The INR ratio, What is the denominator?

[wcasey5]

I have my inRatio2 and when I test, I have both the INR and the PT displayed as a result. To get the INR it looks like the machine is dividing by 10 and rounding up or down. So a PT of 26.2 gives me an INR of 2.6, a PT of 22.7 gives me an INR of 2.3.

According to my last Vein draw, the PT was 24.1 giving an INR of 2.2 (Dividing by 10.95 exact, or 11 if rounding up). My machine (2 hours previous) gave me a PT of 26.2 and an INR or 2.6. 2.1 seconds longer, .4 higher INR. If I used the same demoninator as the lab I would get an INR of 2.4.

So, what gives? Is my machine using the wrong denominator? Is the lab? Can hemosense adjust this? Does it matter?

Opinions and comments very welcome.

 

[normofthenorth]

Newbie here, far from an expert, BUT my understanding is:

1. INR is a dimensionless ratio, like "clotting time divided by clotting time";
2. It's taken over from PT partly because everybody in the world agrees on how to measure INR, while PT had regional or national differences that caused important confusions;
3. I'd be surprised if you'd get into trouble by concentrating on the INR numbers, even if the gizmos do derive that number from another.

 

[wcasey5]

You're right, the INR seems to be clotting time divided by normal clotting time. Looks like the lab normal clotting time is 10.95 and the machine normal clotting time is 10. I guess I'm just geeking out on the numbers. My concern would be the doctor flipping out on the difference, with my machine reading high.

 

[Protimenow]

INR is a normalized ratio (International Normalized Ratio). The ratio has to do with the thrombin level of the reagent -- it's the thrombin (the reagent) that effects the prothrombin time. In the labs, reagents with a specific value are used for testing, and it's the prothrombin time divided by this standard, specific value, that provides the INR.

Meters do this in a similar way. The CoaguChek S uses a code chip - the chip provides the denominator for the INR (this is the value of the rabbit brain thrombin in the reagent on that batch of strips). The ProTime meters have the denominator and other information coded directly on to the 'strips' (which they call cuvettes). The ProTime cuvettes have the reagent value and an expiration date coded onto them - so when the test is run, the correct value is determined - and if the strips have expired, the meter won't accept them.

The InRatio strips come with a batch number. When a test is run on the InRatio, the meter asks if the number on the strip is the same one as the number stored in the meter. Presumably, this batch number also contains the correct value for the reagent, so the results are accurate. Your InRatio meter SEEMS TO BE dividing by ten and rounding - but it's actually applying the reagent value to the determination - in this case, it's somewhere around 10. It may also be adjusting for temperature, which makes it a bit less than predictable if you divide the prothrombin time by the INR value for many tests.

So - for ANY test - lab or handheld meter - the value of the reagent is a critical factor in determining the INR.

Prothrombin time DOES vary based on the value of the reagent, so it's not a valuable number. It is possible that the ambient temperature may also be a factor - blood may clot more quickly at a higher temperature than at a lower one - but I'm not entirely sure if this is actually accounted for by the meters (which heat the strips before testing). The NORMALIZED Ratio is the one to look for.

(I've used three types of meters - each with different reagent values. The prothrombin times for tests made with the CoaguChek S were always nearly twice that for the ProTime meters. The Prothrombin times for the InRatio meters were always pretty close to those for the ProTime meters. The prothrombin times weren't useful diagnostically -- it was the coding/reagent values that were significant, and the reagents in each system had slightly different values)

 

[olefin]

I use the Inratio2 but can't answer your question for I have mine set to display only INR. I never paid any attention to PT number back before I got a monitor.

I'm sure Hemosense tech will answer your question.

Also I don't get lab test.

 

[wcasey5]

Ahhhh. I get it now. Thanks!

 

[Cris N]

I don't use the PT either. Just the INR.

 

[Protimenow]

The INR is really the important number here. Out of curiosity, I have it report the actual prothrombin time using the reagent in the strip, but the number is practically meaningless when it comes to anticoagulation management. Of more importance is the reported INR and a confirmation that the quality controls passed.

 

[wcasey5]

When I looked at the formula, it was actual PT/normal PT. I thought the normal PT was a standard number. I now realize it isn't, it depends on the reagent. The reagent in my Inratio strips happen to have a normal time of 10 seconds, the reagent at the lab had a time around 11 seconds. This makes sense, thanks for all the info!