A clinical laboratory receives a new lot of PTT reagent, so clinical laboratory scientists in the laboratory need to establish the heparin therapeutic range for this new reagent lot. How should this be done?

a. Compare PTT results for patient heparinized samples to those for the lot of PTT reagent that is presently being used.
b. Perform chromogenic Xa and PTT assays on patient heparinized samples, and do a statistical analysis of result comparisons.
c. Add heparin at various therapeutic concentrations to normal plasma, and perform PTT on each concentration using the new lot of reagent.
d. Add heparin at high concentration to one normal plasma, make dilutions of this plasma, and then perform PTT on each diluted sample using the new lot of reagent.

ANS: B
Accrediting agencies for clinical laboratories require that the PTT heparin therapeutic range be determined using samples from patients who are receiving heparin therapeutically; they cannot be receiving simultaneous warfarin therapy (thus their PT must be normal). Both a chromogenic anti-Xa assay and PTT are performed on each patient sample, and the paired results are plotted in a linear graph. The range that corresponds to 0.3 to 0.7 chromogenic anti-Xa is the therapeutic range.