Why some PCR Tests give mixed results

The COVID-19 reverse transcription polymerase chain reaction (RT-PCR) test is conducted for the qualitative detection of nucleic acid from SARS-CoV-2 in upper and lower respiratory specimens (such as nasopharyngeal or oropharyngeal swabs, sputum, lower respiratory tract aspirates, bronchoalveolar lavage, and nasopharyngeal wash/aspirate, nasal aspirate, nasal swabs, or mid-turbinate swabs) from individuals suspected of COVID-19.

However there were several incidents where there were both positive and negative results for the same person such as the latest incident reported from Jinthupitiya. Explainaing this behaviour of tests, Dr. Sudath Samaraweera, Chief Epidemiologist at the Epidemiology Unit, Ministry of Health said that any test has its margins. “These margins are sensitivity and specificity. Therefore if 100 people are tested around 10 people who are positive may test negative and vice versa. This is how tests behave. If a positive person is tested negative this means that the test is not sensitive. If a person without the disease tests positive then the specificity is not good. This happens only in a handful of cases. So far around 150,000 PCR tests have been done,” said Dr. Samaraweera. 

While more patients are recovering the public has been advised to wear facemasks to avoid contracting the virus. When asked about people with respiratory illnesses Dr. Samaraweera said that they can remove the masks if they find it difficult to breathe. “But people should wear it properly whenever possible as an important safety precaution,” the doctor said,