Westgard Quality Control Workshop – Part 3

dohI just returned from the Westgard quality Control Workshop, where I was a speaker and have a few blogs worth of comments – this is the third.

EQC – Equivalent Quality Control

This is the CMS proposal (1) to allow clinical laboratories to reduce the frequency of quality control from twice per day to once a month given that 10 days of running QC shows no values that are out (and given some other conditions).

Let’s try to construct a hypothesis to base such a recommendation. For example:

given any possible error condition that could be detected by external quality control, internal quality control would detect the same error 100% of the time.

This is about the best I can think of, which would result in the recommendation:

Stop running external quality control.

What does running 10 days of external QC with no out of control results show? The answer is nothing. This is because one can assume that during these 10 days, there were either no errors or if there were errors, external QC was not able to detect them. (It is possible that internal QC detected errors during these 10 days). In fact, this experiment is guaranteed to be meaningless. To see this, one must realize that internal QC is always “on” and precedes external QC. So to see if external QC is redundant to internal QC for an error, would mean that internal QC would detect the error and either shut down the system or prevent the result – this being the external QC sample – from being reported. However, one can get different information by running external QC for a longer period because if internal QC misses an error but external QC detects the error, then one has proved that external QC is not redundant to internal QC. This was shown to me (2) as out of control results for a range of assays ranging from 1 to 10 per year, where these were real problems. Since controls are run twice per day, the number of affected patients samples is larger.

So a lab that reduces external QC to once a month is risking an even larger number of patient samples which is made worse since the clinician has probably acted on the erroneous results.

Rather than do the experiment suggested by CMS, a lab can simply examine its external QC records for a sufficient length of time.

References

1.       To review, see: See http://www.aacc.org/events/expert_access/2005/eqc/Pages/default.aspx

2.       Personal communication from Greg Miller of Virginia Commonwealth University

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