Variation among commercial activated partial thromboplastin time reagents in response to heparin

The activated partial thromboplastin time (APTT) has been advocated for monitoring heparin effect. This study was designed to determine the in vitro sensitivity to heparin of commercially available APTT reagents. Heparin was added in increasing concentrations to pooled citrated plasma. Fibrometer APTT determinations were performed at each concentration using General Diagnostics, Ortho, Dade, Hyland, and BBL reagents. A tilt-tube kaolin-activated partial thromboplastin time was also tested using a Sigma partial thromboplastin prepared by the method of Bell and Alton. The General Diagnostics, Sigma, and Ortho reagents displayed linear heparin sensitivity; the General Diagnostics APTT was prolonged 1 1/2-2 1/2 times in a plasma heparin range that prolonged a modified in-vitro Lee-White clotting times 2-3 times. The other reagents were either insensitive, too sensitive, or nonlinear in heparin response. Thus, commercial reagents vary widely in their in-vitro sensitivity to heparin.