Is there a “gold standard” for the diagnosis of this disease? Is the presence of hemophagocytosis necessary to diagnose the disease? First of all, what is the phenomenon of phagocytosis? Under normal circumstances, macrophages engulf aging blood cells, such as red blood cells and white blood cells, to keep the blood cells in a young state. When macrophages become over-activated and swallow up normal cells as well as the hematopoietic stem cells inside the bone marrow, causing a decrease in blood cells, this phenomenon is called the hemophagocytosis phenomenon. Previously, phagocytosis was considered an essential diagnostic criterion for the diagnosis of hemophagocytic syndrome. However, it is now believed that excessive activation of macrophages to engulf red blood cells, white blood cells, platelets, etc. is only one part of the inflammatory factor storm (a phenomenon in which infection of the body with microorganisms causes rapid and massive production of multiple inflammatory factors in the body). Therefore, the presence of hemophagocytosis is not necessarily a hemophagocytic syndrome, and not all patients diagnosed with hemophagocytic syndrome necessarily have hemophagocytosis. Phagocytosis is one of the eight diagnostic criteria that are now popular, and in fact, five criteria are sufficient to diagnose phagocytic syndrome. Therefore, the presence or absence of the phagocytosis phenomenon is not absolutely significant in diagnosing and ruling out the disease, but it is of relative significance, which means that this test is still needed. Is a single bone marrow test sufficient to detect hemophagocytosis and confirm the diagnosis? No, it is not. Many articles in Japan and abroad have indicated that in many cases of phagocytic syndromes, phagocytosis is not found at all in early stages of bone marrow aspiration, but may be found in later stages. Even if phagocytosis is not found, the diagnosis must be continued, because early diagnosis means early intervention for treatment, which can save lives. If we must wait until this phenomenon appears before treatment, it may be too late.