Covid-19 Pandemic: Part 3: Understanding the Diagnostic Tests and What Do the Results Mean

Published in December 2021

In this paper we discussed in detail about the diagnostic tests and what the results tell us about the disease. Interpretation of the results of the tests and limitations and strength of the tests are discussed. In what situation is recommended and should we go for is also discussed with explanation.

We discussed historical context of Covid-19 pandemic, followed by the signs, symptoms and how it affects our body. In this month’s edition we discuss what are various types of laboratory tests and what is the meaning of the test being positive or negative.

Diagnostic Laboratory Tests for Covid-19

We may be advised by our doctor or we may choose to get tested for Covid-19 infection, for various reasons. We may also be advised to go for Covid-19 tests when we are planning to travel and the destination center’s demand a Covid-19 negative tests before we are allowed to that area.

The selection and interpretation of Covid-19 tests should be based on the context in which they are being used, including the prevalence of Covid-19 in the population being tested.
Vaccination status does not affect the results of viral testing for Covid-19
This guidance has been developed based on what is currently known and is subject to change as additional information becomes available.

There are three types of tests for covid-19 two of which diagnose the current infection whether you have at present and the third one diagnoses whether you had infection in the past sometimes.

Diagnostic Tests for Current Infection and Its Interpretation (See the Chart)

For the current infection diagnosis, there are two types of tests and they are called viral tests. They are popularly known as NAAT and antigen tests.

NAAT Tests: This is the most sensitive and reliable test and most of us know this as RT-PCR tests but RT-PCR is one of the NAAT tests and there are other versions using the same technology explained below. RT-PCR stands for Reverse Transcription – Polymerase Chain Reaction.

A Nucleic Acid Amplification Test, or NAAT, is a type of viral diagnostic test for SARS-CoV-2, the virus that causes COVID-19. NAATs detect genetic material (nucleic acids). NAATs for SARS-CoV-2 specifically identify the RNA (ribonucleic acid) sequences that comprise the genetic material of the virus. Specimen is collected from the posterior part of the nose by a swab either by the professional health worker or by the person herself or himself under the supervision of health worker. Saliva can also be used for NAAT test. This is non invasive test and there is no needle involved in the testing.

The NAAT procedure works by first amplifying – or making many copies of – the virus’s genetic material, if any is present in a person’s specimen. Amplifying those nucleic acids enables NAATs to detect very small amounts of Covid Viruses in a specimen, making these tests highly sensitive for diagnosing COVID-19. In other words, NAATs can reliably detect small amounts of SARS-CoV-2 and are very less likely to return a false-negative result of Covid-19 infection.

Interpretation RT PCR tests results

Overall, this is the most sensitive and reliable tests but takes relatively longer time from few hours to few days depending upon the tests load and the facilities doing the tests.

This test is a qualitative test largely. It tells you whether you are currently infected or not. Positive test means you have Covid-19 infection present. False negative tests are rare but can happen. Therefore, test should be interpreted along with signs and symptoms. As such this test is considered Gold Standard test. But if the test is negative, it indicates at the time of the tests you did not have active infection. You can develop infection few days later and so the test should be interpreted in light of the symptoms and signs that you have. With signs and symptoms developing, one must repeat the test.

Antigen Tests: Antigen tests are immunoassays that detect the presence of a specific viral antigen or outer protein present on the virus surface. This is a non-invasive test and does not need a needle prick. This is most widely used test for screening.

Antigen tests diagnose absence of the disease equally well as RT-PCR test (specificity of the test), but are slightly less efficient in detecting infection as compared to RT-PCR. Most are less expensive than RT-PCR and can be processed at the point of care with results available in minutes and thus can be used in screening programs to quickly identify those who are likely to be contagious. Because of the performance characteristics of antigen tests, it may be necessary to confirm some antigen test results (a negative test in persons with symptoms or a positive test in persons without symptoms) with a laboratory-based RT-PCR

Interpretation Antigen Tests Results

This is best used for screening or when the results are needed immediately like attending a meeting or large get together and criteria of negative test are set to attend the meeting or entry to a hall etc. Interpretation is similar like RT-PCR, that means, if the test is positive, person has infection, and if test is negative the person does not have Covid-19 infection. However sometimes the test may be false negative (report negative despite person having infection).

Quick guiding points are shown in the picture based on whether your tests are positive or negative

Antibody Tests:

Antibody (also called serology) tests are used to detect previous infection with Covid-19 virus. This test needs a needle prick since blood is required for the test. CDC does not recommend using antibody testing to diagnose current infection nor the past infection with certainty. Depending on the time when someone was infected and the timing of the test, the test might not detect antibodies in someone with a current infection. In addition, it is not currently known whether a positive antibody test result indicates immunity against Covid-19 infection; therefore, at this time, antibody tests should not be used to determine if an individual is immune against reinfection. Antibody testing is being used for public health surveillance and epidemiologic purposes. Because antibody tests can have different targets on the virus, specific tests might be needed to assess for antibodies originating from past infection versus those from vaccination.

Interpretation of the antibody rest result: At public health level, by the large number of these tests, what proportion of population is already infected sometimes in the past can be reasonably well estimated. It does not have personal use interpretation of any great value at present.

Coming Up

In the coming months ahead, we will discuss how to protect ourselves and prevent the spread of the disease, social and economic impacts in the societies, various vaccines and their doses, myths prevailing against vaccine, how best to protect us, including vaccinations and lessons learnt for the future one by one.