The physicochemical basis of urinary catheter encrustation

Objectives To determine the relationship between urinary pH and Ca2+ solubi lity in urine samples from patients who experienced either frequent ('block ers') or infrequent ('nonblockers') catheter blockage by crystalline deposi ts of divalent cation salts. Materials and 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 inc rements between 4.75 and 9.00. The pH at which crystallization occurred (nu cleation) was determined and crystal composition analysed. Results The mean (SD) voided urinary pH of catheter 'blockers' was signific antly 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 C a phosphate and Mg ammonium phosphate crystals. 'Blockers' were further div ided into two subsets with and without urease-based urinary tract infection ; both showed a decrease in the nucleation pH. Conclusion rn the urine from 'nonblockers' there is a wide safety margin be tween voided and nucleation pHs; this margin was less in the urine from 'bl ockers'. This reduction in the safety margin arises partly because the void ed pH in those with a urinary tract infection is more alkaline. However, th e decrease in the nucleation pH also suggests that a fundamental property o f 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.