The Solitary Functioning Kidney: From Pathophysiology to Long-Term Management

Solitary functioning kidney (SFK) is a clinical condition characterized by the presence of a single functioning kidney, arising from either congenital anomalies or acquired causes. It is recognized as an important risk factor for chronic kidney disease (CKD) and, in some patients, for progression to end-stage kidney disease (ESKD). Congenital solitary functioning kidney is commonly associated with renal hypoplasia or dysplasia, genetic abnormalities, and congenital anomalies of the kidney and urinary tract, whereas acquired solitary functioning kidney is often secondary to unilateral nephrectomy for renal tumors, trauma, tuberculosis, severe infection, or living kidney donation. The principal pathophysiological mechanisms underlying the progression from SFK to CKD are compensatory renal hypertrophy and glomerular hyperfiltration in the remnant kidney. Although these adaptive responses may initially maintain renal function, sustained compensation can lead to glomerular injury, proteinuria, tubulointerstitial damage, and renal fibrosis. Early diagnosis relies primarily on imaging modalities, such as ultrasonography and magnetic resonance imaging, together with assessment of renal function and emerging biomarkers, including neutrophil gelatinase-associated lipocalin and cystatin C. Prognostic evaluation of patients with SFK should comprehensively consider etiology, age, compensatory renal growth, associated urinary tract anomalies, comorbidities, and markers of kidney injury. Effective long-term management is essential to prevent or delay progression to CKD and ESKD, with particular emphasis on individualized follow-up, lifestyle modification, blood pressure control, reduction of proteinuria, and avoidance of additional nephrotoxic insults. Previous publications have often addressed pediatric congenital solitary functioning kidney (CSFK), living kidney donors, and disease-related nephrectomy cohorts separately, limiting the translation of heterogeneous evidence into a common clinical approach. This review integrates congenital and acquired SFK across the life course, distinguishes shared markers of established kidney injury from etiology-specific determinants of renal reserve, and proposes an evidence-informed framework for dynamic risk stratification and risk-adapted follow-up and management. This framework is conceptual and requires prospective validation.