Population exposure to virus, time of conducting rapid test important factors, say experts
The Indian Council of Medical Research (ICMR) has put rapid-antibody test kits for novel coronavirus (SARS-CoV-2) on hold. However, sources in the organisation and independent experts said it was better to wait for field validation before terming rapid antibody test kits ‘faulty’ on April 21, 2020.
Seven lakh such kits have been procured by the ICMR. The first lot arrived five days ago.
“Based on a complaint regarding low detection, we asked three states, based on which we have come to know that there is significant variation in detection of RT-PCR positive cases, varying from 6-71 per cent,” ICMR’s epidemiology head R Gangakhedkar said at a press conference on April 21.
RT-PCR are molecular tests that have been used since the beginning if the novel coronavirus disease (COVID-19) pandemic to detect the presence of SARS-CoV-2.
On the other hand, the antibody test detects the presence of ‘antibodies’, tools that the body's immune system forms after 7-14 days of onset of disease usually.
Hence, the presence of antibodies would indicate that one had already been exposed to the virus and developed tools to fight it.
“Since the infection is new, we recognise that we have to refine our approach as we proceed. However, we have decided that these findings regarding variation in the detection of positive COVID-19 cases in RT-PCR samples cannot be ignored,” Gangakhedkar said.
“Variation means that while RT-PCR gave a positive result on a patient, the rapid test gave negative,” Lokesh Sharma, a senior scientist in ICMR, told Down To Earth (DTE).
“However, this is not black and white. Antibodies do not develop before 7-14 days. If somebody has performed a rapid test before that, the result would obviously be negative,” he said.
“Similarly, we have to check that when the test was performed, there was enough time for enough antibodies to be made in the blood. Only two drops of blood are extracted for the test. If the quantity of antibodies is not enough, then again one would have a negative test,” he added.
Teams from eight institutes of ICMR will check on the field as to how the rapid tests are performing. “Besides the factors that I highlighted before; we will also see if the tests were conducted in areas where the exposure to the virus was high. Else, you may not get accurate results as well,” Sharma said.
To verify whether these precautions were taken, DTE reached out to the head of the microbiology department at Jaipur’s Sawai Man Singh Medical College, Nitya Vyas.
It was Vyas’ team that had reported that rapid tests were giving only 5.4 per cent accurate results, as quoted by Rajasthan’s health minister Raghu Sharma. However, she declined to comment, stating it was an official report submitted to the department and she could not divulge the details.
Importantly though, the model of these kits had been validated by ICMR before the order was placed. As many as 75 such kits had been approved, according to an updated list of ICMR on April 17, 2020.
The two companies whose kits are under question, also have CE certification provided by the European Union.
Field versus lab validation of kits
“It is one thing that ICMR validates kits in labs and it is a completely different thing that they are validated on the field. A good result in the former settings is no guarantee of good results in the latter,” Giridhar Babu, head of epidemiology at Indian Institute of Public Health, Bengaluru, told DTE.
“Once the kits go in the community, there are several factors which come into play like duration of onset of disease, level of exposure to the virus and others,” Babu added.
But was it not possible for the ICMR to do a field validation before supplying the kits to states? “The government is under enormous pressure to perform and perform quickly. Therefore, it might have thought it was not prudent to invest time in filed validation,” Babu said.
“You judge how good or bad a test is by something called predictive value of test. A bad test will have a negative predictive value. It also depends on two factors — sero-prevalance; and specificity and sensitivity value of the test,” a senior scientist associated with a prominent research institute of the Union Ministry of Health and Family Welfare, said.
“If the sero-prevalence in a population, that is proportion of population exposed to the virus, is less than 10 per cent and if these kits are deployed there, then you would be getting the accurate results that you would be wanting too,” the scientist added.
Specificity refers to the ability of the kit to not give false negatives and sensitivity refers to ability of a kit to not miss out a positive case.
Gangakhedkar had been clear from the start that these tests were not to be used for diagnostic purposes, or to confirm a COVID case. They were to be used only to understand the surveillance of the disease.
“We have to be very sure that kits are actually faulty before going back to the companies and asking them to take back their kits. So, we should wait for two days before giving any final verdict,” Sharma said.
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