How Covid-19 is diagnosed?

Lab-based investigations are usually used to confirm the presence of COVID-19. The WHO recommends collecting upper respiratory specimens: nasopharyngeal and/or oropharyngeal swab; lower respiratory specimens: sputum or endotracheal aspirate or bronchoalveolar lavage. These samples are then tested by nucleic acid amplification tests such as reverse-transcription polymerase chain reaction. These tests detect a unique sequence of viral RNA (in this case coronavirus RNA). One or more negative result does not rule the possibility of COVID-19. If there is a high suspicion of COVID-19 in the patient, then a sample is recollected (if the earlier sample was collected from the upper respiratory tract then a new sample will be collected from the lower respiratory tract) and tested again.

Blood investigations

Blood investigations are usually performed when nucleic acid amplification tests are negative but there is a strong epidemiological link to COVID-19 infection. These investigations can support the diagnosis of COVID-19 as well. One type of serological test is the rapid antigen identification test. If the antigen is present in sufficient quantity in the sample, it will bind to the antibodies present in the test and produce a visually detectable signal. The other type of serological test is based on the detection of the host antibody. This test involves detecting antibodies in the blood of people that may have been infected with COVID-19 usually via RT-PCR.

Imaging

Typically, x-ray and chest CT imaging techniques are used in COVID-19 cases. Chest radiographs of COVID-19 patients reveal bilateral air-space consolidation. During the early phase of the disease, chest radiographs may be unremarkable in some patients. Chest CT images from COVID-19 cases show bilateral, peripheral ground-glass opacities. The chest CT pattern seen in COVID-19 cases is similar to that seen in some other respiratory diseases. Some studies have also revealed that chest CT abnormalities were presented in a few patients before the detection of SARS-CoV-2-RNA. Therefore, chest x-ray or chest CT alone should not be used for confirming the diagnosis of COVID-19. However, imaging tests can prove a useful tool in the follow-up of COVID-19 cases for analyzing recovery.

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https://www.who.int/emergencies/diseases/novel-coronavirus-2019/technical-guidance/laboratory-guidance

https://www.mohfw.gov.in/pdf/NotificationofICMguidelinesforCOVID19testinginprivatelaboratoriesiIndia.pdf