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PT % Activity

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!

Comments (5)
Screening Assays
Feb 16, 2016 7:32am

 From Tom Exner, Haematex
 From Tom Exner, Haematex Research, Hi George, Thanks for your interesting “provocative” posts. I can add a little to the one on % PT. I agree with the previous responders that it is based on a calibration curve of PT versus normal plasma dilution. A patient PT can then be interpolated onto this curve to derive a % PT. The problem with this method was what to use as a diluent. Some labs used saline, others used a 5% albumin solution. Yet others used an adsorbed normal plasma deficient in vitamin K-dependent clotting factors.
There was an international survey carried out many years ago, co-ordinated (I think) by Leon Poller comparing these various methods for expressing results on true warfarin plasmas with the use of a reference thromboplastin (British Comparative Thromboplastin or Manchester reagent). The use of a reference thromboplastin was shown to be more reliable than use of a dilution curve. Obviously varying the diluents together with varying PT reagents resulted in more variable results.
This led to the adoption of the “reference reagent” PT system with ISIs and ultimately INRs for the expression of PT results for oral anticoagulant monitoring at least in UK-centric countries. There was a minority report disagreeing with this approach issued by some participants who understood the problem.
Thanks for the opportunity and best wishes from Tom Exner.

Feb 16, 2016 2:29am

I believe Vadim and Joyce
I believe Vadim and Joyce have said what needs to be said; PT % activity is widely used in Europe, and is based on the Quick method, involving a standard curve based on dilutions of normal plasma, but the rest of the world have moved to the INR and PT ratio. As EIC of STH, I am often asking European authors to introduce either PT, PT ratio or INR into tables to accommodate the rest of us. It could be that the Europeans have a useful way of expressing PT for some patient types that PT, PT ratio or INR is inferior for, but we remain in blissful ignorance in Australia and America. Joyce mentioned another procedure previously used in Australia, based on the control PT/patient PT x 100, but that was before my time! Joyce will hit me for saying that next time she sees me.

Feb 15, 2016 1:02am

Dear George,

Dear George, A long long time ago when I was trained to do manual coagulation in the waterbath the PT, historically called the Quick Time, was expressed as a % (also called prothrombin index or PI ). It was the control PT in seconds divided by the patient PT x 100. There was no concept of a mean normal PT then, only a control PT and we used a commercial normal plasma. This was Australia. However there was also in Europe and elsewhere, a test involving using a calibration curve with normal plasma being diluted (normal plasma being 100%) and reading the patient PT activity in percent off the calibration curve. Does that help?

Feb 12, 2016 5:53pm

According to the article
According to the article below, the prothrombin activity in % is most reliable test for estimation of liver failure independent of thromboplastin ISI, while INR, PT in seconds and PT ratio values are not:
Robert A, Chazouillères O. Prothrombin time in liver failure: time, ratio, activity percentage, or international normalized ratio? Hepatology. 1996;24:1392–4.

Feb 12, 2016 5:09pm

Prothrombin (PT) activity in
PT % activity was used widely in Europe and calculated as % from normal pooled plasma. PT in seconds is measured as 100% in undiluted NPP, 50% of the value of NPP when diluted 1:1 with buffer, 25%, 12.5% and 6.25%. Then PT in seconds is plotted vs % of plasma dilution (prothrombin activity). Doubly increased PT compared to MNPT corresponds to 25% of prothrombin activity (i. e. NPP diluted with buffer as 0.25:0.75 vol/vol). More information is detailed here:
Lindahl TL et al. INR calibration of Owren-type prothrombin time based on the relationship between PT% and INR utilizing normal plasma samples. Thromb Haemost. 2004;91:1223–31.

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