Should acidification of urine be performed before the analysis of calcium, phosphate and magnesium in the presence of crystals?

BACKGROUND

Acidification of urine has been recommended before testing for calcium, phosphate, and magnesium. We investigated the necessity of pre-analytical acidification in both crystallized and non-crystallized urine samples.

METHODS

From 130 urine samples obtained via routine urine analysis, 65 (50%) samples were classified as non-crystallized. All samples were divided into three groups: untreated samples, acidified samples with HCl, and acidified samples after 1h room-temperature incubation. Urine samples were measured for calcium, phosphate, magnesium, and creatinine using Modular P800 and were examined for crystals using light microscopy.

RESULTS

In crystallized samples, acidified samples with 1h incubation had significantly higher Ca/Cr, P/Cr, and Mg/Cr than did untreated samples with mean differences of 0.04, 0.03, and 0.01 mg/mg, respectively (P<0.001). In acidified samples that were analyzed immediately, crystallized samples had lower calcium concentrations than those of acidified samples with 1h incubation and a mean difference of 0.21 mg/dl (P = 0.025). None of the sample differences which exceeded the critical difference of urinary Ca, P and Mg was observed.

CONCLUSIONS

Acidification of urine should be performed before the measurement of Ca, P, and Mg in the presence of urinary crystals. However, the lack of an acidification process does not result in a clinically significant change.