A patient is stabilized on warfarin therapy and being monitored using the prothrombin time (PT) followed by calculation of the international normalized ratio (INR). The formula for calculating the INR is:
INR = (PTpatient/PTnormal)ISI (where ISI = international sensitivity index)
What is used for the PTnormal?
a. Arithmetic mean PT for the control
b. Geometric mean PT for the reference population
c. Arithmetic mean PT as found in a standard reference textbook
d. Geometric mean PT reported by the manufacturer of the PT reagent being used
ANS: B
The PTnormal is the geometric mean for the PT reference interval determined by the clinical laboratory scientists in the laboratory where the testing is being performed. Using the wrong value in this calculation has led to major reporting errors that have resulted in bad patient outcomes.
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Fill in the blank(s) with correct word
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