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Changing Reagent Lots

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.

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