[Antibody tests for COVID-19: What the results tell us].

In the context of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, the detection of virus-specific antibodies (AB) will play an increasing role. The presence or absence of such antibodies can potentially lead to considerations regarding immunity and infection. How reliable are inferences from positive or negative test results regarding the actual presence of SARS-CoV-2 specific antibodies? Calculation of the probability that, depending on the pretest probability (prevalence of SARS-CoV-2 infection) and test properties, antibodies are present or absent in the case of positive or negative test results. Sensitivity and specificity of different SARS-CoV-2 AB test systems vary between 53 % and 94 % and between 91 % and 99.5 %, respectively. When using a test with high test quality, the positive predictive value (PPV) is 42 % and 7 9%, respectively, with a pre-test probability of 1 % to 5 %, as can currently be assumed for the general population in Austria or Germany. For persons with an increased pre-test probability of 20 %, e. g. persons from high-risk professions, the PPW is 95 %, with a pre-test probability of 80 % the PPW is almost 100 %. The negative predictive value (NPV) is at least 99.7 % for persons with a low pre-test probability of up to 5 % and 79.1 % for persons with a pre-test probability of 80 %. When using test systems with lower sensitivity and specificity, the reliability of the results decreases considerably. The PPV is 5.9 % with a pre-test probability of 1 %. A sufficiently high sensitivity and specificity are prerequisites for the application of antibody test systems. Positive test results are often false if the pre-test probability is low. Depending on the assumed prevalence of a SARS-CoV-2 infection, there are substantial differences in the significance of a concrete test result for the respective affected persons.

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