In order to avoid repetition, ‘laboratory error’ has been omitted from the differential diagnoses in this chapter. However, it should always be borne in mind that factors such as mislabelling or misidentification of samples, errors introduced by the laboratory machinery (especially certain in-house laboratories where quality control is inadequate) and errors due to ageing samples or incorrect collection techniques can all cause apparent abnormalities. Where a test result is unexpectedly abnormal, it should be repeated, preferably by a different method. It is also important to remember that reference intervals are usually based on the values into which 95% of the healthy population would fall, so small changes outside these values may not be significant. Finally, each laboratory establishes its own reference intervals, due to differences in testing methodology and local factors, and thus when comparing results over a course of time, it is best to use the same laboratory.
