Serum phosphate in stone formers: what does a low serum phosphate level mean?

Metabolic evaluation is indicated in stone formers to identify and subsequently modify risk factors for nephrolithiasis. Some believe that serum phosphate may be an important indicator of nephrolithiasis risk. We determined whether serum phosphate is an independent risk factor for recurrent calcium stone formation and whether it may be used as an early marker for occult disease, such as hyperparathyroidism.

The charts of all patients with regular long-term stone clinic followup were reviewed. Initial serum phosphate levels were compared with stone recurrence, complications due to stones, initial visit urinary laboratory values and the medical diagnoses made during followup. Unpaired 2-sample t tests, and correlation and logistic regression analyses were performed with statistical significance at p <0.05.

Data were available on 51 men and 25 women 22 to 74 years old (mean age 52). Of the patients 50 were recurrent and 26 were single stone formers. Serum phosphate levels were lower in women than in men. No consistent associations existed between initial or future serum phosphate levels and stone recurrence, complications from stones, urinary values or other medical diagnoses made during followup.

Serum phosphate does not appear to be an independent risk factor for urinary tract stone recurrence or complications, or a reliable early predictor of occult disease.