Abstract Purpose: We examined the stone composition, 24-hour urinary risk factors, and insurance status in patients evaluated in two regional stone clinics to further investigate the relationship between the socioeconomic status and kidney stone formation.
Materials and methods: We performed a retrospective review of stone formers who completed a 24-hour urinalysis as part of a metabolic evaluation for nephrolithiasis. Insurance status was determined by billing records and those with state-assisted insurance (SAI) were compared to patients with private insurance (PI). Multivariate analyses were performed adjusting for known risk factors for stones.
Results: Three hundred forty-six patients were included. Patients with SAI (16%) were significantly more likely to be female (55% vs.38%, p=0.026) and younger (43.5 vs.49.2, p=0.003). Among those with stone composition data (n=200), SAI patients were as likely to form calcium phosphate (CaPhos) as calcium oxalate (CaOx) stones (46.9% vs.31.3%, p=0.44). PI patients were significantly less likely to form CaPhos than CaOx stones (10.1% vs.77.4%, p<0.001). On multivariate analysis, among those with calcium stones, the odds of forming CaPhos stones over CaOx stones were ten times higher among SAI patients compared to PI, odds ratio 10.2 (95% CI 3.6, 28.6, p<0.001). Further, patients with SAI had significantly higher urine sodium, pH, and supersaturation of CaPhos, and a lower supersaturation of uric acid compared to patients with PI.
Conclusions: SAI was associated with a greater likelihood of a CaPhos stone composition and increased urinary risk factors for CaPhos stones. These findings may reflect dietary or other unmeasured differences, and have important implications for resource allocation and counseling, as treatment may differ for these groups.