From Cathy Labare:
We are changing reagent lot numbers for our PTs and PTTs. How close should the normal ranges of the new lot be to the current normal range?
Hi, Cathy, thank you for your question. We use a limit of 10%, meaning if the normal range (reference interval) limits from the new lot fall within 10% of the limits generated from the old lot, you need not publish a change. However, we also watch for trends. For instance, if last year the limits rose 7% and this year they rose 8%, we would publish the change. The same is true for PTT therapeutic range, which can vary rather widely from lot to lot.
This seems to be more a rule of thumb than a CAP or CLSI guideline, so I would like to learn if others use a different limit.
in my experience you always must change the curve for PT wit
in my experience you always must change the curve for PT with least 20 samples to get a PPP. You obtain your 100% of activity and the reference values range from 70 (dilution) up to 120% (obtained by extrapolation).
However with PTT you can test 20 samples (healthy people, outpatients) and if less of 3 exceed the manufacturers values it is permitted to uses the insert RV. Try to use the CaCl2 provided in the reagent set.