Covid-19 Antigen Testing in the Age of the Omicron: Understanding Reliability, Results and False Negatives | United States

Covid-19 tests that use lateral flow technology to quickly check for the presence of antigens (proteins on the outer envelope of the SARS-CoV-2 virus) have become the primary means of detecting infections in the variant era. omicron. In Spain, where the health system is under pressure from the sixth wave of coronavirus, authorities now recommend carrying out rapid tests and self-isolating in the event of a positive result.

Rapid antigen tests are not as accurate as PCRs, which are handled in the lab, but they have several advantages: they are faster, cheaper, can be done at home, and have been shown to be sensitive enough when the person has a load. viral higher and therefore more contagious (although the spread of the omicron variant complicates this picture).

Tests may produce two lines after the sample is collected and added in droplets to the kit. The first row is marked (C) for the control and the second is (T) for the test. The C line simply indicates that the test was performed correctly, while the appearance of a T line indicates that a viral load was detected in the sample. Whatever the erasure or erasure of this second line, if it appears the result is positive.

If a second line does not appear, it does not necessarily mean that there is no infection. A positive result is reliable enough to detect an infection, but a negative result may simply mean that the viral load is not yet sufficient to be detected and that the virus is incubating in the person’s body.

The omicron variant adds to the further complication that symptoms may appear earlier than a positive result. Following an outbreak in Norway, it was found that most of the infected people started showing signs of the disease (headache, body aches, sore throat) three to four days after contact. “The relationship between symptom onset and peak viral load has changed,” says epidemiologist Michael Mina. “This could be because of the characteristics of the variant or because it causes more re-infections and our bodies react differently to them.”

If you suspect an infection, it is advisable to do more than one test, as in the first few days the test will not detect the virus. If you have symptoms, even if the test is negative, you must take strict precautions, as you could be infected and infect others. A recently published small preliminary study concludes that “most cases of omicron were infectious for several days before being detectable by a rapid antigen test.” It is worth being careful. “If you are symptomatic and negative […] you have to be very, very careful. Isolate yourself if possible and get tested the next morning or the same evening, ”explains Mina.

The higher the viral load detected by the test, the darker and lighter the second line will appear. During the first and last days of infection, it will appear lighter and darker when the infection is at its peak.

In Spain, a seven-day isolation period is required upon onset of symptoms, or upon obtaining a positive result in the case of asymptomatic people. This should be followed regardless of whether the person suspects they are at an earlier or more advanced stage of infection, and whether the second line is faint or very clear.

If the test is performed at home, the patient should also notify the Spanish health authorities. If symptoms are mild, self-isolation is sufficient and can be completed seven days after self-diagnosis, provided the patient has been asymptomatic for three days. Otherwise, isolation should be extended to 10 days.

Authorities in the United States and Mexico recommend five days of isolation from the onset of symptoms or a positive result. This drops to seven days in Argentina and Colombia.

If symptoms persist or worsen (high fever, breathing difficulties), consult a doctor.

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