasma samples. In addition, intrinsic characteristics of the individual specimens exerted substantial effects on the test results for all specimen types. Therefore, one cannot automatically assume that phenomena that hold for one form of plasma preparation are applicable to another. The nature and source of the heparinized plasma specimen must be taken into account when interpreting aPTT results.