Abstract Body: Background: Leukocyte count has been linked to cardiometabolic risk and reduced cardiorespiratory fitness (CRF). The neutrophil-to-lymphocyte ratio (NLR), derived from complete blood counts, integrates innate and adaptive immune activity and may better reflect low-grade inflammation associated with impaired CRF. Bogotá (2,640 m above sea level) presents chronic hypobaric hypoxia, which reduces maximal oxygen uptake (VO₂max) and alters hematologic parameters. The relationship between NLR and VO₂max at high altitude has not been previously described.

Methods: We conducted a cross-sectional analysis in adults aged 18–80 years participating in a comprehensive health evaluation program at a tertiary-care medical center located at 2,640 m in Bogotá, Colombia (2015–2023). VO2max (ml*kg^-1*min^-1) was estimated using a standardized Bruce treadmill protocol. Correlations between VO₂max and hematologic and clinical variables were assessed using Spearman’s rank test, and sex differences were analyzed with the Mann–Whitney U test.

Results: A total of 684 adults (mean age 52.6 ± 12.9 years; 63.6% men; BMI 25.9 ± 3.9 kg/m²) were included. VO₂max showed significant inverse correlations with age (ρ = –0.51, p < 0.001), BMI (ρ = –0.32, p < 0.001), higher number of comorbidities (ρ = –0.28, p < 0.001), total leukocyte count (ρ = –0.16, p < 0.001), neutrophils (ρ = –0.17, p < 0.001), and NLR (ρ = –0.12, p = 0.0012). In contrast, hemoglobin (ρ = 0.15, p < 0.001) correlated positively with VO₂max. VO₂max was significantly higher in men than in women (z = –3.02, p = 0.0025).

Conclusions: At 2,640 m of altitude, higher leukocyte and neutrophil counts, as well as elevated NLR, were associated with lower VO₂max. These findings indicate that leukocyte-derived parameters, particularly NLR, may reflect the level of cardiorespiratory fitness among high-altitude residents. Although the cross-sectional design precludes causal inference, these associations support the potential use of hematologic markers as accessible indicators of aerobic fitness and provide a rationale for future multivariate and longitudinal studies to validate these relationships.