Lab tests have error and sometimes very large errors. As the last blog entry showed, patient harm can result from certain lab errors. In this blog entry, lab error means an error large enough to result in patient harm. But it is not the error itself that causes harm, it is the clinician acting on the result. When harm occurs from lab error, one can infer that the clinician has not questioned the accuracy of the test. Thus, it’s up to the patient to question the result.
Some examples of how lab results can have error:
Interferences – The HAMA interference from the previous blog is just one example of an interference and can occur on assays other than hCG.
Known bias – Example: PSA values can differ by 22% on average depending on the manufacturer. This is due to how the assay is standardized. Say one’s PSA value was 3.3 as assayed by manufacturer ABC. If next year the assay value remained at 3.3 by manufacturer ABC, but a different manufacturer were used (that had this 22% different standardization), the reported value would be 4.0 on average. Actually, taking into account the ~5% CV of these assays, 95% of the time the value would be between 3.2 and 4.8 (e.g., half the time greater than 4). Unless questioned, this might lead to a biopsy. Often, the manufacturer is not listed on the lab report. To find this out, one must call the lab.
Problems with newer tests – Molecular testing with arrays is the newest type of testing. A recent article (subscription required for full article) showed that the reproducibility was often greater than 30% CV. If one translates this to a glucose test with a true value of 100 mg/dL, 95% of the time, values would be between 40 and 160 mg/dL! Another report showed that different generations of probe sets often had close to 0 correlation. Back to glucose, this is equivalent to running a method comparison between a newer and older machine and getting random scatter rather than a typical result of a correlation > 0.9.
Other problems – There are many other potential problems that would give an error such as a patient sample mix-up, an undetected instrument error, sample pretreatment problems, and so on.
When to question lab tests – In principle, any lab test could be questioned, although this could be (is) impractical. Moreover, the above problems will be unknown to patients not familiar with laboratory medicine and even people who are familiar with error causes may be unaware that an error has occurred.
Two scenarios are suggested to question lab tests:
- Before a treatment is started, especially a treatment with risks such as surgery
- If symptoms persist and a lab test was negative
How to question lab tests – Unfortunately, simply repeating a lab test will not always help. It depends on the error source. If the error source is random, then simply repeating the test will help. If the error source is not random, such as caused by an interference, then repeating the test by the same procedure in the same lab will not help. In situations with HAMA interference, part of the problem was that serial measurements gave the same wrong answers which prompted clinicians to continue (the wrong) treatment.
The safest way then is to request a test to be repeated by a different laboratory and preferably a reference laboratory, if one exists for that assay.
And remember – A wrong lab test is a rare event – whereas a result is worth questioning, the likelihood that a lab test is wrong is extremely low.
NO MEDICAL ADVICE: Material appearing here represents opinions offered by non-medically-trained laypersons. Comments shown here should NEVER be interpreted as specific medical advice and must be used only as background information when consulting with a qualified medical professional.