Studies on the crystallization of magnesium ammonium phosphate in urine.

The crystallization of magnesium ammonium phosphate (MAP) was studied in salt solutions and dialysed urine at similar levels of MAP supersaturation. At pH levels of 7.1 or higher crystallization occurred to the same extent in solutions with and without urinary macromolecules. Whereas crystals in the size range 3.5-5 microns were observed in the salt solution at pH 7.0, this was not so in dialysed urine. When the crystal size distribution was determined after 30 min larger crystals were observed in dialysed urine, indicating a promoting effect by urinary macromolecules on the formation of MAP crystals. A modified AP(MAP) index was formulated based on calculations with the EQUIL 2 programme in order to improve the relationship between this simplified estimate and the ion-activity product of MAP (APMAP). This index had the following form: [formula: see text] for 24-h values of magnesium (Mg), ammonium (NH4), phosphate (P), pH and urine volume (V). The APMAP required for the formation of 2,000 crystals in the size range 3.5-5 microns varied between 226 x 10(-15) and 293 x 10(-15) (mmol/l)2 in dialysed urine. An experimental system was designed based on the measurement of pH during the addition of NH4OH. At the point assumed to correspond to the start of crystallization, AP(MAP) index values between 409 and 903 were recorded. Such a test might provide useful information on the crystallization properties in urine.