The activated partial thromboplastin time as a monitor of heparin therapy: a warning.

The degree of correlation between the whole blood clotting time (WBCT) and the activated partial thromboplastin time (APTT) was evaluated in 42 patients receiving intermittent intravenous heparin therapy. Correlation between the two tests was found to be poor (correlation coefficient 0.52), primarily for reasons related to characteristics of the APTT system employed. The relationship in vitro between the APTT and the level of heparin was evaluated and found to vary widely over a range of heparin concentrations. This study emphasizes the marked variability between various APTT systems and the dangers involved with substituting the APTT for the WBCT to monitor heparin without appropriate initial evaluation.