<div>COVID-19 IgM/IgG false-positivity in a rheumatoid arthritis patient
negative for RF and ACPA: exist of other serological interference?</div><div>To the editor:</div><div>The outbreak of coronavirus disease 2019 (COVID-19) which rapidly spread
throughout the world is still threatening the global health safety.
Diagnostic tests for COVID-19 divided into two main categories:
detection of viral RNA and serological detection of anti-SARS-CoV-2
immunoglobulins including specific IgM and
IgG<sup>[1]</sup>.</div><div>Recently, we came up with an 89-year-old RA patient. She was diagnosed
of rheumatoid arthritis for interphalangeal joint pain and rheumatic
factor (RF) positive over 15 years ago and had been taking prednisone
5mg per day. Since the spread of COVID-19, we gave her the test of
SARS-Cov-2
IgM/IgG and nucleic acid routinely. The confusing results showed
IgM
and IgG positive and nucleic acid negative. This old woman had no
epidemiological history due to poor lung function. To further clarify
the diagnosis, we retest SARS-Cov-2 nucleic acid from
throat,
nasopharyngeal and anal swab specimen and got negative results again.
Diagnosis of COVID-19 was eliminated. 6 months later, this lady was
readmitted with persistent positive result of specific IgM and IgG and
negative result of nucleic acid. Serological test of RF and anti-cyclic
citrullinated peptide antibody (ACPA) showed negative.</div><div>According current reports, serum IgM arrived peak need 16-30 days and
rapid decayed, while IgG remained relatively stable up to 105 days
post-symptoms
onset<sup>[2,3]</sup>.
In our reported case, the IgM and IgG had persisted for more than 6
months.</div><div>Recent study reported that no cross-reactivity was observed between
autoantibodies in autoimmune disease and SARS-CoV-2 antibodies<sup>[4]</sup>.
Controversially,
the
false-positivity of
anti-SARS-CoV-2
IgM was reported in RF IgM-positive serum<sup>[5]</sup>. Considering
this case, some unknown factors besides RF IgM and ACPA may exist in
serum of RA patients which cause persist false positivity of
anti-SARS-CoV-2 immunoglobulin.</div><div>In conclusion, medical history combined with nucleic acid tests still
played the key role in diagnosis of COVID-19 when come with positive
result of SARS-Cov-2 IgM and IgG. The false positivity could involve
either IgM or IgG which may be caused by other cross-reactive factors
besides RF antibody and ACPA. The potential mechanism remained further
explored.</div>