Most self-testing kits rely on the same principle, called ‘an immunoassay’, that is used by hospital laboratories. Usually, in these types of tests, specific antibodies have been manufactured and labelled with enzymes or coloured particles so that they can be easily detected. The amounts of enzyme or particles present in the immunoassay are proportional to the amount of analyte (the substance being measured). Even today, immunoassay procedures used in hospital laboratories can take anything from a few minutes to a day or so to provide a result.
Unlike laboratory tests which usually give a number for the amount of biomarker present in a sample (quantitative test), most self-test kits will give a yes or no result based on whether the amount of biomarker detected in the sample is higher or lower than a clinically relevant cut-off (qualitative test).
What is meant by accuracy?
The accuracy of a test is calculated by working out the number of false positive and false negative results. Scientists and doctors are aware that all medical diagnostic tests, whether used in the laboratory, or at home with a self-test kit, will give a small number of false results.
Most manufacturers of self-test kits will demonstrate the performance of their test kit versus a hospital laboratory equivalent method, which can provide doctors with the reassurance they need that the test their patient has used is reliable.
For example, pregnancy tests used in the hospital lab and by self-test kits, measure the hormone hCG in urine. Pregnancy self-test kits often claim that they are more than 99% accurate. This means that if you measured 100 urine samples from women who may or may not be pregnant with the self-test, and compared this with a laboratory test that also measures hCG, then we would expect one sample to give a different result.
Nevertheless, neither the laboratory test, or the self-test, will tell whether the patient from which the urine sample was taken was pregnant - only that the sample contained raised levels of hCG, and that these raised levels of hCG are associated with pregnancy (there can be other reasons for raised levels of hCG).
The same often applies to other biomarkers, for example, prostate specific antigen (PSA), which is used as a biomarker of prostate cancer. It is possible for a self-test kit to claim that it is more than 99% accurate at detecting PSA, when compared with a corresponding laboratory testing system, but this claim could be misleading if you were not aware that PSA is more likely to be raised in conditions other than prostate cancer, such as prostatitis or some urine infections.
A more ‘sensitive’ test is not necessarily good news
Another question often asked is “How sensitive is the test?”. The implication being that the more sensitive the test, then the better it is.
In diagnostic terms, sensitivity means the likelihood of the test being able to detect samples that are known to be positive. Whereas the detection limit of the test being the lowest amount of biomarker that the test can reliably detect. Some tests may have different detection limits, or cut-offs, depending on the application.
For example, with bowel cancer diagnostics using the faecal immunochemical test (FIT) principle, two cut-offs may be used clinically, a higher cut-off for bowel screening, and a lower cut-off for use when bowel cancer is suspected from a review of the patient’s clinical history by their doctor. The cut-off for some self-test kits can also vary markedly depending on the specific country (population) that the test has been designed for.
Test kits that have been manufactured for one country may be overly 'sensitive' compared to tests used in another, or intended for diagnosis rather than screening.
A test designed for diagnosis with a very low cut-off, could produce more false positive results than a test designed for screening with a much higher cut-off, potentially causing unnecessary worry by the patient, and further pressure on the NHS with unnecessary referrals for gastroenterology.
Specificity is another term used in medical diagnostics, and means the likelihood of the test showing that samples from healthy people have a negative result. The overall accuracy is usually determined by combining the results for both sensitivity and specificity.
Responsibility of manufacturers and retailers of self-testing kits
Although the manufacturing and technology of medical diagnostics is heavily regulated in the UK and EU, manufacturers and retailers of self-test kits should show responsibility when it comes to the marketing and advertising of these products, taking care with claims of accuracy.
A medical test by itself is not a diagnosis and should only be interpreted together with the patient’s clinical history. Nevertheless, self-test kits can help foster better informed patients and prompt action so that an early diagnosis can be made.
Further information of the SELFCHECK range of self-test kits can be found at selfcheckmedical.com