India’s fight against the pandemic started on a commendable note. The government responded early by issuing travel advisories, conducting screening at airports, imposing lockdowns, and putting in place a series of testing strategies devised by the Indian Council of Medical Research (ICMR) based on the progression of the disease in the population.
The use of rapid antigen PoC
A recent development in the testing strategy devised by ICMR was the adoption of rapid antigen point-of-care (PoC) tests, which provide test results within 30 minutes. Delhi has used this in substantial numbers, as is evident from its bulletin reports, to improve the testing numbers, in the last three weeks. And Delhi has shown a steady reduction in the daily number of confirmed cases (from 3,788 on June 24 to 1,781 on July 11).
Also read | COVID-19 tests in India cross one-crore mark
SD Biosensor, which manufactures these kits, has the Food and Drug Administration’s approval dated April 23 for the Standard M nCoV Real-Time detection kit (a RT-PCR kit) but not for the Standard Q COVID-19 Ag (rapid antigen PoC test) which ICMR has approved based on independent validations by AIIMS and itself. The rapid kits are to be used across India in containment zones, hospitals and labs under the NABL. The manufacturer claims sensitivity (ability of a test to correctly identify those with the disease) of 84.4% and specificity (ability of the test to correctly identify those without the disease) of 100%. The test procedure requires a nasopharyngeal sample (taken from a part behind the nasal cavity which forms the upper throat). The kit needs to be maintained at a temperature of 2°C- 30°C for optimal results.
The negative results reported by these rapid antigen kits in Delhi need to be interpreted with caution. Individuals need training and expertise in collecting nasopharyngeal samples, else the swab will only collect nasal secretions. Temperatures in Delhi now average 40°C-45°C; hence the cold chain used for transporting and storing these kits in the community needs to be monitored closely. Low sensitivity could be worsened further by these factors and pave the way for ‘false negatives’. Although ICMR has said that all rapid antigen test negatives should be re-tested in RT-PCR, the Delhi bulletin report doesn’t show how many of these negatives have been tested again. The ideal sensitivity and specificity in community settings can be estimated if there’s data showing how many of these negatives turned out to be positive in RT-PCR. Additionally, if these kits were used in low prevalence settings, the validation parameters (sensitivity and specificity) get disturbed by Bayesian laws.
Details in break-up
Delhi should start giving a break-up of these test numbers so that a scientific conclusion can be made on the success of these kits. Else it could be possible that ‘false negatives’ continue to spread the virus unknowingly. The break-up could include a) symptomatic/asymptomatic tested; b) RT-PCR/rapid antigen test used; c) first test/second test of the individual. Since re-tests are mandatory for all rapid test negatives, it’s possible that the same individual could be counted twice — once on the day of rapid testing and then on the day of RT-PCR.
Data | Are States testing enough compared to the size of their coronavirus outbreaks?
Positivity rate is a metric that measures the adequacy of testing capacity in a given region. According to the World Health Organization, if the positivity rate is less than 5% for 14 days, the region can be classified as having good testing. Taken on face value, Delhi’s epidemic appears to have been controlled very well (positivity rate is now below 10%). However it’s important to get the break-up of these numbers to understand the reality behind the scenes.
The COVID-19 fight would require real-time data reporting for insights. It also requires synchronisation of data between communities, laboratories and hospitals to plan and prioritise resources. Data collection, analysis and interpretation can have layers of bias. Hence, monitoring the process during this phase of the epidemic is of paramount importance.
Dr. Giridara Gopal, M.D., is pursuing a Ph.D. and working as a research scientist in AIIMS
