Q: What is the clinical utility of doing free plasma Hemoglobin? 

Answer: Plasma normally contains no free hemoglobin. Significant amounts of hemoglobin occur in plasma following hemolysis either from a transfusion reaction or any mechanical fragmentation of red blood cells.

Caution should be taken not to perform test on serum, as hemoglobin gets liberated from red blood cells during clotting. Moreover, elevated bilirubin levels interfere with the quantitation of the total plasma hemoglobin.

Reference:

Fairbanks VF, Ziesmer SC, O'Brien PC: Methods for measuring plasma hemoglobin in micromolar concentration compared. Clin Chem 1992;38:132-140