Q&A: QC with a new control lot; manufacturer mean

Posted by Sten Westgard, MS

Some recent questions we got via email:

"When a new lot number of control was started in a machine, the insert sheet range will be followed until 20 or 30 days then our own range will be settled. After doing this some of the tests run during this period become outside the settled range. Is this accepted?"

"Sometimes our own range is outside the insert sheet range e.g. the insert sheet range is 20-30 and our own range is 18-25. is this accepted or we have to be inside the insert sheet range"

The answers, after the fold...

"When a new lot number of control was started in a machine, the insert sheet range will be followed until 20 or 30 days then our own range will be settled. After doing this some of the tests run during this period become outside the settled range. Is this accepted?"

It is unusual to find that many values of the 20-30 day "settling" period would have values out-of-control. But in any case, the mean that you have established is for the future, not for the past. It is not so useful to use this new mean to look backward on data from many days ago.

In theory, you have been continuing to operate with the old lot of control during this phase-in of the new control, so those "settling" period data points will not have caused the release of any bad test results. Thus, the merit of declaring them out-of-control is limited. If this is a persistent occurence, however, I would suggest that you talk with the manufacturer.

"Sometimes our own range is outside the insert sheet range e.g. the insert sheet range is 20-30 and our own range is 18-25. is this accepted or we have to be inside the insert sheet range"

The manufacturer range is usually only to be used for the mean. That is, your mean should fall within the manufacturer's range. Your range, however, will probably be different than the manufacturer's range - and that is not a problem. Does the manufacturer give you different instructions on what to do with their supplied mean and range? Do they describe how they determine that mean and range?

We hope these answers helped. We encourage you to to send in your own questions, just please bear in mind that we do not always have the time to answer immediately. 

Comments

[mike toyoshima]

Sten,
I hope you are well.
These are excellent questions - ones that should lead to some questions for the lab (type of test/instrument - antibody mediated, for example; is this a test that can be calibrated are 2 intial questions I'd ask). I'm also interested in those values outside the range...more of a poor precision distribution, or perhaps a bias issue. This is why peer group data is so helpful, particularly for common instruments using popular control lots.
Questions like these should've been proposed to their instrument or QC provider, or both.
Sten, please edit as necessary.
mike