False negatives — when a person carrying a pathogen tests negative — have been reported several times during the Covid-19 pandemic. These results have come up during nasal swab testing of patients who have seemingly recovered from the disease — but have later been found to be still carrying the virus.
Now, researchers have reported that testing of oropharyngeal secretions — secretions from the part of the throat at the back of the mouth — may reduce the number of false negative results. They have published their findings in the Journal of Dental Research.
The study was led by Jingzhi Ma of Tongji Medical College, Wuhan. A small number of patients who had tested negative through nasal swabs were found to be positive through the testing of oropharyngeal secretions. The study included 75 ready-for-discharge Covid-19 patients who tested negative using nasal swabs. Because of detection of potential false-negatives in that cohort, a second study paired oropharyngeal and nasopharyngeal samples collected from 50 additional Covid-19 recruits during their recovery stage.
Oropharyngeal secretions obtained from 2 of the 75 subjects in the first study yielded positive results for SARS-CoV-2. In the second study, oropharyngeal samples missed only 14% of positive cases, compared with 59% for the nasal samples.
Sampling of oropharyngeal secretions is a simple procedure that can be performed in any quarantine setting. It minimises contact between healthcare workers and patients, thereby reducing the risk of virus transmission, the researchers said.
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“The NPS test has a risk of sending home more patients who still have the infection while the OS test will make such errors in fewer patients. Although OS sampling improves the accuracy of SARS-CoV-2 nucleic acid testing, it must be emphasised that this conclusion is based on a very small sample size,” Ma said in a statement.
Source: International Association for Dental Research
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