Table 1.

Diagnostic criteria of persistent inflammatory sero-negative COVID^a

A patient is defined as having persistent inflammatory sero-negative COVID if they fulfil the following criteria and no alternative diagnosis:
1. Host criterion	B-cell depleting disease or therapy, including the following: •Primary immunodeficiency causing hypogammaglobinaemia (X-linked agammaglobulinaemia, common variable immunodeficiency, other primary hypogammaglobinaemia). •Secondary immunodeficiency - anti-CD20 treatment in the past year; chronic lymphoblastic leukaemia, non-Hodgkin lymphoma, multiple myeloma accompanied by hypogammaglobinaemia or receiving immunotherapy directed against B cells (bi-specific antibodies or antibody-drug conjugates against CD19, CD20 or BCMA); chimeric antigen receptor T-cell therapy or allogeneic or autologous haematopoietic stem cell transplantation within 1 y.
2. Clinical criterion	Prolonged or remitting fever (total >7 d) with elevated CRP levels plus either one of the following: prostration, non-resolving cough and dyspnea (total >14 d), abnormal chest imaging showing pneumonitis (bilateral ground glass opacities).
3. Virological criterion, defined as either of the following	•Persistent or intermittent positive SARS-CoV-2 RT-PCR result over >21 d.b •Positive SARS-CoV-2 RT-PCR result in the last 90 d + sero-negativity for SARS-CoV-2 14 d after the initial infection in monoclonal antibody-naïve patients.c

B-cell maturation antigen (BCMA), CD, cluster of differentiation; COVID, coronavirus disease; CRP, C-reactive protein; Real-time PCR (RT-PCR); SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

^aBeing sero-negative before and at the time of the onset of acute infection (regardless and despite vaccination) is a characteristic of this entity. It was not comprehensively included in the criteria for diagnosis because of practical reasons; the diagnosis can be made without a specialized blood test.

^bA positive SARS-CoV-2 result from either a nasopharyngeal swab or lower-respiratory specimen demonstrating the same variant using sequencing supports the diagnosis but is not mandatory.

^cUndetectable levels or low titres according to a local serology platform; patients who were treated with monoclonal antibodies for prevention may have higher titres.