To contribute to the development of a reference reagent for monitoring heparin therapy, a lyophilized partial thromboplastin time (PTT) reagent was prepared from synthetic dioleoylphosphatidylcholine, dioleoylphosphatidylserine, and dioleoylphosphatidylethanolamine, with colloidal silica as activator. The reagent, coded 91/558, was contained in sealed glass ampoules; it deteriorated in a heat degradation experiment, but its activity remained constant for at least 4 years when stored at −70 °C. Within- and between-run precision with this reagent complied with the requirements proposed by the International Committee for Standardization in Haematology (ICSH) Panel on PTT. The response of this reagent and of two other reagents to heparin added to pooled normal plasma was nonlinear. Citrated samples from 58 patients receiving intravenous heparin and from 24 apparently healthy volunteers were tested with reagent 91/558, with Automated APTT (Organon Teknika), with Manchester APTT reagent, with an anti-factor Xa assay, and with an anti-factor IIa assay. The correlation of APTT with anti-Xa and anti-IIa activity was poor. The best correlation was observed between reagent 91/558 and the Organon Teknika reagent. Correlations were improved when individual patients’ samples were replaced by pooled plasmas from heparinized patients, in whom the effect of oral anticoagulation was minimal. These results suggest that preparation of a lyophilized synthetic phospholipid reagent is feasible for use in monitoring heparin therapy.

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