The stem or our November 2022 Quick question asked, If you use the PT when no warfarin is being measured, how do you report the results? Our 51 participants responded as follows:
- Report both PT and INR: 34 [66%]
- Report PT/MRI ratio: 0
- Report PT only: 14 [27%]
- Report INR only: 4 [7%]
Answer 2 was an attempt to learn if any facility chooses to use the pre-INR reporting system. It appears this approach is no longer in use.
Although most reporting systems report both PT and INR, some of us stick with the principle that INR is only designed to monitor stable warfarin therapy, whereas PT in seconds should be used for all other applications. Conversely, a few of us report only the INR, perhaps on the principle that most providers are accustomed to making clinical decisions on this basis anyway. We look forward to some additional discussion on this topic from our participants.
We don’t normally report PT, instead we report PR on patients not on warfarin in our laboratory
I agree completely with Dr. Brace in his assessment of using seconds for reporting the prothrombin time (PT). There were once approximately 300 different methods of reagent/instrument combinations to perform this assay. http://clsjournal.ascls.org/content/ascls/15/2/91.full.pdf. This makes the comparison between laboratories using the PT to measure screening, coagulation factor levels and Vitamin K-antagonist therapy a “shot in the dark.’ Using the international normalized ratio (INR) calculation only can stop the misinterpretation of the PT results using seconds. In one institution I previously worked at we switched the PT thromboplastin reagent to a lower international sensitivity index (ISI). We then dispensed extensive information in oral seminars, e-mail and handouts to the entire hospital system explaining to clinicians to use the new reference data that should be used to monitor the PT/INR results. We explained that the new reagent/instrument system may cause longer PT times than had been seen with the previous ranges for normal values. It took no longer than 3 days that myself and the Hematology lab officer had to respond to a Pediatrician who said he couldn’t practice medicine this way. When we asked what the issue was we found he was using a 10 year old coagulation manual from Johns Hopkins hospital for his PT ranges. We were a Department of Defense facility. This is a problem anytime you change a component of a test. Dr. John Olson MD once experienced a similar issue when his institution switched to a more sensitive PT reagent with a low ISI. The physicians started seeing longer PT times of 15 seconds or slightly longer that still corresponded to non-critical INRs. TheSurgeons would then start ordering blood components such as packed red blood cells, fresh frozen plasma, platelet concentrates and cryoprecipitate. Dr. Olson then had the coagulation laboratory only report out the INR value. Only on special request when a factor deficiency was suspected would the PT in seconds be reported. In one years time this institution saved approximately $500,000 in wasted blood products by reporting out the INR.
I would like to take this further by suggesting that because the INR range above 4.5 is not accurate due to the limitations of the method use a chromogenic factor X assay. The clinician and the laboratory can then pinpoint the warfarin therapy to match the specific patients coagulation situation. See the following manuscripts that show how the INR can be inaccurate due to certain disease states and pre-analytical variables that will affect the PT/INR results.
In my opinion, the PT in seconds is a useless parameter because of the widely different sensitivities of the reagents used. Remember that the INR is a mathematical manipulation of the PT in seconds to account for the different sensitivities of the reagents used. Example: I work at two different hospitals; one uses a PT reagent with an ISI of 1.0 and the other an ISI of 2.0. A PT of 17.0 seconds has different meanings in these two settings. However, whatever the INR is has exactly the same meaning in both. It tells you the degree of coagulation derangement, no matter what the cause. Remember, when cases are being discussed, the normal range in seconds is often not readily available during the discussion. On the other hand, the meaning of the INR is standardized across clinical settings. An INR of 2.5 at one lab should mean exactly the same as an INR of 2.5 at another lab. Yes, I know that the INR was initially developed to monitor patients stably anticoagulated on vitamin K antagonists. Using the INR ONLY for monitoring oral vitamin K antagonists is a bit like saying that a newly approved anticancer drug can only ever be used for any other cancer than the one it was approved for. We all know that is not the way things work. So to go back to my opening comment, the PT in seconds is a useless parameter and can actually be misleading if you don’t have each institutions normal range in seconds readily at hand. The PT in seconds should be retired as a no longer useful parameter.