Public health experts and epidemiologists are puzzled by the current obsession among administrators as well as a huge section of doctors about the ‘rapid tests’ for COVID 19, which are not a primary diagnostic modality but are useful largely for epidemiological investigations.
The terminology ‘rapid test’ essentially means that the test process is fast, simple, not lab-dependent and the results are rapid. It, however, cannot rapidly detect COVID-19 as soon as one contracts the infection.
“In the middle of an epidemic, when what is required is early diagnosis and treatment, the rapid test has no role. It only results in unnecessary testing and rakes in money for private laboratories,” a public health expert points out.
Detection process
Infectious disease diagnosis relies upon the detection of a pathogen protein (antigen) by ELISA methods or its nucleic acid by Nucleic Acid Tests (like PCR). If these tests are not available, then one can make the diagnosis by looking for antibodies produced by the body against the pathogen as an immune response.
Antigen or nucleic acid is present in the body from the very beginning of a disease whereas the antibodies are produced only after a week of onset of the disease symptoms. Thus PCR tests are preferred for early detection of the disease. Antibody-based rapid tests, on the other hand, can detect the disease only much later.
Repeat test
A person with a possible exposure to COVID-19 will test negative if the test is done just two days after the symptoms start. Thus a repeat test is needed after a week, when the body would have produced enough antibodies, to ascertain if the person actually has the infection.
“We know that 80% of COVID-19 patients have only mild symptoms and that many could be asymptomatic. But they could be spreading the infection in the community. So wherever there is a cluster of severe acute respiratory infection (SARI), the antibody-based rapid tests are useful to check if the community has had pre-exposure to SARS-CoV-2,” he explains.
“In case of some diseases like dengue, both antigen detection and nucleic acid detection tests are available. The rapid test for dengue allows rapid detection on the first day of fever, before antibodies appear five or more days later. However, for COVID-19 such rapid antigen detection tests are not available,” E. Sreekumar, a senior scientist at RGCB explains.
In fact, the excitement over the introduction of rapid tests, as though it will change the epidemic scenario in Kerala is not warranted because it cannot help in early detection of COVID-19 and it costs more than PCR test. The cost of PCR test is ₹1,200 per test, while that of antibody-based rapid tests is ₹1,500.
Not available here
On the practical side, none of these antibody-based rapid test kits are available in India now. Given the current situation of lockdowns across nations and restrictions in international travel, none of these are expected to reach India any time soon, says Dr. Sreekumar.
Public health experts also point out that only a small fraction of the PCR test capacity available in the State is being utilised now. Due to the nationwide demand, the PCR kits available at the State’s disposal is limited and these are used judiciously to test only suspect cases with mild symptoms.
