From Dr. Larry Brace, Edward Hospital, Naperville, IL: I need an opinion. Here is my view on prothrombin time (PT) in seconds. We should no longer have to do a normal range for the PT in seconds. We know that the general consensus is that the international normalized ratio (INR) is considered normal to at least 1.2 (maybe 1.25 if you calculate to 2 decimal points). I think that the PT in seconds should be eliminated and we should view the INR in the same way we view cholesterol. We don’t do normal reference intervals for cholesterol, we just adopt them based on ATP guidelines because cholesterol is a “standardized” assay. Isn’t the INR a “standardized” assay? We say it is, and that every laboratory that reports INR should match every other laboratory that reports INR. If someone would really need to have a normal reference interval for the PT in seconds (and I don’t know why they would), the upper end of the normal range would be the PT in seconds that correlates to an INR of 1.2 and the lower end of the reference interval would be the PT in seconds that corresponds to an INR of 0.9.
If you adopt as normal an INR of 0.9–1.2, but determine independently the reference interval for the PT in seconds, you invariably get situations in which the INR is normal but the PT in seconds is not, or vice versa. This causes no end of grief because it is almost impossible to explain to clinicians why one result is flagged as normal and the other as abnormal.
I bring this up because during my most recent CAP inspection at Edward, the CAP inspectors had a fit about the upper end of our normal range in seconds (which corresponded to an INR of 1.25). (Yes, we still report both, but I don’t know for how much longer). BTW, the upper limit of 1.25 was based on clinician input because they were unhappy chasing clinically irrelevant “abnormal” results. Personally, I would be just as happy with an INR upper limit of 1.2 and will lobby for that with the newest lot of reagent (see below). When we did our last normal range verification, it comes as no surprise that our normal range in seconds did not match the upper and lower limits based on the INR. I simply adjusted the PT in seconds to match the INR. We just completed our evaluation of a new lot of PT reagent. We have 3 Stago instruments in 2 locations – the Satellite is at an off-site. Here are the results of the reagent validation:
Compact range: NEW LOT PT= 11.9–13.5, INR 0.92–1.08; OLD LOT PT= 12.2–15.4, INR 0.93–1.26
Evolution range: NEW LOT PT= 12.2–13.4, INR 0.94–1.06; OLD LOT PT= 12.2–15.4, INR 0.93–1.26
Satellite range: NEW LOT PT= 11.6–13.6, INR 0.90–1.10; OLD LOT PT= 12.4–15.7, INR 0.93–1.26
What would you do? There is no way that an INR upper limit of 1.1 would ever work.
From Geo: Dr. Brace and I have had this discussion on one or two occasions in the past, and I’ve stubbornly hewed (hewn?) to “conventional wisdom,” asserting that the INR was only developed for monitoring Coumadin. To screen for extrinsic pathway coagulopathies, we must use and report the PT in seconds compared to our locally developed PT reference interval. However, I’ve never been able to refute his argument, presented above with eloquence. It is true that most clinicians who have entered the profession in the past 20 years only know the test by “INR,” and, in fact, the INR is nothing more than a ratio computed directly from the PT in seconds, so why not just use it? I’d like to hear from our participants on this subject.