Underfilling of specimen tubes containing 129 mmol/L (3.8%) buffered citrate prolongs prothrombin time (PT) and activated partial thromboplastin time (APTT) values. We studied this phenomenon by using 109 mmol/L (3.2%) buffered citrate as the anticoagulant, anticipating some increase in tolerance to underfilling. Venous blood drawn from 12 healthy subjects and 30 patients receiving long-term oral warfarin therapy was mixed with 109 mmol/L buffered citrate solution in proportions equivalent to filling the collection tubes from 52% to 100% of capacity. Accurate PT values were obtained from normal specimens if the tubes were filled to 65% or more of capacity. Accurate PT results in the therapeutic range were obtained only with filling to 80% or more ofThe results of prothrombin time (FT) and activated partial thromboplastin time (APTT) tests are known to become prolonged if there is too high a concentration of citrate, as may occur if the specimen collection tube is insufficiently filled, and the proportion of whole blood to anticoagulant solution is less than 9:1. 1 -2 Guidelines based on this observation have been established in most clinical laboratories for the rrtiriimum acceptable volume of blood in specimen tubes submitted for coagulation tests. The National Committee for Clinical Laboratory Standards recommends a final citrate concentration of 10.9 to 12.9 mmol/L in the specimen, corresponding to complete filling of a tube containing one of the two common types of buffered citrate anticoagulant.
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