Molecular weight for oxalic acid: 90.04
Molecular weight for citric acid: 192.12
Parameters:
(1) 24 hour urine volume in mL
(2) 24 hour urine calcium excretion in mmol per 24 hours
(3) 24 hour urine oxalate excretion in mmol per 24 hours
(4) 24 hour urine magnesium excretion in mmol per 24 hours
(5) 24 hour urine citrate excretion in mmol per 24 hours
activity product index =
= 3.8 * ((calcium excretion) ^ 0.71) * (oxalate excretion) / (((magnesium excretion) ^ 0.14) * ((citrate excretion) ^ 0.10) * ((urine volume in mL) ^ 0.1.2))
Interpretation:
• The activity product index correlates well with the ion-activity product for calcium oxalate when urinary electrolyte values are within the normal range.
Limitations:
• With extreme combinations of electrolytes the index does not correlate ion-activity product for calcium oxalate, but these conditions are considered unusual.
