Effect of Clot-Detection Methods and Reagents on Activated Partial Thromboplastin Time (APTT): Implications in Heparin Monitoring by APTT.

Abstract

Two automatic coagulometers, ACL 810 (Instrumentation Laboratory), a laser-nephelometric centrifugal analyzer, and KoaguLab 40 A (Ortho Diagnostics), an optical automatic coagulometer, were compared with the manual tilt-tube method for the performance of activated partial thromboplastin time (APTT). Seven commercial APTT reagents were used for duplicate determinations in 30 normal controls, 26 patients with liver disease, and 33 patients on full-dose heparin treatment. Clotting times were longer with the manual method than with ACL 810 and, to a lesser extent, with KoaguLab 40 A. Average imprecision of duplicate determinations (coefficient of variation [CV]) was less with ACL 810 (less than 1.5%) than with KoaguLab 40 A (2.9%) and the manual method (2.4%). Differences in slope of the regression curves of clotting times obtained with the coagulometers over the tilt-tube method were observed with all the reagents tested (P less than 0.01). Transformation of clotting times of controls, patients with liver disease, and patients on heparin therapy to APTT ratios did not eliminate the bias resulting from the different reagents (P less than 0.001) and clot-detection methods (P less than 0.001); in controls, significant (P less than 0.001) reagent-method interaction was also observed. The in vitro heparin sensitivity differed with the APTT reagents evaluated and was influenced by the clot-detection method used. Transformation of APTT ratios of anticoagulated patients to apparent plasma heparin levels--as derived from in vitro dose-response curves--effectively eliminated the bias resulting from the different clot-detection methods but had no effect on the bias resulting from the different APTT reagents. In vitro heparin activity curves thus have little, if any, relevance for the ex vivo monitoring of heparin treatment.

The inhibition of blood coagulation by heparins of different molecular weight is caused by a common functional motif—the C-domain

“…Specific activity is defined as activity per amount of material and, for heparins, is usually expressed in international units (IU) per mg. One milligram of a natural heparin contains an unknown amount of molecules that do not contain the AT binding pentasaccharide and therefore weight is not an exact measure of the amount of active material. The activity of any heparin relative to a standard is strongly dependent upon the test method and experimental conditions [27–29]. So neither IU nor mg can be used as units to express specific activity with high precision.…”

Problems In Laboratory Monitoring Of Heparin Dosage

“…The results of APTTs determined for heparin dosage monitoring can be influenced by the method of end-point detection (D'Angelo et al, 1990). We have demonstrated that use of one particular coagulometer (ACL) with one reagent was not associated with differences between APTT ratio when compared to manual end-point detection , whereas with the same instrument, a significant increase in APTT ratios was obtained using another reagent (manual mean APTT ratio of patients 2´5, compared with ACL mean 3´3).…”

Dose‐effect relationship for several coagulation markers during administration of the direct thrombin inhibitor S 18326 in healthy subjects

“…APTT is the test most widely used to monitor heparin treatment [6, 7] and many studies have shown that it is suitable for adjusting the dose of unfractionated heparins [8, 9]. Given the known variability among APTT reagents [10, 11], we initially compared five commercial reagents in vitro and chose the reagent yielding a linear increase in clotting time over a wide range of S 18326 concentrations, leading to an increase up to 7‐fold the basal APTT (Figure 1).…”