The ‘RAT’tling effect of Rapid Antigen Test
By Sambit Dash
If you have to make a choice between two diagnostic processes — one that is time consuming, more expensive, requiring specialised workers and machinery and the other that is quick, less expensive, not requiring specialists and machinery — which one would you prefer? Well, the answer is not straightforward. It depends on several factors such as purpose, sensitivity and specificity of the diagnostic test. The two diagnostic tests are the reverse transcription polymerase chain reaction (RT-PCR) and rapid antigen test (RAT) that have become household terms in this pandemic. Of late, States have increased their reliance on RAT to ramp up testing, with Odisha conducting 90 per cent of these tests. While RAT has specific purpose, increased reliance on this test is misplaced at this stage of the pandemic. It is essential that testing should adopt the gold standard RT-PCR.
Since testing is the cornerstone of tackling Covid-19 pandemic, it is important to adopt testing methods based on sound science and clear policy. It is essential to understand that no test is 100 per cent accurate and depending on sensitivity and specificity, diagnostic tests may throw up false negative and false positive results. In simple terms, a person may be carrying the coronavirus but could test negative, or may not be carrying the virus and yet test positive. In RT-PCR, the chances of such false results are low but in RAT, they are quite high. This is well known and ICMR, which has been guiding testing protocol, in its recent guideline on September 4 had stated that symptomatic individuals who test negative on RAT are to be definitely tested by RT-PCR while asymptomatic, who test negative, are to be followed up if they turn symptomatic and the test must be repeated.
RAT replacing RT-PCR
Herein lies the problem. The Union Health Ministry was concerned over many States not retesting symptomatic individuals whose RAT showed negative. Why is this happening? In the earlier days of the pandemic, when biotechnology companies had not developed these rapid antigen tests, RT-PCR was the sole and reliable testing method. The worldwide chorus on increasing the number of testings grew, and even though much capacity development in RT-PCR occurred, it did not achieve the satisfactory scale. Then came the mass produced (mainly by South Korean SD BioSensor), less expensive, not requiring any of the high capital, labour-intensive input rapid antigen tests. The only hitch was that RATs have pretty low sensitivity. So the two dots, demand to increase testing and the availability of RAT, joined. Low hanging fruits are easy to pluck after all.
Low sensitivity of RAT
It is important to understand what purpose RATs can serve. The biggest advantage of RAT is quick turnaround time (TAT) and this feature can be used, as has been the case already in many offices and institutions in the western countries, to allow entry, get access to laboratories, hospitals and other areas where there are large number of people. However, this too is not as simple as it sounds and certainly not as convincing. The low sensitivity of RAT means it could throw up false negatives which, in turn, means that even if people are tested by RAT, they may be still be carrying the virus. It is argued that even if the false positives are low in RAT, applying it to large populations, and in regular intervals, will lead to a large number of false positives anyway.
Demanding right strategies
At a time when India is reporting more than one lakh cases per day, and we are not on the downhill of the curve, and in an environment when there are widespread reports of ICU beds being saturated, the overdependence on RAT may not be an ideal testing strategy. There is still the need for the more accurate diagnosis by investing in RT-PCR facilities, which needs supply of chemicals, establishing and maintenance of machines, training and employing personnel; all of which need increased investment and importantly, clarity from the Government vis-à-vis the policy on pricing, rules for testing, etc.
There must also be the constant demand to split RT-PCR and RAT since combing these two very different strategies swell the number of tests making it look good, but do not help in putting in place the public health strategies.
As science advances, there will be specificity of RAT with further decrease in TAT. In the coming days they may become cheaper. These tests will also become the mainstay as phase-wise reopening of different sector occurs. However, currently, RAT may not be the best armour in battling the pandemic.
__ The author is Senior Grade Lecturer, Department of Biochemistry, Melaka Manipal Medical College (Manipal Campus). He comments on public policy, healthcare and issues of social interest. He tweets at @sambit_dash