State level variations in nephrology workforce and timing and incidence of dialysis in the United States among children and adults: a retrospective cohort study

BACKGROUND Multiple factors influence timing of dialysis initiation. The impact of supply of nephrology workforce on timing and incidence of dialysis initiation is not well known. METHODS We determined the number of pediatric and adult nephrologists in each state using data from the American Medical Association and American Boards of Internal Medicine and Pediatrics. We ascertained state population data from the 2010 US Census. United States Renal Data System (USRDS) data were used to determine estimated glomerular filtration rate (eGFR) at dialysis initiation and dialysis incidence for adults (≥18 years) in 2008 and children (<18 years) in 2007-2009 by state. RESULTS Across all states, there were a median of 3.0 (IQR 2.3 to 3.4) adult nephrologists per 100,000 adults and 0.5 (IQR 0.2 to 0.9) pediatric nephrologists per 100,000 children. The median eGFR at start of dialysis was 9.8 mL/min/1.73 m2 (IQR 7.1-13.1) in adults and 8.5 mL/min/1.73 m2 (IQR 6.2-11.4) in children. Neither the number of adult (Spearman r of 0.02 [95% CI -0.26-0.30], p = 0.88) nor pediatric (Spearman r of -0.13 [95% -0.39-0.15], p = 0.38) nephrologists per state population was associated with mean eGFR across states. The number of nephrologists per state population was associated with incident dialysis cases per state population in adults (Spearman r of 0.50 [95% CI 0.26-0.68], p = 0.0002), but not in children (Spearman r of -0.06 [95% CI -0.33-0.22], p = 0.67). In linear regression models, the association between nephrologists per state population and incident dialysis cases per state population remained statistically significant (p = 0.006) after adjustment for provider characteristics. CONCLUSIONS Nephrology workforce supply is aligned with demand but does not appear to be associated with timing of dialysis initiation.