Nepal’s Covid-19 tally on Sunday reached 38,561, with 1,221 new cases in the last 24 hours. With 14 more fatalities, the coronavirus death toll has soared to 221. Kathmandu Valley reported a record 429 new infections, with 372 cases in Kathmandu district, 45 in Bhaktapur and 12 in Lalitpur, according to the Ministry of Health. As Covid-19 patients in need of critical care also continue to rise, infectious disease experts and doctors say there is growing evidence of community transmission of the virus. Epidemiologists say the only way to confirm or rule out community transmission is through expanded testing and it needn’t be polymerase chain reaction tests. Serology or antibody tests can also provide a perspective about the scale of transmission and help the government and public health agencies chart an effective Covid-19 strategy, they say.

According to the World Health Organisation, serology tests would help investigate the ‘occurrence of infection in the population, how many people have mild or asymptomatic infection and who may not have been identified and the proportion of the population who may be protected against infection in the future’.

Here’s everything you need to know about the serology test in the context of Covid-19:

What is a serology test and why is it used?

A serology test examines the liquid part of the blood to detect antibodies made by the immune system in response to a threat, such as SARS-CoV-2 which causes Covid-19.

The test looks for either Immunoglobulin M (IgM) or Immunoglobulin G (IgG), the two antibodies developed by the body against the virus. The IgM is the first antibody that is developed by the immune system so it is detected on the test first, within a week or two followed by the IgG, which is detected on tests two weeks after the infection. While the IgM usually disappears from the blood within a few months after recovery, the IgG can be detected for years.

How is the test performed and what do the results mean?

A blood sample of a person is collected and sent to the laboratory for testing. A positive test would mean that the person was infected with SARS-CoV-2 in the past and their immune system developed antibodies as a defence mechanism. But in communities which have reported sporadic cases, experts say there is a higher chance that a positive test is a false positive. Similarly, in communities which have reported clusters of cases, there is a high chance that a positive test is a true positive.

If a test returns negative results, it would mean that the person may not have contracted Covid-19 in the past, but there remains a possibility that the antibody test was conducted too soon to test positive. This is the reason a serology test is not used to diagnose an active Covid-19 case but to determine if there was a past infection.

However, serology tests are both subject to sensitivity and specificity and may produce unreliable results while in some cases, tests could cross react with other coronaviruses that are not SARS-CoV-2, leading to false results.

How is the serology test helpful in the context of Covid-19?

Since a serology test can identify if a person had contracted Covid-19 in the past, regardless of the case being symptomatic or not, the presence of the IgM or IgG antibodies could help determine when the illness had occurred. A serology test at the community level could help the government and public health agencies understand what percentage of the population may have already contracted the disease.

A serology test could also help identify individuals who qualify to donate convalescent plasma, which is currently being tested as a Covid-19 treatment.

How can serology tests help the government and public agencies devise a strategy to break the chain of transmission?

Depending on the serology test results, the government and public health agencies can further investigate the antibody-positive individuals in line with the trace and treat principle, stressed by epidemiologists as the only way to break the chain of transmission and a polymerase chain reaction test can be performed on those who have been exposed to the antibody-positive individuals. The data from serology tests can also help public health agencies plan vaccine trials in the future.