title: The physicochemical basis of urinary catheter encrustation.
abstract: OBJECTIVE
  To determine the relationship between urinary pH and Ca2+ solubility in urine samples from patients who experienced either frequent ('blockers') or infrequent ('nonblockers') catheter blockage by crystalline deposits of divalent cation salts.
  
  
  METHODS
  Fresh urine samples from 'blockers' and 'nonblockers' were collected and the ionic calcium concentration ([Ca2+ ]) measured using a Ca2+-selective electrode whilst the urinary pH was increased in 0.25 increments between 4.75 and 9.00. The pH at which crystallization occurred (nucleation) was determined and crystal composition analysed.
  
  
  RESULTS
  The mean (sd) voided urinary pH of catheter 'blockers' was significantly more alkaline than that from 'nonblockers', at 7.63 (0.64) and 5.97 (0.80), respectively (P=0. 001). The nucleation pH of catheter 'blockers' was significantly more acid than in 'nonblockers', at 7.43 (0.73) and 6.45 (0.65), respectively (P=0.005). Urine from 'blockers' had significantly more Ca phosphate and Mg ammonium phosphate crystals. 'Blockers' were further divided into two subsets with and without urease-based urinary tract infection; both showed a decrease in the nucleation pH.
  
  
  CONCLUSIONS
  In the urine from 'nonblockers' there is a wide safety margin between voided and nucleation pHs; this margin was less in the urine from 'blockers'. This reduction in the safety margin arises partly because the voided pH in those with a urinary tract infection is more alkaline. However, the decrease in the nucleation pH also suggests that a fundamental property of urine is altered, which reduces Ca2+ solubility at more neutral pH values. The long-term goal is to increase the nucleation pH of catheter 'blockers' and increase the margin of safety.
authors.affiliation: Institute of Urology and Nephrology, Department of Chemical Pathology, University College, London, UK.
authors.forename: S K
authors.initials: SK
authors.lastname: Choong
authors.forename: P
authors.initials: P
authors.lastname: Hallson
authors.forename: H N
authors.initials: HN
authors.lastname: Whitfield
authors.forename: C H
authors.initials: CH
authors.lastname: Fry
mesh_terms: D002118:Calcium / Q000737:chemistry*; D002408:Catheters, Indwelling; D003460:Crystallization; D004868:Equipment Failure; D006801:Humans; D006863:Hydrogen-Ion Concentration; D014546:Urinary Catheterization / Q000295:instrumentation; D014556:Urine / Q000502:physiology*; D014570:Urologic Diseases / Q000628:therapy / Q000652:urine
chemical_list: D002118:Calcium