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The effects of 3.2% and 3.8% sodium citrate concentration on the results of routine coagulation assays (prothrombin time [PT] and activated partial thromboplastin time [aPTT]) were evaluated by means of two sets of reagents, one responsive and the other nonresponsive. Five groups were entered in the study: healthy volunteers; outpatients receiving stable oral anticoagulant therapy; and hospitalized patients receiving intravenous (IV) heparin therapy, both IV heparin and oral anticoagulant therapy, or no anticoagulant therapy. With use of nonresponsive PT and aPTT reagents, varying the citrate concentration has little clinical significance except in patients receiving IV heparin therapy. In contrast, when responsive PT and aPTT reagents are used, the concentration of Many preanalytic variables may affect the results of routine coagulation assays. To improve the precision and accuracy of laboratory testing, it is important to identify these variables and realize their potential effects on testing. Preanalytic variables pertinent to routine coagulation testing can be classified into three major categories: specimen collection, specimen transportation and storage, and specimen processing. Variables associated with specimen collection alone are numerous. Some have little to no effect; others significantly affect assay results. Few studies document the influence of preanalytic variables. Such studies are difficult to reference; many have been published in nonpeer-reviewed publications such as Manuscript received May 15, 1996; revision accepted August 9, 1996.A d d r e s s reprint requests to Dr Adcock: D e p a r t m e n t of Pathology, 16601 E Centretech Pkwy, Aurora, CO 80011.sodium citrate anticoagulant has a significant effect on assay results. Eighteen percent of samples from patients receiving stable oral anticoagulant therapy demonstrated a change of less than 0.7 INR (International Normalized Ratio) units between citrate concentrations. Nineteen percent of patients receiving IV heparin therapy had a greater than 7-second difference when aPTT results were compared. These data demonstrate that citrate concentration affects the results of coagulation tests. On the basis of these data, it is recommended that 3.2% citrate be used for all coagulation tests.