Pre-, post- or no acidification of urine samples for calcium analysis: does it matter?

Background Measuring 24 h-urine calcium concentration is essential to evaluate calcium metabolism and excretion. Manufacturers recommend acidifying the urine before a measurement to ensure calcium solubility, but the literature offers controversial information on this pre-analytical treatment. The objectives of the study were (1) to compare pre-acidification (during urine collection) versus post-acidification (in the laboratory), and (2) to evaluate the impact of acidification on urinary calcium measurements in a large cohort. Methods We evaluated the effects of pre- and post-acidification on 24-h urine samples collected from 10 healthy volunteers. We further studied the impact of acidification on the calcium results for 567 urine samples from routine laboratory practice, including 46 hypercalciuria (≥7.5 mmol/24 h) samples. Results Calciuria values in healthy volunteers ranged from 0.6 to 12.5 mmol/24 h, and no statistical significance was found between non-acidified, pre-acidified and post-acidified conditions. A comparison of the values (ranging from 0.21 to 29.32 mmol/L) for 567 urine samples before and after acidification indicated 25 samples (4.4%) with analytical differences outside limits of acceptance. The bias observed for these deviant values ranged from -3.07 to 1.32 mmol/L; no patient was re-classified as hypercalciuric after acidification, and three patients with hypercalciuria were classified as normocalciuric after acidification. These three deviant patients represent 6.5% of hypercalciuric patients. Conclusions Our results indicate that pre- and post-acidification of urine is not necessary prior to routine calcium analysis.