Two kinds of tests are available for COVID-19: viral tests and antibody tests.
- A viral test tells you if you have a current infection.
- An antibody test might tell you if you had a past infection.
A) Test for Current Infection (Viral Test)
1, Rapid Antigen Detection Test for COVID-19
There is urgent need of a reliable point-of-care rapid antigen detection test with good sensitivity and specificity for early detection of the disease.
- It reduces the burden of relying on just RT-PCR tests to identify COVID-19 patients.
“Antigen testing is useful because even if it’s less sensitive, it is rapid and the results that are positive will be positive “So, patients who test positive can get into isolation faster.”
- Antigen tests remove about half of the positives from the testing load.
- Antigen tests are also inexpensive compared to RT-PCR.
- An antigen test can only reveal whether a person is currently infected with SARSCoV2. Before or after the infection has passed, antigens won’t be present.
- Since antigen testing doesn’t involve any processes of amplifying the virus or its genetic material, a swab sample may have too little antigen to be detected. This could produce a false negative result. As a precaution, a negative test should be followed up by the more accurate RT-PCR test, to confirm a true negative for COVID-19.
- Accuracy is the single largest problem with antigen tests, which are much less sensitive than RT-PCR as a diagnostic tool.
Real Time RT-PCR is the Gold standard test for detecting cases of COVID-19. The advantage of this platform lies in its accuracy of detection.However, wherever available, it is advised to use real time RT-PCR as the front line test for diagnosis of SARS-CoV-2.
An RT-PCR test is considered very reliable because it can detect even a single virus particle in swabs taken from inside the mouth or nose, where the virus particles are most prevalent.
Advantages of RT-PCR testing
- Sensitive and specific: There are real-time variations of the RT–PCR technique that are quick, super sensitive, and specific. They can deliver a reliable diagnosis in three hours. That said, the process on average takes around eight hours to produce a conclusive result.
- More accurate, less prone to contamination: Compared to other methods to isolate and detect viruses, RT–PCR is much faster and less likely to be contaminated or cause errors during the testing process.
- Severity of infection can be estimated: A standard real-time RT–PCR set-up usually goes through 35 cycles, which means that, by the end of the process, around 35 billion new copies of the sections of viral DNA are created from each strand of the virus present in the sample.
Limitations of RT-PCR testing
Only detects ongoing infection: The main limitation of RT-PCR testing is that it cannot be used to detect a past infection with SARS-CoV-2, which is important for understanding the development and spread of the virus, as viruses are only present in the body for a specific window of time.
3,The TrueNat and CBNAAT systems
- These have also been deployed for diagnosis of COVID-19 in view of availability of customized cartridges. These platforms have a quick turnaround time (30-60 minutes) but only 1-4 samples can be tested in one run, limiting the maximum numbers that can be tested to 24-48 samples / day only.
- TrueNat is a chip-based, portable RT-PCR machine.The device is on its way to becoming one of the main diagnostic tools for COVID- 19 as India looks to expand its testing capacity. It is the fastest available PCR-based technique recommended by ICMR till date.
B) Test for Past Infection (Antibody Test)
IgG Antibody test for COVID-19 (Only for surveillance and not diagnosis):
IgG antibodies generally start appearing after two weeks of onset of infection, once the individual has recovered after infection and last for several months. Therefore, the IgG test is not useful for detecting acute infection. However, detection of IgG antibodies for SARS-CoV-2 may be useful in the following situations:
- Serosurveys to understand the proportion of population exposed to infection with SARS-CoV-2 including asymptomatic individuals. Depending upon the level of seroprevalence of infection, appropriate public health interventions can be planned and implemented for prevention and control of the disease. Periodic serosurveys are useful to guide the policy makers.
- Survey in high risk or vulnerable populations (health care workers, frontline workers, immunocompromised individuals, individuals in containment zones etc) to know who has been infected in the past and has now recovered.
Antibody testing for COVID-19 may be done if:
- You had symptoms of COVID-19 in the past but weren’t tested
- You’re about to have a medical procedure done in a hospital or clinic, especially if you’ve had a positive COVID-19 diagnostic test in the past
- You’ve had a COVID-19 infection in the past and want to donate plasma, a part of your blood that contains antibodies that can help treat others who have severe cases of COVID-19
Results of COVID-19 antibody tests may not always be accurate, especially if the test was done too soon after infection or the test quality is questionable.