Objective To investigate the prognostic value of albumin-corrected anion gap (ACAG) in patients with advanced lung cancer receiving immune checkpoint inhibitors (ICIs).

Methods A total of 637 patients with advanced lung cancer who received ICI treatment were enrolled. Baseline clinical data and laboratory parameters were collected, and ACAG was calculated. The optimal prognostic Cut-off value for ACAG (11.58 mmol/L) was determined using X-Tile software, and patients were divided into low (≤11.58 mmol/L) and high (>11.58 mmol/L) ACAG groups. Propensity score matching was performed to control for confounding factors. Survival curves were plotted using the Kaplan-Meier method. A Cox proportional hazards regression model was employed to analyze prognostic factors, and subgroup analyses were conducted.

Results Among 637 patients, 440 patients remained in the matched cohort. Multivariate Cox analysis showed that after adjusting for clinical and laboratory parameters, ACAG >11.58 mmol/L was an independent risk factor for OS (HR=1.74, 95%CI: 1.02–2.98, P=0.044) but was not an independent predictor for PFS. After adjusting for different sets of covariates, stratified analysis further confirmed that ACAG >11.58 mmol/L was consistently associated with a significantly shortened OS. Survival curves demonstrated that the median OS in the high ACAG group (47.0 months) was significantly lower than that in the low ACAG group. Subgroup analysis indicated that the predictive value of ACAG for OS was more pronounced in males, patients aged <65 years, and patients with squamous cell carcinoma than in other patients.

Conclusion In patients with advanced lung cancer treated with ICIs, a high baseline ACAG level (>11.58 mmol/L) is an independent risk factor for shortened OS but is not significantly associated with PFS. ACAG may serve as a simple and effective biological marker for assessing overall survival prognosis in patients with advanced lung cancer undergoing immunotherapy.