This came up in correspondence yesterday and challenged an assumption. The PT is reported in seconds and INR, of course, and it was previously reported as the PT ratio (PTR), which is defined as a ratio of the patient’s PT to the mean normal PT (MNPT). But what is the PT % activity? I have assumed it was simply the PTR expressed as a percentage, but the math doesn’t work. Next I thought it was the difference of the patient’s PT and the MNPT divided by the MNPT, expressed as a percentage, but that doesn’t work either. There’s a current pub by Dr. Tripodi on this subject, but I’ll withold it for a few days so I can learn if I’m the only one who didn’t know how to determine PT % activity without looking it up. Please give me your advice!