SII index

SII index

The SII index stands for the systemic immune-inflammation index and is a number calculated from a routine blood count. It combines platelets, neutrophils and lymphocytes in a single measure using a simple formula: platelets times neutrophils divided by lymphocytes. This calculation captures the balance between inflammatory drivers (neutrophils and platelets) and the body's immune reserve (lymphocytes). A higher value generally indicates stronger inflammation and a weaker adaptive immune response, while a lower value suggests the opposite. Clinicians and researchers use the SII index as a prognostic marker in conditions where inflammation matters, such as cancer, cardiovascular disease, and serious infections. Its appeal comes from being inexpensive, easy to obtain from standard lab tests, and useful for tracking changes over time. The index is not disease-specific, so it must be interpreted with other clinical information rather than used alone. Different studies use different cutoff points, so what counts as a "high" SII can vary by disease and patient group. Still, it can help doctors estimate risk, monitor response to treatment, and guide decisions about further testing or therapy.