title: Differences in the free Ca2+ in undiluted urine from stone formers and normal subjects using a new generation of ion-selective electrodes.
abstract: OBJECTIVE
  To measure the ionized Ca2+ in urine from normal subjects and patients with urinary tract stones.
  
  
  METHODS
  Urine samples were obtained from 37 normal subjects and 52 stone-formers. Ca2+ was measured using plastic dip-cast ion-selective electrodes; total Ca and other variables were measured by standard analytical techniques.
  
  
  RESULTS
  The ionized Ca2+ and the total Ca were greater in stone-formers' urine although the difference was more significant with the Ca2+ data, especially at a standardized pH. Absolute values of the Ca2+ were dependent on the type of reference electrode used. The Ca2+ was a function of urine pH; lower values were recorded as pH increased. Measured values of the Ca2+ did not correlate well with calculated values using a standard computer program.
  
  
  CONCLUSIONS
  Measurement of the Ca2+ in spot urine samples at a standard pH provides a more discriminative test than total Ca for the presence of urinary tract stones. The pH dependence of the Ca2+ may have important consequences in the formation of urinary tract stones. The poor correlation between measured and computed values of Ca2+ emphasizes the importance of accurate measurement of this variable.
authors.affiliation: Department of Physiology, UMDS, St Thomas' Campus, London, UK.
authors.forename: S E
authors.initials: SE
authors.lastname: Langley
authors.forename: C H
authors.initials: CH
authors.lastname: Fry
mesh_terms: D002118:Calcium / Q000652:urine*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D017736:Ion-Selective Electrodes; D012016:Reference Values; D012680:Sensitivity and Specificity; D014545:Urinary Calculi / Q000652:urine*
chemical_list: D002118:Calcium