Summary
The activated partial thromboplastin time (APTT) is the most commonly used test for
laboratory monitoring of unfractionated heparin therapy. Since there are differences
between APTT reagents in respect of responsiveness to heparin the widely used therapeutic
range of 1.5-2.5 (APTT ratios) may not be appropriate for all reagents.
The aim of this study was to assess the relationship between 6 different APTT reagents
using a manual technique, 2 of these reagents used in combination with a coagulometer,
a heparin assay by protamine titration and a chromogenic anti-Xa assay. Samples from
42 patients treated with unfractionated heparin for thromboembolic disease were studied,
12 of whom were receiving warfarin therapy with International Normalised Ratios (INR)
of >1.3.
For normal subjects, APTT results were highly dependent on the method used and statistically
significant differences were noted. The ratio of patient to mean normal APTT was calculated
for each APTT method. When 30 samples from heparinised patients (with INRs of <1.3)
were analysed manually, the APTT ranges equivalent to 0.2-0.4 u/ml heparin by protamine
titration (by regression analysis) were 1.6-1.9 for Boehringer reagent (the least
responsive) up to 2.2-2.9 for Instrumentation Laboratory reagent (the most responsive).
The concentration of heparin associated on average with APTT ratios of 1.5-2.5 varied
approximately twofold to threefold between reagents.
