Independent studies from Italy confirm a reliable and accurate detection of anti-SARS-CoV-2 IgM and IgG antibodies [1, 2]. Positive results were confirmed using RT PCR. Of particular note is the high positive rate for IgG detection in patients with known start of symptoms especially in the early (<5 days) and middle (5‑10 days) stages after the appearance of symptoms.
Table 1: Rate of positivitiy of the different methods in the 48 patients in whom the date of symptom onset was available.
|5 days||1 / 30 (3.3%)||0 / 30 (0%)||1 / 30 (3.3%)||1 / 30 (10%)|
|> 5-10 days||4 / 13 (30.8%)||2 / 13 (15.4%)||2 / 13 (15.4%)||7 / 13 (53.8%)|
|> 10-21 days||5 / 5 (100%)||5 / 5 (100%)||3 / 5 (60%)||5 / 5 (100%)|
Giuseppe Lippi et al. 2020
The study by Padoan et al.  confirms the previously published, very good analytical performance of the Maglumi system, and the 2019-nCoV IgM and IgG chemiluminescence immunoassays. This is very important for measuring the course of IgG so that even small titer changes can be detected reliably. The kinetics of antibody occurrence in COVID-19 patients was also confirmed.
High specifity for accurate results
A commonly known problem in diagnostics is a lack of specifity when using (too) sensitive methods, leading to false-positive results. Especially when diagnosing SARS-CoV-2, attention should be paid to the reliability and accuracy of the obtained results. An independent study from Belgium attested an impressive 100-% specifity (n = 198) for the IgM and IgG antibody tests supplied by medac .
The importance of specifity is illustrated by the following example. In a case where a positive result was obtained for IgA only but not IgG as the marker for an infection of the recent past a low specifity of the IgA test causes difficulties in the interpretation of the diagnosis.
EC-compliant validation of the tests
The anti-SARS-CoV-2 IgG and IgM antibody tests were successfully validated in compliance with the criteria set out by the European Commission (EC) in April 2020 .
Consequently, the antibody tests were investigated respectively concerning the following interferences:
•Endogenous influences: Haemoglobin, bilirubin, triglycerides, rheumatoid factor and HAMA
•Interaction with the following medicinal products: Acetaminophen, acetylcysteine, adefovir, ampicillin sodium, Aspirin, cefoxitin, entecavir, ibuprofen, lamivudine, metronidazole, rifampin, telbivudine, tetracycline and theophylline
Exclusion of cross-reactions
Based on the tests of different kit batches, cross-reactions with the following related human coronaviruses can be excluded for the IgG and IgM antibody tests [5, 6]: