Are the antigen tests reliable to detect omicron? Guide to avoid false negatives

After a Christmas marked by the omnipresence in the families of the antigen tests, one of the main doubts that arise, before the growing number of false negativ

Are the antigen tests reliable to detect omicron?
 Guide to avoid false negatives

After a Christmas marked by the omnipresence in the families of the antigen tests, one of the main doubts that arise, before the growing number of false negatives by Covid, is: Are the rapid tests reliable to detect the omicron variant?

The debate is growing and experts have not yet reached a definitive and secure conclusion. It seems that, in general, the antigen tests are capable of detecting omicron with the same error margin as any other variant, always considering that they have a slightly lower sensitivity than PCR. In practice, however, this margin of error implies a greater number of false negatives, with the danger of propagation that this entails.

There is a characteristic typical of omicron that could effectively affect these results: the growing evidence indicates that omicron accumulates mainly in the throat and not on the nose, and this could be the reason why the test-based tests taken in The nose sometimes do not detect the variant at first. However, at the moment it is only a hypothesis.

A recent empirical study followed 30 people probably exposed to omicron in the US, and found that the saliva tests could detect COVID cases three days before the antigen tests, which use nasal swabs. These results derive from a small study and have not been reviewed by peers, but confirm the suspicions of the FDA, the drug regulatory agency in the US, which at the end of December warned that, although it does detect ÓMICRON, the antigen tests can Have a "reduced sensitivity".

Another study conducted in South Africa (not yet revised) also shows that the antigen tests detected a higher rate of omicron infections when saliva samples were used instead of nasal samples.

What does this imply? Antigens tests, seeking a specific protein from the coronavirus, are very effective in confirming positive cases, but when the result is negative in some cases it could be incorrect. You have to be attentive to the symptoms and the risk contacts and, if any doubt, resort to the PCR or repeat the test. That said, the rapid tests can offer results in minutes and continue to be an important public health tool.

There are two other practical factors that could affect the results of the antigen tests: the moment the test is performed and how the sample is collected.

The antigen test (same as PCR) must be carried out from 48 hours after a contact considered of risk up to five days later, ideally, ideally, it would repeat it at 5 days. If the test is done too soon, it is in danger is to have a "false negative", because in the first two days the virus is not detectable by the tests, from 48 to 72 hours it is only detectable through PCR and from 72 hours as well by means of antigen test.

The second problem is who performs the collection of samples: Go to a laboratory minimizes the margins of false negatives. The Auto Test of Covid is quite risky because one may fail in the collection of samples.

To minimize false negatives, research shows that the antigen tests are highly accurate when they are performed sequentially: since the incubation period with omicron seems to be shorter, the faster tests are reduced the possibilities of A false negative.

The Council is to make a first quick test and then wait from 24 to 36 hours for a second test. However, in case of symptoms, the test can be performed immediately.

Updated Date: 10 January 2022, 05:17

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