Myths of EQC (Equivalent Quality Control)

mythCMS has established EQC (Equivalent Quality Control) as a way for clinical laboratories to reduce the frequency that they perform quality control, provided they meet certain guidelines. I have previously commented on the problems with this: see my 4/20/07 blog “Beware of Equivalent Quality Control” at and also an AACC expert session at

The purpose of this entry is to deal with the fact that although the expert session dealt with myths of EQC – these myths persist in comments by CMS and by people who are preparing CLSI documents about risk management. Hence, I will repeat here some of the myths.



1.       Internal QC is new – What is meant by internal QC is algorithms and associated hardware to detect and prevent incorrect lab results. Internal QC has been around since the days of SMAC. Whereas it is always being improved, it is not new. “Internal QC is new” is often used as a justification for implementing EQC as in … because modern analyzers are now using new, sophisticated …

2.       External QC is redundant to internal QC – It is often implied that one is justified in reducing external QC because it is redundant to internal QC. This is not always true. As an example, say an algorithm looks at the response to determine if the sample is too noisy. If so, the sample will be rejected. But algorithms such as these do not work 100% of the time. If the algorithm fails on a calibration, the calibration will go through and all subsequent samples could have a shift. External QC is different and when run will likely detect the shift. An example of redundancy is to have five blood gas systems in the laboratory. If one system fails, the other systems can be used. (See also #5, #6).

3.       External QC doesn’t work for unit use systems – Here, it is implied that each sample run is completely unique in unit use systems so that external QC can only inform about one sample. This is not true. Unit use systems are manufactured and the manufacturing process can have drift and bad lots, so that a batch of unit use devices are bad. External QC will detect this condition.

4.       Internal QC always works (is 100% effective) – See #2 – internal QC often has the properties of a medical test – there are some false positives and false negatives. One can see that this point is missed in writings about internal QC. Thus, if one has data from an internal QC experiment, such as success was achieved in 100 out of 100 tries, one must realize that whereas the point estimate is 100% (effective), the confidence interval is not. One has not proved 100% effectiveness. The value of external QC is that is uses a different mechanism and can catch errors that internal QC misses.

5.       Because one performs FMEA and other risk management tools, one has thought of everything  – Of course, there will be no associated internal QC for a failure that no one has thought of. But with external QC, one does not require knowledge of failures for external QC to work. One need only review the list if FDA recalls to see that manufacturers have not thought of all ways a system can fail. (see also #2, #6).

6.       No one makes human errors – In reviewing the list of FDA recalls, some errors are human errors – such as releasing a lot of reagent that has failed. Once again, external QC can catch these errors.

There will be many conditions where internal QC catches errors that would be missed by external QC, but there is no scientific evidence that one can reduce the frequency of external QC without increasing the risk of medical errors.


One Response to Myths of EQC (Equivalent Quality Control)

  1. […] This figure came from previous considerations about equivalent QC, which are here, and here. […]

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