The conversion of plasma to serum results in a variable loss of the coagulase-reacting factor (C.R.F.) of the plasma.

The C.R.F. loss is incurred during the process of prothrombin conversion: conditions which favor the most effective prothrombin conversion result in maximal C.R.F. loss, while factors which interfere with prothrombin conversion spare the C.R.F.

In a system containing an adequate concentration of calcium, thrombo-plastin, and prothrombin, the C.R.F. loss reflects the amount of prothrombin conversion-accelerating substances (factor V or AC globulin).

If fibrin clots are produced directly by thrombin, and prothrombin conversion is excluded, there is no significant C.R.F. loss.

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