In one sentence
Neutrophils times platelets divided by lymphocytes, three numbers from an ordinary blood count folded into one that rises when the inflammatory side of the immune system is up and its regulating side is down.
Why it matters
The index was built in cancer surgery, where in 256 patients with liver cancer a value at or above 330 ×10⁹/L separated those with worse survival after resection (Hu 2014), and it has since been followed in whole populations. In 42,875 US adults in NHANES free of cardiovascular disease and cancer at baseline, followed for up to 20 years, those with an index above 656 had 29 percent higher all-cause mortality and 33 percent higher cardiovascular mortality than those below 335 (Xia 2023); in 25,955 adults from the 2005 to 2018 survey years the top quarter carried 24 percent higher all-cause mortality than the bottom, the relation was J-shaped, and cancer mortality was higher at both ends (Wu 2025). It is cheap, it comes from a test everyone already has, and it is noisy, in that order.
What moves it
Everything that moves the three counts: an infection (neutrophils up, lymphocytes down, so the index jumps), the hours after hard exercise, acute stress, smoking, corticosteroids, iron deficiency (platelets up), and the time of day. It also drifts upward with body fat and with age.
How to measure it well
From a blood count taken under the panel's rules: not within two weeks of being unwell or of a vaccination, not within 48 hours of hard training, in the morning. One value is a snapshot; two or three draws under the same conditions are a baseline.
How Vitality Tracker uses it
Computed from one draw only (the same-draw rule) and shown on the labs dashboard beside its three inputs and the neutrophil-to-lymphocyte ratio, with the cohort reference points stated as the cohorts' ("upper tertile in NHANES, above about 600") and never as a personal figure. There is no laboratory reference range for an index, and VT draws no working optimal line; the index sits in the inflammation cluster of the pre-visit report as a supporting figure, and it is excluded from a draw taken while unwell.
How far to trust the number
Calculated from three precise counts, so the arithmetic is exact and the biology is not: a value doubles with a cold and halves a fortnight later. Read the trend across clean draws.
Sources
- Hu B, et al. Systemic immune-inflammation index predicts prognosis of patients after curative resection for hepatocellular carcinoma. Clin Cancer Res 2014;20:6212–22. doi:10.1158/1078-0432.CCR-14-0442
- Xia Y, et al. Systemic immune inflammation index (SII), system inflammation response index (SIRI) and risk of all-cause mortality and cardiovascular mortality: a 20-year follow-up cohort study of 42,875 US adults. J Clin Med 2023;12:1128. doi:10.3390/jcm12031128
- Wu S, Liu Z, Li X, Gao S, Xia P. Association between systemic immune-inflammation index and the risk of all-cause, cancer and non-cancer mortality in the general population: results from NHANES 2005–2018. BMC Public Health 2025;25:227. doi:10.1186/s12889-025-21423-1
- Vitality Tracker, Clinician report content specification (derived indices).