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PT/INR Results Using ISI 1.0 Reagents

From Terezinha Paz Munhoz: we started to use recombinant thromboplastin,RecombiPlasTin 2G (ISI 1.0) on our IL ACL TOP 500. The PT results are quite different from rabbit brain reagent. The high sensitivity shows a wide range: 8–220 seconds. We know the method linearity in this equipment is 8,0–320 seconds, and the percentage activity (that we still report) is “crazy” with values from 5% or less to 140%. INRs are very high too. What is the importance in reporting these results? Should we limit as <10% and >130% or less? If we decide to stop reporting percent activity we still have seconds and INR in a very wide range. We can describe prolonged results as incoagulable, but there is a clot. On the other hand, some samples could be activated, though unlikely that they appear daily.

When we tested donors samples (n = 20) the results ranges (med ± 2 SD) were 9,28–12,3 sec; activity 83,5%–127,3%; and INR 0,8–1,09. With our patients we’ve been having results like I told above. Including more normal patients from the lab we achieved a bit larger range that we are using: 12,1–16,5 sec; 75,6–130,4% and INR 0,8–1,14. How can we manage this situation? Sorry for too long message and  questions. Best regards,
Terezinha Paz Munhoz
Laboratório de Patologia Clínica
Porto Alegre, Brazil

Hello, Ms. Munhoz, and thank you for your question, which was definitely not “too long!” The results you are seeing are typical of synthetic low international sensitivity index (ISI) thromboplastins. I discussed your question with reagent expert Dave McGlasson, and we agree that you should probably set your instrument to cut off at 200 seconds, as INR values that are generated from prothrombin times longer than 200 seconds have little clinical significance. Some choose even shorter times. You are also wise to continue to educate your clinicians and anticoagulation clinic experts on the value of the international normalized ratio (INR) over the use of PT results expressed as % activity.
Also, to establish your reference interval (normal range), your healthy laboratory professional subjects probably more closely match your patient population than do donor samples and provide you with a more workable interval, thus 12.1–16.5 seconds may be realistic. I hope this helps, and I encourage assistance from our participants.

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