Comparison of Two APTT Methods of Monitoring Heparin Therapy: APTT Ratio and Heparin Response of Pooled Normal Plasma

Abstract

The sensitivity of the reagents for activated partial thromboplastin time (APTT) test varies greatly. Consequently the physicians who prescribe heparin based on certain APTT ratios may order different doses of heparin and produce different levels of anticoagulation in their patients, depending on the sensitivity of APTT reagent used by the laboratory. The authors have been recommending that physicians in their hospital use a therapeutic range for heparinization based on the sensitivity of the APTT reagent and keep the APTT of patients within the range of APTT of pooled normal plasma containing 0.2-0.4 units of heparin per milliliter. Because the patient's response to heparin in vivo may be different from that of pooled normal plasma, the authors planned to compare the effect of these two methods of heparin monitoring on heparin usage and complications of heparin therapy. In a retrospective study, the authors reviewed the hospital records of patients treated with continuous intravenous heparin for the management of thromboembolic disorders during two periods: one period in which the laboratory used an APTT reagent with low in vitro sensitivity to heparin (LSH) and another period in which the authors used an APTT reagent with high sensitivity to heparin (HSH). The authors found that there were no significant differences between the incidence of bleeding or thrombotic complication in the two periods. Furthermore, they found that in both periods, the patients had received similar total doses of heparin during the first 72 hours of therapy. However, as was expected from in vitro sensitivity, the APTT of patients during the LSH period was significantly lower than those during the HSH period. More heparin would have been used during the LSH period compared to the HSH period of physicians were to use the APTT ratio method for monitoring the therapy. The authors conclude that using the therapeutic range for monitoring heparin therapy based on the heparin response of pooled normal plasma will result in a more comparable level of heparinization from year to year and from center to center than by using the APTT ratio method.