Association of Urine Specific Gravity With Hemoglobin Concentration in Chronic Kidney Disease, Independent of eGFR and Proteinuria

Background:Chronic kidney disease (CKD) affects over 10% of the global population.Current staging relies on glomerular markers (estimated glomerular filtration rate [eGFR] and proteinuria), which may not fully capture kidney dysfunction.Urine specific gravity (SG) is a functional marker of urinary concentrating ability that may provide complementary information.We evaluated whether lower urine SG is independently associated with lower hemoglobin concentration after adjustment for eGFR, proteinuria, and other clinical covariates. Material/Methods:This retrospective cross-sectional study included 1064 patients with CKD not receiving dialysis with paired morning urine SG and hemoglobin levels within 7 days at a tertiary center in Warsaw, Poland (2019Poland ( -2025)).Patients on dialysis, with transplantation, or active malignancy were excluded.Anemia was defined as hemoglobin < 13.0 g/dL for men and < 12.0 g/dL for women.Multivariable linear regression assessed the SG-hemoglobin association, adjusting for eGFR, age, sex, diabetes, heart failure, acute kidney injury, and dipstick proteinuria. Results:Urine SG was independently associated with hemoglobin (b = 0.607 g/dL per 0.010-unit increase; 95% CI, 0.396-0.817;P < 0.001), with a graded association across groups (P for trend < 0.001).In advanced CKD (eGFR < 45 mL/min/1.73m 2 ), median hemoglobin concentration was 13.0 g/dL in the high-SG group and 11.9 g/dL in the low-SG group (Hodges-Lehmann location shift, 0.9 g/dL; 95% CI, 0.3-1.6;P = 0.008).Results remained robust in sensitivity analyses. Conclusions:Higher urine SG was independently associated with higher hemoglobin in non-dialysis CKD.This cross-sectional association does not establish causality or support using SG alone for anemia assessment or treatment decisions.Prospective validation is needed.