Serum Phosphate within Reference Levels Associates with All-Cause Mortality in Non-Dialysis CKD Patients: A 5-Year Pro-spective Cohort Study

Cardiovascular (CV) morbidity and mortality are markedly increased in non-dialysis patients with chronic kidney disease (CKD). Thus, the precise management of CV risk factors involved in CKD is crucial to improving outcomes. Serum phosphate (Pi) and FGF-23 levels have been linked with a higher risk of CV events in CKD. However, the exact thresholds of Pi and FGF-23, at which the risk of adverse events increases, remain unknown. We evaluated the expression of intact FGF-23 (iFGF-23) and Pi in a non-dialysis CKD patient population (n=82; 42M:40F; median age 61 years) and investigated their association with CV and renal outcomes, in a 5-year follow-up period. At baseline, the median estimated glomerular filtration rate (eGFR), iFGF-23, and Pi were 45 ml/min/1.73m2 (IQ 26.6-73.1), 69.9 µg/ml (IQ 33-117) and 3.4 mg/dL (IQ 3.3-3.9), respectively. Univariate analysis showed a strong association of both iFGF-23 and Pi with age, Charlson Comorbidity Index, hypertension, and diabetes. In addition, iFGF-23 and Pi were both associated with the composite outcome (major CV and cerebrovascular events – MACCEs, hospitalizations, and all-cause mortality) during follow-up. Moreover, Pi was independently associated with all-cause mortality during follow-up. The segmentation of the population in terciles, according to Pi (<3mg/dL; 3-3.6 mg/dL; ≥3.7 mg/dL) within reference serum levels, showed a distribution of the fatality of 0%, 20% and 80% (p =0.034), respectively. Our results reinforce the association of both iFGF-23 and Pi with composite CV outcomes in non-dialysis CKD patients and further suggest that Pi, within current reference levels, may behave as an independent risk factor for mortality in this population. It is suggested that a reassessment of Pi reference levels for early therapeutic intervention in this population may be justified.