Coronavirus: Why An Infected Person May Test Negative In 40% Of Cases

A reverse transcription-polymerase chain reaction (RT-PCR) test is the only way to identify an infected COVID-19 person. Since it takes 24 to 36 hours to achieve results, the Indian Council for Medical Research (ICMR) recently introduced Chinese-made rapid tests kits which can produce results in 10 to 15 minutes. However, this exercise didn’t prove successful because these test kits showed erratic results.

A lot of patients have complained that their first RT-PCR test was negative but later on, when the COVID-19 symptoms persisted and when they went for the second test, it came positive. They blame the diagnostic labs for these inconsistencies.

Diagnostic labs, which ICMR has approved for COVID-19 tests, say that the RT-PCR test can only detect 65 out of 100 positive patients because it has its own limitation. A whole gamut of factors decides the accuracy of the test.

This also explains why many people who have tested negative initially become positive later on.

Experts say that the presence of the virus in per microlitre of nasal and throat swab plays the key role.

In the RT-PCR test, a paramedic takes the nasal and throat secretions of a person through the special swab. Viruses from these swabs are extracted and put in test tubes. Various reagents are added in tubes to identify the presence of the virus.

Many companies manufacture testing kits whose ability to read virus load varies. A test kit with 99 per cent accuracy fails to detect a viral load of 50 per microlitre or lesser than that. It means that if there are 50 viruses per microlitre in the swab of a person, such RT-PCR kits give a negative result.

“A person with less virus load can either be asymptomatic or shows very mild systems of COVID-19. In due course of time, when the load of virus increases with the decline of an individual’s immune system, he or she tests positive,” Dr Deepak Sadwani, Founder, Prognosis Lab, said.

Experts also say that besides virus load, a lot depends on the sample collection, sample handling and lab examination. In many districts in India, samples are transported to over 200 km to testing centres as every district doesn’t have testing facilities.

For instance, only Patna, Darbhanga and Muzaffarpur in Bihar have testing centres and samples from other far off districts are transported to these places.

Though ICMR has provided sample collection, packaging and transportation guidelines for COVID-19 yet a minor violation can vary the result.

“Many times, a paramedic who takes the secretions through a swab might not collect it from deep into the nose (nasopharynx)  or mouth closer to the throat (oropharynx) so that the sample will have a high virus load,” Dr Rajeev Ranjan from Lab Medicine in AIIMS, Delhi, said.

Paramedics in rural areas are not so much trained to collect the samples accurately and hence it leads to differences in result.

“While interpreting the results of a COVID-19 RT-PCR test, the referring physician must keep in mind the window period of the infection,” Dr Arjun Dang, CEO, Dr Dangs Lab, said.

For instance, according to the diagnostic test sensitivity, if a person is tested between one to seven days after the onset of symptoms, there are about 70 per cent chances of detection of infection. If the same test is conducted after 8 to 14 days after the onset of symptoms, the detection percentage goes down to 54 per cent.

Dr Dang says that labs should follow robust training programmes for sample collection officers and it should be done in such a way that it can maintain its integrity and ensure the highest accuracy.