COVID-19-associated cytokine storm syndrome and diagnostic principles: an old and new Issue.

SARS-CoV-2 has claimed 2,137,908 lives in more than a year. Some COVID-19 patients experience sudden and rapid deterioration with the onset of fatal cytokine storm syndrome (CSS), which have increased interest in CSS's mechanisms, diagnosis and therapy. Although the prototypic concept of CSS was first proposed 116 years ago, we have only begun to study and understand CSS for less than 30 years. Actually, diseases under CSS umbrella include familial/primary and secondary hemophagocytic lymphohistiocytosis (HLH), macrophage activation syndrome (MAS), infection-associated hemophagocytic syndrome, cytokine release syndrome (CRS), and cytokine storm (CS). Hematologic malignancies and autoimmune diseases that cause CSS are named malignancy-associated hemophagocytic syndrome (MAHS) and MAS, respectively. In-depth research on the pathogenesis of HLH/CSS has greatly increased the number of patients that were able to be definitively diagnosed with HLH/CSS. However, it should be emphasized that HLH/CSS diagnosis is difficult at the early stages due to the non-specific clinical signs and symptoms, which tends to result in missed and incorrect diagnoses. Therefore, clinicians should not only possess extensive clinical experience to ensure high sensitivity to the characteristics of HLH/CSS but must also be familiar with HLH-2004/2009 diagnostic criteria, and HScore methods. The paper concisely comment evolution of CSS classifications, cytokines associated with CSS, evolution of CSS diagnostic criteria and importance of the correct identification of hemophagocytes in diagnosing CSS, which is timely and may benefit clinicians familiar HLH-2004/2009 diagnostic criteria, and HScore methods. In addition, clinicians must also understand that there are some limitations to these diagnostic criteria. Abbreviations: aBMT: autologous bone marrow transplantation; CAR-T: chimeric antigen receptor-engineered T-cell; COVID-19: coronavirus disease 2019; CSS: cytokine storm syndrome; HLH: hemophagocytic lymphohistiocytosis; MAS: macrophage activation syndrome; CRS: cytokine release syndrome; CS: cytokine storm; MAHS: malignancy-associated hemophagocytic syndrome; IAHS: infection-associated hemophagocytic syndrome; fHLH/pHLH: familial/primary hemophagocytic lymphohistiocytosis; sHLH: secondary hemophagocytic lymphohistiocytosis; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2; TCR-T, T-cell receptor-engineered T-cell.