FX has been claimed to be of clinical relevance in monitoring long term treatment with anticoagulants. The aim of this investigation was to study the behaviour of FX at the beginning of the treatment.Seventeen patients under heparin therapy received a daily dose of 3 mg of Acenocoumarol. Blood samples were obtained at 1, 2, 4, 6, 8 days (Dθ - 8), after starting the treatment. Heparin was stopped when prothrombin time (PT) was less than 40 per cent of normal value. FX was evaluated using an

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... ated amidolytic method. Factor VII (FVII) and PT were measured in a clotting assay.The disappearance of FX from plasma could be fitted to an exponential decay pattern during 6 days in 17 patients, 8 days in 11 patients. Concerning FVII, the exponential decay was limited to the first 2 days, then a stable level was reached. The decrease rates of FVII and FX during the first two days were correlated (r = 0,76). The regression line In (FX) = f (D) calculated from D0, D1, D2 could be used for predicting FX concentration at or D8. A highly significant linear correlation existed between the observed and predicted levels (D8 : r = 0,97) (D6 : r = 0,92).These results suggest that FX amidolytic assay could be useful for monitoring the initiation of anticoagulant treatment. The decrease rate of FX could be a valuable guide for an early determination of the maintenance dose of anticoagulant.