Comparison of anticoagulants for the preservation of prothrombin time specimens

The value of different anticoagulants as preservatives of specimens for prothrombin time ratios is evaluated and discussed. An increase in the ratio was demonstrated with all anti-coagulants after 24 hours and this was significant when coagulometer error was taken into account. Both solid and liquid sodium citrate were superior to sodium oxalate as preservatives at 24 hours after taking off samples, but differences were slight and were not significant when coagulometer error was considered. It was concluded that for routine clinical practice, where a range of the prothrombin time ratio of 1.8/1 to 2.5/1 is allowable, there would be no significant advantage in any of the anticoagulants considered.