On the usefulness of point-of-care antibody tests for severe acute respiratory syndrome coronavirus 2 in community screening settings.

We are pleased with the discussion of our article and would like to take the opportunity to discuss the benefits of antibody testing in screening scenarios in more detail.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a particularly complex pathogen because especially mild forms of the disease are often non-specific or asymptomatic, and therefore, symptom-based risk management is not possible.

The present letter to the editor reports a polymerase chain reaction (PCR) sensitivity of about 70%, but of course this depends on whether RNA-negative persons are also defined as COVID-19 cases based on certain symptoms. However, this definition is country specific and has been updated several times during the pandemic.

PCR is the gold standard for acute infections, and we agree with the comment that an antibody test based on IgG is not suitable for adequately testing the immune response owing to the time course of the immune reaction.

But screening of asymptomatic or unspecifically symptomatic SARS-CoV-2–infected persons by means of IgM antibodies seems to be worthwhile, considering the relevance of investigational speed.

As mentioned in our study, we could not see a single “IgM-only” response in our rapid tests or a single “IgG only” response. It could be that we happen to see only freshly infected people before IgM antibodies are formed and infected people in antibody class swap. However, there could also be a technical problem with the test on the IgM band.

Therefore, besides the development of suitable rapid tests based on PCR or antigens with high sensitivity for screening and quarantine decisions, it should be considered whether rapid antibody tests could be useful as an addition to PCR.

In our study, 11 individuals with a positive rapid test result would have been quarantined, and 3 would have been released after a negative PCR result. Fourteen more people would have been quarantined 24 h later after a positive PCR result. We would have reduced the transmission possibilities for 24 h by 36.4%, with opportunity costs of three false positives at an extremely high prevalence of 44.9% (by today's case numbers). With a lower prevalence and therefore a lower positive predictive value (PPV), these costs would even increase.

Therefore, we summarize our statement that antibody tests should not be used in community screenings to derive public health measures such as quarantine. This is without prejudice to the importance of antibody testing for the decision to lift quarantine or other de-escalation measures.