Testing for Covid: How its done
NIP weighs in
28 July 2021 | Health
Following recent public outcry regarding false positives and negatives in Covid-19 testing Dr Iyaloo Konstantinus of the Namibia Institute of Pathology (NIP) shed some light on the matter.
“The reliability of diagnostic tests refers to their ability to identify positive and negative cases accurately,” says Konstantinus, adding that this is defined by two parameters: Sensitivity and specificity.
Sensitivity refers to the likelihood that a test can accurately test positive when a patient is indeed positive, while specificity refers to the likelihood that a test will accurately reveal negative cases.
Currently, polymerase chain reaction (PCR) testing is the most accurate test for Covid-19 but despite a specificity rate above 95%, there is still a small window in which false results can occur.
However, it is still more reliable than an antigen test which meets the World Health Organisation’s (WHO’s) minimum performance requirements of over 80% sensitivity and over 97% specificity.
“Antigen tests have a lower sensitivity compared to PCR tests and are therefore less reliable in detecting Covid-19 in patients with low viral load,” says Konstantinus.
Viral load refers to the quantity of a virus in a given amount of a sample.
According to Konstantinus, the possibility of a false result could depend on both pre-analytic (circumstances prior to testing such as during sample collection) or analytic performance.
“The pre-analytic factors during sample collection will depend on the amount of virus at each site. This means you can get swabbed at two different sites and one might come out negative, while one might come out positive and this usually depends on the stage of infection you are in,” he says.
Konstantinus says other factors that could affect sample analysis includes the duration of transport and storage conditions while the sample is in transit. These are all pre-analytic factors.
A notable analytic factor is the kind of PCR testing kit that is used. Some PCR kits have higher sensitivity due to the fact that they test for two or more genes, which makes them more accurate.
What to do if you suspect a false result
Konstantinus recommends that members of the public who have been tested using antigen tests opt for a PCR test if they suspect a false result, especially if they are symptomatic but tested negative with an antigen test.
Should a patient be symptomatic but have a negative PCR test, they can request nasopharyngeal and oropharyngeal testing, thereby increasing their chances of an accurate result.
“False positives are highly unlikely for PCR because of its high specificity above 95%, The RT-PCR test is highly specific. That is, if someone truly doesn’t have the infection, there is a high probability the test will come out negative,” Konstantinus says.
When to use which test
The different methods of Covid-19 testing could present false results at different stages of infection as the viral load increases or decreases.
“The PCR test detects the presence of small fragments of genetic material from the virus and hence it is useful at all stages of Covid-19 infection and sensitivity will range from when the body has low viral load to when the body has high viral load,” Konstantinus says.
According to him, antigen tests are more accurate when the patients viral load is higher, which is usually on to three days before symptom onset and in the early symptomatic phases or 5 to 7 days after infection.