Measurement of urine calcium, magnesium and phosphate: Preanalytical considerations in the elimination of the...

Traditionally, urine analysis of calcium (Ca), magnesium (Mg) and phosphate (Phos) requires acidification of the sample. This study aims to assess the need for acidification and evaluate preanalytical factors that influence the accurate measurement of these analytes in urine.

A total of 107 spot urine samples from patients with a median age of 9 years (95% ≤ 21 years of age, range 5 days to 46 years) were analyzed for Ca (n = 94), Mg (n = 97), and Phos (n = 102) with and without acidification. Analyses without acidification resulted in 27 outliers for Ca, 6 for Mg, and 17 for Phos that exceed the acceptability limits: 4% or ± 0.5 mg/dL for Ca, 12.5% or ± 0.1 mg/dL for Mg, and 5% or ± 0.15 mg/dL for Phos. Sample turbidity was found to be a contributing factor to these outliers. In a subset of 37 samples, centrifugation improved the correlation between acidified and unacidified samples (R² > 0.99) for Ca (n = 35), Mg (n = 35), and Phos (n = 37). Only 3 outliers were found for Ca, 0 for Mg and 1 for Phos. The recovery of Ca in urine containing calcium oxalate (CaOx) (n = 5) was not affected by the addition of a centrifugation step in the sample pretreatment.

Our results suggest that urine acidification is not necessary for the analysis of Ca, Mg and Phos, but that centrifugation should be performed to overcome any turbidity interference.