Sensitivity of testing for SARS-COV-2 infection is not as great as you think: it may be only 32-72% compared with broncho-alveolar lavage obtained at bronchoscopy.

photo courtesy of Gerd Altmann (geralt) via pixabay.com

A comparative study of RT-PCR (reverse transcriptase-polymerase chain reaction) testing for COVID-19 found widely varying sensitivity for different test sites.  A total of 1070 tests were done in 205 patients.  Broncho-alveolar lavage specimens (obtained at bronchoscopy, a risky procedure in these cases) were positive in 93% (14/15) tests.  Here is a quote from the article:

Most of the patients presented with fever, dry cough, and fatigue; 19% of patients had severe illness. Bronchoalveolar lavage fluid specimens showed the highest positive rates (14 of 15; 93%), followed by sputum (72 of 104; 72%), nasal swabs (5 of 8; 63%), fibrobronchoscope brush biopsy (6 of 13; 46%), pharyngeal swabs (126 of 398; 32%), feces (44 of 153; 29%), and blood (3 of 307; 1%). None of the 72 urine specimens tested positive.

The article goes on to say that nasal swab specimens that were positive had the largest amounts of virus present.  While virus was found in feces and another article suggests that fecal-oral transmission might be an issue, attempts to isolate “live virus” in cell culture from feces were unsuccessful in yet another study (which I couldn’t find after seeing it a few days ago– there are already over a thousand studies on BioRxiv).

This result gives us cause for concern about official figures for the new virus and our policy of isolating people until they get negative test results.  Bayesian analysis (statistical decisions based on “prior probability”, or likelihood based on what you already know before testing) suggests that patients with cough, fever, and other consistent symptoms who have negative tests for influenza should be presumed to have the novel coronavirus even if an initial nasopharyngeal swab test is negative.  For example, noncontrast CT (computer-assisted tomography) could be done to detect the typical ground-glass opacities (areas of darkening) found in the lungs of COVID patients– this is what the Chinese did during their epidemic.

Detection of SARS-COV-2 in asymptomatic patients still depends on nasopharyngeal swabs.  Once the recently approved blood antibody test becomes widely available, we may discover that many more people have had asymptomatic or mild infections than we suspected.

Finally, “potent neutralizing antibodies” specific to SARS-COV-2 (not to SARS-COV) were found in this March 21 study, suggesting that infection with COVID-19 will result in immunity after recovery.  This is a good indication that people who survive the infection will be able to avoid re-infection and need not be further isolated.