The Prothrombin Time Test: Effect of Varying Citrate Concentration^[*]

Summary

Blood samples from 12 normal subjects and 46 patients on oral anticoagulants were divided so that each was anticoagulated with four concentrations of trisodium citrate solution in the range 0.09–0.15 M, 9 vol blood being added to 1 vol citrate solution in each case. Citrated blood haematocrit and plasma prothrombin time was measured on each subsample; 0.025 M CaCl₂-solution was used for recalcification throughout. Three laboratories participated, each using a different prothrombin-time technique.

The haematocrit of the citrated blood fell progressively as the molarity of the citrate increased, showing a mean reduction of 0.2%/0.01 increase in molarity in the citrate reagent. With rising citrate concentration, two laboratories showed a progressive lengthening of the prothrombin time in the plasmas of anticoagulated patients, but one did not.

A uniform concentration of trisodium citrate solution should be used: 3.2% trisodium citrate. 2H₂0 (0.109 M) is suggested. A chart is provided for obtaining a constant proportion between citrate and plasma volumes, from the patient’s haematocrit.

^* This paper was prepared for the Expert Panel on Oral Anticoagulant Control (International Committee for Standardization in Hematology; International Society for Thrombosis and Haemostasis). The members of the panel are listed at the end of the paper.

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