Normal, coumarin, factor II deficient, factor VII deficient, factor X deficient, factor IX deficient, abnormal factor IX and abnormal factor X (factor X Friuli) plasmas were diluted 1:1, 1:2, 1:3, 1:5 and 1:10. Variable aliquots of such dilutions were then added to several tissue thromboplastins and the clotting time measured. Four thromboplastin reagents were used: Thrombotest, Normotest, Manchester Thromboplastin and Simplastin A. Using Manchester Thromboplastin absorbed normal plasma was added to the system too. The "inhibitory effect" present in the system was evaluated by means of conventional units "i". Using Thrombotest, Manchester Thromboplastin or Simplastin A a variable but clear "inhibition" ("i">0.5 units) was found in all plasmas. Using Normotest a slight "inhibition" was present only in coumarin plasmas and in factor VII or factor X deficient plasmas. In factor IX deficient plasma no inhibition could be assessed since in all cases it gave a dilution curve identical to the normal plasma one. The studies indicate that the dilution curve analysis has no value in the case of single congenital prothrombin factors deficiencies since these plasmas "behave" in the system in a way similar to coumarin plasma.
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