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Validating the ISI

From “swagner.”

George, can you please describe how to calculate and validate a reagent-analyzer specific prothrombin time (PT) reagent international sensitivity index (ISI)? QMP-LS, Ontario’s mandatory proficiency testing program, publishes information on reagent validation. Included is the requirement to establish an analyzer and reagent-specific ISI using a certified reference plasma. I’m using Stago’s Etaloquick® and the insert contains these reference values:

  • spec#1 81.0%
  • spec#2 29.0%
  • spec#3 20.0%

What is this % and how do I get it back to a prothrombin time in seconds?

I’ve been following the method described in this paper: Van Den Besselaar AMHP, et al, Guildelines on preparation, certification, and use of certified plasmas for ISI calibration and INR determination. J Thromb Haemostas 2004;2: 1946–53. The method is based on a graph of the log of the PT in seconds on the X axis vs the log of the reference PT in seconds on the Y axis. You get a straight line y=mx+b where m = the slope. The analyzer-reagent specific ISI = m X vendor’s ISI. I’ve made a nifty spreadsheet, and now I just need to get the reference in PT in seconds and I’m set.

Stago has provided a differerent 2-stage method, where first you measure the difference and if its (message terminates here).

From George: Hello, and thank you for your question. I went to my colleague Stephen Duff, Co-CEO of Precision BioLogic Inc for assistance, as he has done extensive work on PT calibrators.

I’ll begin by asking, are you are literally required to establish an instrument-reagent specific ISI, or must you simply verify the accuracy of the international normalized ratio (INR) with the current reagent ISI-instrument specific combination? The Clinical and Laboratory Standards Institute (CLSI) Guideline H54-A, of which Mr. Duff is a co-author, recommends the laborious local ISI calibration only if the accuracy of the INR cannot be verified locally. Methods for both that and verification are described in the guideline, which is available from CLSI, linked here. I recommend you first determine if you want to do a full calibration or verification, then follow the stepwise instructions in H-54.

Meanwhile, those percentage figures published in  Stago’s Etaloquick package insert are ratios based on a single calibrator that is established at 100%; Stago will provide you with the results in seconds that you need.

Again, thank you for your question, you’ve driven me to the books and the experts! Please let me know how your project comes out, and watch here, as we are likely to see comments from others facing INR verification issues. By the way, check your message and resend with the portion that became truncated, there may be more we can discuss. Geo

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