A simplified estimate of the ion-activity product of calcium phosphate in urine

Eur Urol. 1984;10(3):191-5. doi: 10.1159/000463786.


A computerized program was used to calculate the activities of the calcium, phosphate, and hydrogen phosphate ions. The most important determinants for ion-activity products of calcium phosphate and brushite were calcium, phosphate, citrate, urine volume, and pH. These urine variables were used to derive indices corresponding to ion-activity products of calcium phosphate (AP[CaP] index) and calcium hydrogen phosphate (AP[Bru] index). Factors were given to adapt these indices to collection periods shorter than 24 h. Relationships between the AP(CaP) index and ion-activity products of hydroxyapatite and octacalcium phosphate were also formulated. With urine electrolytes within the normal range a very good correlation was obtained between these indices and corresponding ion-activity products. The coefficient of correlation was better than 0.99. The presented indices might be useful in the evaluation of patients with renal calcium stone formation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Calcium Phosphates / urine*
  • Computers*
  • Crystallization
  • Electrolytes / urine
  • Humans
  • Hydrogen-Ion Concentration
  • Kinetics
  • Software*
  • Urinary Calculi / urine*


  • Calcium Phosphates
  • Electrolytes
  • alpha-tricalcium phosphate
  • tetracalcium phosphate
  • calcium phosphate, monobasic, anhydrous
  • calcium phosphate
  • calcium phosphate, dibasic, anhydrous