Debunking Myths on COVID-19 Testing

Science, experience, and learnings pertaining to COVID-19 test is evolving at a global level. From what we have already learnt at our COVID Lab at Metropolis, we present to you this useful information which may help you understand the nuances of the test.

Is it possible for a person to test positive without having symptoms at all?

Yes, it is possible. Globally, it has been observed that few patients remain asymptomatic and yet test positive.

A positive result means that these individuals are silent carriers of the infection. A recent ICMR study said that 80% cases tested positive in India are asymptomatic.

If your result comes positive, as per protocol your result is notified to the ICMR and the state health authorities. It is important not to panic because of a positive result and follow the guidelines as advised by the authorities.

How reliable is a POSITIVE test result?

Positive results are considerably reliable based on following criteria:

1. The RT PCR Test involves two parts; a screening test as well as confirmatory test before the patient is declared “POSITIVE”. This acts as a double check on the final result that is given to you.
2. During testing, multiple genes are detected for a patient to be labelled “POSITIVE”.

What is a false-negative result and why does it happen?

Currently, the RT PCR is the best test available to test for COVID 19. At times, there is a possibility of having a false-negative result. A false negative result means a patient’s test result is negative even though the patient has the virus in the body.

A false negative result may happen when a patient undergoes testing very early in the infection, that is within one to two days of infection. A repeat test after 2-3 days may become positive.

A false negative result may be reported if the sample is not collected properly. A nasopharyngeal swab is collected from deep inside the nose and an oropharyngeal swab is collected from deep inside the throat. Improper collection leads to not having enough virus in the sample for it to be detected during testing.

Another reason for a false-negative result is when the sample is not stored or transported properly. The RT PCR test involves detection of the Viral RNA. If sample is not transported or stored properly, the RNA in the virus tends to get inactivated or damaged and is undetectable.

The samples need to be transported in a special viral transport medium and at Metropolis, we ensure that sample integrity is maintained by following proper protocols during sample collection, storage, and transport.

Is it possible that a patient is tested POSITIVE at one lab and subsequently tested NEGATIVE in another lab?

This is very much possible and below is the explanation for the same

If testing is done few days apart with fresh samples taken at each lab: When you test for COVID 19 at one lab on a particular day and when you undertake the test at another lab after a few days, chances of a different result is very high because virus in the body gets reduced day by day. Patients who have good immunity clear the virus faster and may test negative within the 4th or 5th day. Studies show that virus can get cleared as early as 4.3 days of infection.2 This is also the reason why some people have no symptoms and still test positive.

If same sample is tested at two labs: If the same sample is tested at two different labs, the results in an ideal scenario should match. However, if the results do not match, there could be multiple reasons. Different labs use different kits, and this has a direct consequence on the test result. At Metropolis, we use kits, subject to availability, that were found to be 100% concordant in true negative and true positives, in study done by ICMR. Also, if the sample is not transported properly from one lab to another, the virus may get inactivated leading to a difference in the result.

If new sample is taken on same day: Suppose you have a negative result from one lab and if a fresh sample is taken and tested at another lab on the same day, the results should match. If the results do not match; the reason could be the use of different kits and if the sample collection is not done correctly as explained above.

Is it possible that a patient is tested NEGATIVE at one lab and then tested POSITIVE at another lab?

1. If a patient gets tested too early (< 2 days of infection), there is chance of the first test being false negative. On repeat testing after a few days, the patient may be reported positive.
2. Improperly collected sample as explained above may result in false – negative and a repeat test may be positive.
3. The RT PCR test involves detection of viral RNA. The RNA particle is extremely labile which means that it disintegrates and gets inactivated if sample is not stored or transported correctly. This may lead to a difference in results.
4. If different type of sample is used for test. RT PCR test can be performed on different types of samples: nasopharyngeal sample, oropharyngeal sample, saliva, or sputum. All these samples have different sensitivity levels. So, if a nasopharyngeal sample was tested negative the first time, if the same person gives a sputum sample in the second test, there will be a difference in test result. In India, testing is not allowed on sputum sample at present.

How does one know that he/she is free from virus?

If you test negative twice 24 hours apart, as per current guidelines, you may be discharged from patient care. However, there are exceptions cited in literature and patient may still shed virus in stool for 4 to 5 weeks after infection. 6 That is why it is important to continue following the safety precautions and guidelines.

References:

• Yang et al. Evaluating the accuracy of different respiratory specimens in the laboratory diagnosis and monitoring the viral shedding of 2019-nCoV infections
• Correlation of Chest CT and RT-PCR Testing in Coronavirus Disease 2019 (COVID-19) in China: A Report of 1014 Cases.
• Kelvin et al. Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study.
• Carl et al. SARS COV 2 viral load and severity of COVID-19
• ICMR revised testing strategy for Covid19 in India, 20 March 2020.
• Yongjian et al. Prolonged presence of SARS-CoV-2 viral RNA in faecal samples
• MCGM, Revised guidelines for COVID-19 testing