RCS-based threshold analysis identified a nonlinear inflection at 24.0 ng/mL of MPO, where each additional 1 ng/mL of MPO below this threshold was associated with a reduced infection risk (OR = 0.959, 95% CI: 0.947–0.971), whereas levels above increased the risk (OR = 1.004, 95% CI: 1.002–1.007). The gene discussed is MPO; the disease is infection.