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.
SummaryThe results of determinations of the prothrombin time (PT) and the activated partial thromboplastin time (aPTT) are frequently used to assess hemostatic function. Accurate results for these laboratory tests depend on many variables, one of which is the ratio of plasma to anticoagulant. We studied 12 patients and 4 normal subjects to determine the effects of sample volume on PT and aPTT. We conclude that underfilling may produce profound effects, particularly on the aPTT. In contrast, overfilling rarely affects the results. The greatest effects of sample volume were observed in specimens in which the true PT or aPTT was elevated. A normal PT or aPTT result on any specimen, regardless of sample volume, strongly suggests that the true value is normal.
Use of the International Normalized Ratio (INR) has been recommended as a means of standardizing prothrombin time (PT) results for management of oral anticoagulant therapy. During the evaluation of a new lot of thromboplastin reagent, however, INR values were obtained that were inconsistent with results obtained with the prior lot of reagent from the same manufacturer. A local normalized ratio (LNR) was substituted for the INR for the new reagent, based on a calculated local sensitivity index (LSI). Validation of the LSI was performed at the three hospitals in the medical community by testing an identical panel of aliquots from 64 plasmas obtained from patients who were chronically receiving oral anticoagulants. Each test result was classified according to the INR value as low, low therapeutic, high therapeutic, or high. Local normalized ratio values obtained at one hospital in the community were in reasonable agreement with INR determinations in the other two hospital laboratories. Classification mismatches occurred using the INR, however, in 84 of 280 (30%) of the paired samples. Thus, the inability to generate consistent INR values within a local medical community raises serious concern about the reliability of the INR concept in its current form.
P r o t h r o m b i n T i m e a n d A c t i v a t e d P a r t i a l T h r o m b o p l a s t i n T i m e C a n B e P e r f o r m e d o n t h e F i r s t T u b eEUGENE L. GOTTFRIED, MD, AND MITCHELL M. ADACHI, MD
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