What is serum ferritin useful for detecting the disease? Serum ferritin is a very important indicator, which is one of the eight diagnostic criteria for hemophagocytic syndrome. In fact, many patients, especially adults with hemophagocytic syndrome, have a serum ferritin value much higher than this number. What is the usefulness of the NK cell activity test in immunology for the diagnosis of the disease? Altered NK cell activity, which is the cause of phagocytic syndrome, means that there is a decrease in NK cell activity before the disease develops. While serum ferritin values fall or rise accordingly as the disease improves or worsens, NK cell activity is not the same, and its values do not always change as the disease improves or worsens. When the disease is well controlled, but NK cell activity remains low, it strongly suggests that the disease may recur. Therefore, NK cell activity is important not only in the diagnosis of phagocytic syndrome, but also in monitoring the prognosis. It is also worth noting that NK cell activity assay refers to the detection of NK cell function rather than the number and proportion of NK cells, and the number and proportion are not indicative of its good or bad function. How is the serum sCD25 (soluble interleukin-2 receptor) test helpful in confirming the diagnosis of the disease? Although some inflammatory infections or immune diseases can cause an increase in serum sCD25, serum sCD25 is very specific in phagocytic syndromes. As with ferritin, serum sCD25 values increase as the disease progresses; as the disease improves, serum sCD25 values decrease. This test is very important for the diagnosis of phagocytic syndrome. Although many hospitals cannot do both NK cell activity tests and serum sCD25, it is recommended to try to get both tests done if the disease is suspected.