Measurement of the risk of calcium phosphate crystallization in urine

A method is described for analysis of the risk of calcium phosphate (CaP) crystallization in urine samples. The pH required for formation of 500 crystals in the size range 3.5 to 5 micron (pHCaP) was determined in a Coulter Counter following addition of sodium hydroxide. The risk of CaP crystallization (CaP-CR) was defined as: 1/(pHCaP--5.8). CaP-CR was determined in 24 h urine collections from 25 patients with calcium stone disease and 26 normal subjects, each urine diluted to a creatinine concentration of 5 mumol per ml. The mean (+/- SD) CaP-CR was 0.71 +/- 0.18 and 0.63 +/- 0.14 respectively and did not differ significantly. This method might be useful for evaluations and follow-up of stone formers with respect to the risk of CaP precipitation and stone formation.