DR SANJAY Zodpey is Vice President,PublicHealthFoundationofIndia(PHFI).A fellow Nagpurian was in city recently during which ‘The Hitavada’ talked to him on science oflaboratory testing.He madeitclearthatthe viewsheexpressed in the interview are his personal and are based on scientific literature.

 

A:If a persontestsnegative-this indicates that probably person did not have

COVID-19 atthe time when sample was collectedorwascollectedtooearly.Rarely this may also happens because of inadequate or inappropriate sample collection technique and transport. A person may also be exposed to COVID-19 after thetestandthengetinfectedandspread

the virus to others. Ifaperson develops symptoms later, then another test can

determine the infection is ofCOVID-19.

 

A: The antibody testing for COVID 19 is carried out to detect past infection. Antibody testing, also called as "serologic testing" or "serology", is typically performedona bloodsample. Antibody tests check for antibodies that appearin the blood between about one and four weeks after infection with the virus and may remain for long period of time. It is not yet known how long these antibodies protect against reinfection with the COVID-19 virus.

IthasbeensuggestedthatIgG AntibodytestforCOVID-19may be administered only for surveillance purposes and not for diagnosis. IgG antibodies generally start appearing after one to four weeks of infection, and last for several months.

Therefore, the IgG test is not useful for detecting current infection. However, detectionofIgGantibodiesforSARS-CoV2 may be useful to understand the proportion of population exposed to infection including asymptomatic individuals to plan appropriate public health interventions.

 

If a person tests positive by antibody test thenitmeanspersonmayhave antibodies from an infection with the virus that causes COVID-19. A person may testpositiveforantibodies even if he/she have neverhadsymptomsofCOVID-19,which happens because of asymptomatic infection. We also need to note that several people remain asymptomaticafterCOVID-19 infection.Havingantibodiesto the virus that causes COVID19 may provide protection. However, we do not know how much protection these antibodiesmayprovideorhowlong this protection may last.

 

A: If a person tests negative - he/she may not have ever had COVID-19 or could still haveacurrent infection. The test may be negative because it typically takes one to four weeks after infection for body to make antibodies.

Sometime, it is observed that if two samplesof samepatientare testedusing two different diagnostic tests or in two different labs or at two different times provide different test results.

A symptomatic or asymptomaticperson may test negative on antigen test and positive on RT PCR on sequential testing. This may happens due to low sensitivityofrapidantigendetectiontest and is already considered in the ICMR advisory that - suspected individuals who test negative for COVID-19 by rapid antigen test should be definitely tested sequentially by RT-PCR to rule out infection,whereas a positive test should be considered as a true positive and does not need reconfirmationbyRT-PCR test. As per scientific literature, standard

COVID-19Agrapidantigendetectiontest has a very high specificity (i.e. ability to

detect true negatives means who do not have disease). However, the sensitivity of the test (i.e. ability to detect true positives means who have the disease) is a bit low.

RTPCRis a gold standard test and has high sensitivity but asymptomatic person may still be reported negative because of sample related characteristics such as low viral titre, inappropriately collected sample, sample collected in very early stage of disease, sample collected from incorrect site,inadequate quantity of sample, or transported to laboratorybyincorrectmethodetc.Even if these sample related issues are addressed, still approximately 20 to 30% of diseased individuals may be tested negative by RT PCR test.

Therearescientificreasonswhysomeone tests positive or negative on a certain test at a given point in time. The health care providers are trained to understand these nuances and appropriately interpret test results and manage cases. (Concludes)