U-shaped association of systemic immune-inflammation index levels with cancer-related and all-cause mortality in middle-aged and older individuals with frailty

Purpose: Frailty is a state of heightened vulnerability to stress, whether from within the body or external factors. We aim to assess the prognostic value of the Systemic Immune-Inflammation Index (SII) in middle-aged and older frail adults. Methods: We used data from the 2003-2018 National Health and Nutrition Examination Survey (NHANES) linked to the 2019 National Death Index (NDI) to study mortality. Cox proportional hazards model and two-piecewise Cox proportional hazards model were used to elucidate the nonlinear relationship between SII level and mortality. Results: Our study included 7,446 frail patients (mean age 65.6) with 2,524 deaths (726 from cardiovascular disease and 458 from cancer) over 49,565 person-years. Elevated SII levels were associated with an increased risk of all-cause, cardiovascular disease (CVD)-related and cancer-related mortality, even after adjusting for potential confounders (adjusted HR (95 % CI) = 1.35 (1.25, 1.46), 1.42 (1.22, 1.65) and 1.26 (1.05, 1.51), respectively). Moreover, a U-shaped correlation was discerned between SII levels and the risks of all-cause and cancer-related mortality, with respective thresholds identified at 334.96 and 348.28. Conclusion: Our findings reveal SII levels positively correlate with frailty, all-cause mortality, CVD-related mortality, and cancer-related mortality in middle-aged and elderly frail individuals in the U.S. The critical thresholds for SII index were 334.96 for all-cause mortality and 348.28 for cancer-related mortality. This study underscores the potential benefits of maintaining a certain low level of SII to effectively mitigate the incidence of frailty and mortality among frail patients.