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!

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!