An internal quality control program must monitor results both in the normal range and in the abnormal range. For each test, there is one control in the normal range and one or two abnormal controls. Abnormal controls may be in the unhealthy but physiologically possible range, or outside what is physiologically possible, or both. Testing in many ranges ensures that the procedures are accurate for a wide range of patient results.

Controls are run at least as often as specified by the instrument manufacturer. Controls should also be run whenever there is concern about the quality of results or stability of the testing system, or if the results of previous controls were not acceptable.

If a problem is discovered, the samples in previous runs of the instrument may also have been affected. Once the problem(s) are corrected, it may be necessary to go back and re-run previous samples working in reverse order, until the retested results match the original results.