An estimate of the ion-activity product of magnesium ammonium phosphate in urine.

Based on the analysis of magnesium (Mg), ammonium (NH4), phosphate (P), urine pH, and urine volume (V), a simplified estimate (AP[MAP] index) of the ion-activity product of magnesium ammonium phosphate (AP MAP) was derived: (Formula: see text). The factor A varies according to the collection period. In 4-hour urine samples more than half of the patients with staghorn calculi had values above 5 in contrast to normal subjects and calcium oxalate stone formers in whom lower values apparently were the rule. The AP(MAP) index might be of value in the evaluation and follow-up of patients with staghorn calculous disease.