Semi-quantitative assays measure approximate concentrations. At this time, getting a “positive” result on the serological COVID-19 test does not mean one should avoid the guidance for masking and social distancing provided by the CDC. Researchers are still trying to understand whether these antibodies to the virus could protect people from getting re-infected with the virus and how long this immunity lasts. Although such assays do not tell us the level or concentration of antibodies present in the sample, they help us understand whether the patient has developed humoral (B-cell) immune response against the virus. Such qualitative assays use a single calibrator to establish a cutoff value and provide a simple “positive” or “negative” result. Most SARS-CoV-2 antibody tests report a qualitative result, denotating a “yes” or “no” answer to whether antibodies (immunoglobulins G, A, and/or M) that bind viral antigen are present or absent in a sample. The results are not expressed in numerical values, but as descriptive terms such as “positive,” or “reactive,” and “negative,” or “non-reactive.” (1) An example of a qualitative test is a general pregnancy test which determines the presence or absence of human chorionic gonadotrophin in the patient's urine but does not quantify the amount present. Qualitative assays provide a simple “yes” or “no” answer by measuring the presence or absence of a substance or target. Quantitative assays that determine antibody concentrations allow longitudinal monitoring of adaptive or humoral immunity in response to infection or vaccination. Most serologic assays are qualitative and semi-quantitative but quantitative serologic assays are also being developed. While qualitative reverse transcription polymerase chain reactions (RT-PCR) are used to diagnose COVID-19, serologic tests for antibodies specific to the SARS-CoV-2 virus are used to detect past infection.
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