ASSESSMENT OF THE INFLUENCE OF CITRATE CONCENTRATION ON THE INTERNATIONAL NORMALIZED RATIO (INR) DETERMINED WITH 12 REAGENT-INSTRUMENT COMBINATIONS

Citrate concentration is one of the variables that can affect coagulat ion tests. However, few studies have so far been performed to assess t he magnitude of this effect on coagulation tests in general and PT in particular. The aim of this study was to assess the extent of influenc e of citrate concentration on the PT test with results expressed as IN R. Twelve reagent-instrument combinations (systems) were calibrated vs . the Reference Preparation BCT/441 using plasmas collected in either 105 mM or 129 mM citrate from normals and anticoagulated patients (OAT ). PTs for plasmas collected in 129 mM citrate were longer than those collected in 105 mM both for normals and patients on OAT, but the rati os (patient-to-normal clotting times) for the two citrate concentratio ns were significantly different in many instances, implying that the I nternational Sensitivity Index (ISI) is also different. ISIs for calib rations with plasmas collected in 105 mM were greater (up to 10%) than those with plasmas collected in 129 mM citrate. When PT ratios were t ransformed into INR using crossover ISIs (i, e., plasmas collected in 105 mM and ISI determined with plasmas collected in 129 mM citrate, or vice versa) we found that an INR of 4.5 could be up to 20% apart from the value that would have been obtained if the appropriate ISI was us ed. Moreover, if the ISI determined with the manual technique was used to convert PTs obtained with a particular instrument into INR, the ef fect of citrate concentration was even greater (INR difference up to 6 4%). Should these observations be valid for other systems, they might provide additional explanations for the frequent reports which documen t discrepancies in the INR determined with different systems to which incorrect ISI might have been applied. World-wide consensus on a singl e citrate concentration to collect patients' as well as lyophilized pl asmas to be used in External Quality Assessment Schemes and for local system calibration is therefore urgently needed.