The Value of the Standardized Partial Thromboplastin Time in Detecting Heparin during Low‐Dose Prophylaxis

Abstract

Summary. The sensitivity of the standardized partial thromboplastin time technique in detecting circulating heparin during low‐dose calcium heparin prophylaxis in gynaecological surgery has been evaluated and compared with an anti Xa assay and a chomogenic substrate method. The PTT test and anti Xa test were performed daily on plasmas from eight control and seven heparin treated patients. Both the PTT and anti Xa test showed significant prolongation in heparin‐treated patients receiving the 5000 units regime 12‐hourly. In control patients the PTT test showed a significant shortening, maximal on the fifth day after operation. This interferes with the detection of heparin by the PTT during this period. Nevertheless, the effect of small concentrations of heparin is observable during low‐dose prophylaxis by its prevention of the post‐operative acceleration of the PTT test. By contrast, the anti Xa test is relatively unaffected by the post‐operative acceleration and is more prolonged by the low‐dose heparin therapy.The heparin levels in 17 plasmas from gynaecological surgical patients receiving low‐dose therapy were determined by PTT, anti‐Xa and chromogenic substrate. The chromogenic substrate and anti Xa assays showed reasonably good intra‐class correlation. Heparin levels determined by the PTT test were invalidated by the post‐operative acceleration of the test. If, however, this post‐operative acceleration is of clinical significance and relates to a prethrombotic state, its detection by the PTT may be important and may be a valuable indicator for the need for an increase of the heparin dosage. Unless it is shown that the anti Xa effect of heparin is of more importance in the prevention of thrombosis than the prolongation of the PTT, the standardized PTT appears to offer a simple method of control of low‐dose heparin administration.