C4A and chronic kidney disease: Kynurenine had the highest OR for CKD (OR: 13.81; 95% CI: 7.38–25.86), followed by citrulline (OR: 10.42; 95% CI: 5.34–20.14), methionine (OR: 5.35; 95% CI: 2.78–10.29), arginine (OR: 4.94; 95% CI: 2.66–9.20), C4 (OR: 4.76; 95% CI: 3.06–7.40), phenylalanine (OR: 4.33; 95% CI: 1.57–11.88), alanine (OR: 4.37; 95% CI: 2.06–9.29), isoleucine (OR: 4.29; 95% CI: 2.00–9.19), leucine (OR: 3.83; 95% CI: 1.65–8.92), and asparagine (OR: 3.75; 95% CI: 1.72–8.15).