What causes a false negative in the nucleic acid test for COVID-19?

What is a COVID-19 false negative? It means that the result of a certain test should actually be positive, but the result is negative due to reasons such as operation, equipment, personal physical characteristics, etc.

Positive results were obtained after repeated negative tests, throat swab samples were negative for multiple tests but finally positive results were detected in respiratory lavage fluid samples, nucleic acid tests were negative but CT imaging screening showed the characteristics of new coronavirus pneumonia.

A sample that should have been positive actually tested negative.

So, what causes a false-negative nucleic acid test for COVID-19?

• The problem of low viral load

The impact of sampling period: in the early stage of infection, after the patient gradually recovers, and patients with intermittent detoxification carry a low viral load, which may cause “false negatives” in the test.

The influence of sampling site: bronchoalveolar lavage fluid, deep sputum > nasal swab > pharyngeal swab The sampling depth and intensity all affect the viral load.

• Problems with detection reagents

The efficiency and purity of nucleic acid extraction, differences in the sensitivity of in vitro diagnostic reagents, and virus variability.

• Other reasons

The operation is not standardized enough, and the clinical laboratory is not standardized enough.

How to deal with “false negatives” in laboratory tests

1. The patient’s disease course

During different disease stages, the viral load in the patient’s body varies. For example, in the early stage of infection, the patient’s own viral load is low, which may result in a “false negative” test.

After the patient gradually recovered, the viral load gradually decreased, and the positive rate of nucleic acid test decreased accordingly. Faced with this situation, it is recommended to carry out multiple nucleic acid tests at different times, conduct regular nucleic acid tests for key populations, and make clinical observations and data summaries.

2. Sampling site

The collection of bronchoalveolar lavage fluid and deep sputum is better than nasopharyngeal swab, and the collection of nasal swab is better than oropharyngeal swab. In order to ensure the accuracy of the results, it is recommended to use the nasopharyngeal + oropharyngeal swab method, that is, to collect nasopharyngeal and oropharyngeal swabs at the same time for testing.

3. Sampling accuracy

The depth and intensity of the sampling should be sufficient. For the collection of nasopharyngeal and oropharyngeal swabs, it is necessary to obtain the deeper epithelial cells in order to detect the presence of the virus.

4. Detection of kit quality differences

There are some differences in the quality of test kits today. With joint efforts and continuous optimization and progress of technology, it will gradually improve. False negatives or false positives caused by reagent problems can be replaced by reagents without batch numbers or manufacturers for comparative testing.

5. Other methods

Suspected cases with a negative nucleic acid test for the new coronavirus can be tested for IgM antibodies as an indicator for supplementary diagnosis. (The new coronavirus IgG antibody test is also used to determine whether you have been infected)