Summary
Blood is incoagulable for several hours after therapeutic doses of intravenous heparin when tested by the glass clotting time (CT) or activated partialthromboplastin test (APTT). A simple modification of the APTT is described which allows the quantitative measurement of plasma heparin levels at any time during therapy. In principal the test plasma is diluted in normal plasma before assay, thereby reducing the heparin concentration to a range which can be measured by the APTT. The corresponding plasma heparin concentration is estimated by reference to a single standard curve which applies for concentrationsfrom 0.1 to 10 I.U./ml of plasma. Since all samples are diluted in normal plasma, storage of blood at 4° or even 37° for up to 48 h does not significantly alter the results. The assay is applied in the study of plasma heparin disappearance time and in the control of anticoagulant therapy forthromboembolic disease.
