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What are the different tests to detect COVID19 and why none of them are 100% accurate?

As we are struggling through this COVID19 pandemic and bracing ourselves for the peak, we all may have come across various tests for detecting the virus. Some of you may have got tested yourself and your friends or relatives may have got tested too. Let us know more about what tests are available, how they work and why multiple tests may be necessary sometimes to confirm the diagnosis.

RT-PCR test

This is the most commonly used test for diagnosing COVID19 worldwide. It has been named the gold standard by the WHO, CDC and ICMR.

Basically this test involves the collection of a swab (looks somewhat like a long ear bud) from the back of your throat and nose by a trained healthcare person. The sample collected is put into a sealed sterile container and carefully transported to a lab which has the necessary facility to perform the test.

This test identifies the genetic material of the virus from within your throat and nasal secretions. It requires fairly sophisticated equipment and skilled lab personnel to perform it. Thereby this is the most expensive among the available tests. It is being offered at low cost at government centers but may take few hours to 1-2 days for the report because the processing takes time especially if there are higher number of samples that need to be processed.

Just because it is labelled the "gold standard" does not mean it is the most accurate! Among the tests available this is the best one. At every step there could be factors at play that could compromise the results.

Firstly the test itself is not 100% sensitive. Research has shown that it has a 2-29% chance of showing a false negative report (testing negative inspite of actually having covid)

Secondly other factors could affect its accuracy:

  • Testing too early – Minimum 3-5 days are required for the virus to multiply in your body and appear in the secretions in a way that can be detected by the test. If you test too soon the test may be negative but you may still have the disease.

  • Testing too late – A person who may have recovered from COVID may still have inactive fragments of the virus in their nose and throat. Such a person may test positive but may not be actively infected.

  • Improper sample collection Faulty technique while collecting the sample may produce a negative result

  • Improper storage and transport of the sample – ideally the sample once collected should be kept dry and below 28 degree C. If not it could compromise the virus such that the test is unable to detect it and you will get a false negative report

So ideally if you have got yourself or your loved one tested by the RT PCR test for COVID19 and you have got a negative report, speak to your doctor. If you have strong symptoms, if you have come in contact with a positive person, if the doctor has a high suspicion after examining you or based on other tests or if you fall under the high risk category, continue to isolate yourself and take precautions. You may require a repeat test.

Rapid Antigen Test (RAT):

This is another test that is quickly becoming popular due to its speed, convenience and lower cost.

This also requires a swab to be collected from the back of the nose and throat. Instead of detecting genetic material, it detects certain proteins (antigens) which are present on the surface of the virus. (spike glycoprotein and the nucleocapsid protein). This involves less tedious processes and simpler equipment. It can be performed on site and does not need to be transported to a major lab

That is why it costs lesser. The report is typically available within 30-60 min.

But this test also comes with its limitations. Since it detects surface proteins of the virus, it could cross react with other proteins that have a similar structure and give a false positive result (test positive without actually having COVID).

It could also not detect the proteins if the viral load is too low or if it is too early in the course of the infection giving a false negative result.

This test result is usually confirmed by an RT PCR test.

So even with this test a clinical correlation is necessary. Your doctor will have to assess your symptoms and risk level and correlate with the test report to identify if you actually have the disease. It is not always a straightforward yes or no.

Antibody test for COVID

When you are infected with a virus or bacteria your body produces proteins called antibodies to fight against the infection. So this test detects the presence of these proteins in the bloodstream. It is a blood test and does not need a throat swab.

It is a good test for research purposes and to screen populations that are regularly exposed to the virus like healthcare workers, transportation workers, lab personnel etc to make decisions regarding quarantine and work hours but it is not useful to identify active illness among the general public.

CT scan of chest for COVID

A CT (computerised tomography) scan is a type of medical imaging which uses x rays taken from different angles to produce computerised cross sectional images of different organs of the body.

A simple X ray of the chest to check damage to the lungs is commonly advised to all patients of COVID. But a CT scan also has its place in making a diagnosis.

It has been found that the pneumonia caused by COVID19 results in certain specific changes in the lungs that can be visualised using a CT scan. Using this a scoring system has been developed which can grade the likelihood of covid infection in a person based on the CT scan images.

It is useful in cases where the doctor has a high suspicion but the RT PCR test shows variable results, it is delayed or not available. It could be an incredibly valuable tool to identify covid early in high risk populations.

CT scan findings are not to be taken just by themselves and need to be collaborated with clinical symptoms and signs.

Hope this article has enlightened you about the different tests available for COVID and the reasons why a single test cannot be relied upon in certain situations.

Stay safe, wear a mask and practice social distancing!

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