Frequency of QC in the clinical laboratory

Kent Dooley has written an interesting essay, which is here. One of the points he makes is that not all clinical laboratory errors result in patient harm because clinicians will not always act on the erroneous result. So if an assay result doesn’t agree with other clinical data, the clinician may suspect the result might be wrong and ask to have it repeated. Dooley suggests that the minimum QC frequency should follow the time course for the likelihood of a clinician requesting a repeat sample, so that upon repeat, if the result had been in error, the new result will be correct (because now QC has been run).

Now, I am unencumbered by the knowledge and experience of working in a lab but my view of things is somewhat different. It seems to me that there are several error/detection/recovery possibilities as shown in the figure below. (Note, better pictures are here).

The problem of waiting for a clinician (of for that matter a patient) to question a result, before running QC is that it doesn’t take advantage of the purpose of QC, which is shown below.

That is, one runs the assay and at some time QC. If the QC is ok, then the results are released to the clinician. If not, one troubleshoots the assay including possibly rerunning patient samples. Using this scheme, QC frequency should not be determined by a retest time course but rather by the turn-around-time requirement for the assay.


Now if the clinician requests a the assay to be repeated, and QC had already been run, it is unlikely that running a second QC will detect anything. QC has limitations in its ability to detect error (see figure below). Random biases and random patient interferences will not be detected by QC.

This figure came from previous considerations about equivalent QC, which are here, and here.

Besides suspecting assay error, many assay results are repeated because a condition is being monitored. Delta checks are a type of QC that is performed on these samples to determine whether the difference between results is expected. Exactly how the clinical laboratory could act on the knowledge that the clinician suspects that something is wrong with the assay result is a topic for clinical laboratorians to answer.

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