Abstract: Ｏｂｊｅｃｔｉｖｅ：To investigate the relationship of renal fractional excretion of solutes or electrolytes (FEx) and glomerular filtration rate (GFR) in chronic kidney disease (CKD) patients, and compare FEx in CKD patients with in acute kidney injury (AKI).
Ｍｅｔｈｏｄｏｌｏｇｙ：From January 2020 to August 2022, patients diagnosed as CKD or AKI in National Clinical Research Center of Kidney Diseases were retrospectively screened of whom with full records of blood and urine biochemical tests. Estimated GFR (eGFR) and fractional excretion of sodium (FENa), potassium (FEK), chloride (FECl), phosphorus (FEP), calcium (FECa), urea (FEUN) and uric acid (FEUA) were calculated. The relationship between FEx and eGFR in CKD was analyzed. Comparisons of FEx in AKI and CKD patients were made after propensity score matching (PSM).
Ｒｅｓｕｌｔｓ：261 CKD patients and 53 AKI patients were included in the study. Most of FEx increases significantly with the progression of CKD stage. A close relationship of FEx (FENa、FEK、FECl、FEP and FEUA) and eGFR was found. The ratio of individual FEx and the mean value of FEx in CKD stage 1 patients was calculated as relative FEx (RFEx), and nonlinear curvefitting equations of eGFRRFEx were established in Na, K, Cl, P, and UA. Scatter plots of eGFRFEx in AKI patients show similar patterns as seen in CKD patients. With a much lower eGFR in AKI group, significant differences were found in all FEx between AKI and CKD group. After PSM focusing on sex, age and eGFR, a significant difference between AKI and CKD group was only seen in FEUN in patients with eGFR≥60 mL/(min·173m2), and FEUA in patients with eGFR<60 mL/(min·173m2). The optimal cutoff value to distinguish AKI from CKD in patients with eGFR≥60 mL/(min·173m2) is FEUN<1295％ (AUC 065, sensitivity 700％, specificity 571％); while in patients with eGFR<60 mL/(min·173m2), it is FEUA>1141％ (AUC 074, sensitivity 758％, specificity 735％).
Ｃｏｎｃｌｕｓｉｏｎ：eGFR has a distinct impact on FEx, with most of FEx increase progressively along with the decline of eGFR regardless of CKD or AKI. It has a diagnostic value for FEUN in patients with eGFR≥60 mL/(min·173m2) and for FEUA in patients with eGFR<60 mL/(min·173m2) to distinguish AKI from CKD.

Key words: fractional excretion, glomerular filtration rate, electrolytes, acute kidney injury, chronic kidney disease