How Does How Accurate is Sars-cov-2 Antibody Igg Work?

Background

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COVID-19, caused by the SARS-CoV-2 virus, spreads easily among individuals, similar to the common cold or flu. While most individuals experience mild-to-moderate respiratory illness, some may remain asymptomatic. Others may develop severe symptoms requiring specialist treatment and intensive care.

The immune system responds to COVID-19 infection by producing antibodies—proteins that attack the virus present in the bloodstream. Vaccinated individuals also produce these antibodies. Various tests are available to detect antibodies in people's blood, which may indicate current or past infection with COVID-19 or vaccination status. This review primarily focuses on tests identifying past infections.

Importance of Accurate Testing

Accurate testing is vital for identifying individuals who need to isolate to prevent the spread of the virus or might require treatment. False negatives, or undetected COVID-19 cases, can delay necessary treatments, allowing further virus transmission. Conversely, false positives can lead to unnecessary tests, treatments, isolation, and distress. Recognizing individuals who have previously contracted COVID-19 is essential for understanding disease spread and evaluating public health measures.

To assess the accuracy of antibody tests in identifying COVID-19, results are compared from individuals confirmed to have had COVID-19 against those who have not. The criteria to ascertain COVID-19 status is termed the “reference standard.” Many studies utilize reverse transcriptase polymerase chain reaction (RT-PCR) as a reference standard, taking samples from the nose and throat. Alternative methods may include symptom evaluations and imaging tests, such as chest X-rays. Sometimes, individuals without COVID-19 are identified through archived blood samples taken before the pandemic or from patients with confirmed non-COVID-19 diseases.

Aims of the Review

This review aimed to determine whether antibody tests:

• Can effectively diagnose infections in symptomatic and asymptomatic individuals.
• Are suitable for confirming past COVID-19 infections.

The studies included evaluated three antibody types: IgG, IgM, and sometimes IgA. Most tests measure IgG and IgM, while some assess individual types or combinations of all three.

Review Methodology

We searched for studies measuring the accuracy of antibody tests for current or past COVID-19 infections, comparing them to reference standards. Given the variety of available tests, we included any antibody test against any reference standard. Participants were tested either in hospital settings or the community. Samples included confirmed cases, suspected cases, or individuals who had been ruled out for COVID-19.

Study Findings

We identified 178 relevant studies conducted across Europe (94), Asia (45), North America (35), Australia (2), and South America (2). Seventy-eight studies involved hospitalized patients with confirmed or suspected COVID-19, while 14 focused on community settings. One hundred forty-one studies included recent infection cases within the first three weeks of symptoms, and many also assessed late-stage infections (from day 21 onward).

Key Results

Among participants who tested one week post-symptom onset, only 27% to 41% of infections were detected by antibody tests. The detection rate increased to 64% to 79% in the second week and 78% to 88% in the third week. Tests specifically evaluating IgG or IgM antibodies were the most accurate, with a 93% detection rate when samples were collected 21 days after the onset of symptoms. False positive results occurred in about 1% of individuals without COVID-19.

We present two hypothetical scenarios regarding the test results:

1. For individuals tested for IgG or IgM antibodies during the third week after symptom onset, if only 20 (2%) truly had COVID-19:
• 26 would test positive, with 8 being false positives (31%).
• 974 would test negative, with 2 being false negatives (0.2%).
2. In asymptomatic individuals tested for IgG antibodies, if 500 (50%) previously had COVID-19 more than 21 days earlier:
• 455 would test positive, with 6 being false positives (1%).
• 545 would test negative, with 51 being false negatives (9%).

Reliability of Study Results

We express some limitations in confidence regarding the evidence. Frequently, sample sizes for each week post-symptom onset were small, and study methodologies varied. Participants often consisted of hospitalized patients who might have experienced more severe COVID-19 symptoms. Thus, the accuracy of antibody tests could differ significantly for those with milder or moderate symptoms.

Applicability of Findings

The majority of participants were hospitalized, potentially featuring more severe diseases than non-hospitalized cases. Limited studies focused on individuals without symptoms, making the results more pertinent for those with severe conditions.

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Difficulty arises from the review studies not consistently reporting participant symptoms at the time of testing, complicating the differentiation between early and later-phase infections. Nonetheless, given the range of studied test methods worldwide, we expect consistent results across various regions.

Conclusion of the Review

This review indicates that antibody tests can be instrumental in determining previous COVID-19 infections, but the timing of testing is crucial. They can validate COVID-19 cases in individuals exhibiting symptoms for over two weeks, providing essential insights for potential treatments. Additionally, antibody tests can aid in quantifying those who previously contracted the virus. However, the accuracy in mild or asymptomatic cases, as well as following vaccinations, remains uncertain.