Acidification and urine calcium: is it a preanalytical necessity?

This version was published on 1 November 2009

Ann Clin Biochem 2009;46:484-487
doi:10.1258/acb.2009.009027
© 2009 Association for Clinical Biochemistry

 

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Original Articles


R Sodi1,
L B Bailey1,
J Glaysher2,
L Allars1,
N B Roberts1,
E M Marks1 and
W D Fraser1


1 Department of Clinical Biochemistry & Metabolic Medicine, Royal Liverpool & Broadgreen University Hospital, Prescot Street, Liverpool L7 8XP;
2 Department of Clinical Biochemistry, Booth Hall Children’s Hospital, Charlestown Road, Manchester M9 7AA, UK


Corresponding author: R Sodi. Email: ravsodi{at}yahoo.com


Background: It has been suggested that for the accurate measurement of calciumin urine, samples must be collected into bottles containingacid. Acidification poses risks to both patients and laboratorystaff. Here we reappraise whether acidification is a preanalyticalnecessity.

Methods: Twenty-four-hour urine samples were collected from 133 patientsinto bottles without acid or preservatives. In a subset of 29patients, 10 mL aliquots were prepared to test the effect onurine calcium of 0.1, 1.0 and 5.0 mol/L hydrochloric acid (HCl).Calcium was then measured immediately after acidification, after12 h and seven days storage at 4°C. In a separate study,urine calcium concentrations in paired control (non-acidified)and acidified (with 5 mol/L HCl) samples were compared in 133patients. When available, we recorded the time from start ofurine collection to time of analysis. Calcium was measured usingthe cresolphthalein complexone colorimetric endpoint assay onthe Roche Modular system.

Results: There was no significant difference in the calcium concentration in the 29 cases studied between the varying acid concentrations tested compared with non-acidified urine (P = 0.987). Overall, in 133 patients there was no difference between control and acidified samples (P = 0.888). We found no correlation betweenbasal urine pH and urine calcium at all time points studied.

Conclusions: Our results suggest that the acidification of urine samplesis not a preanalytical necessity for the measurement of urinecalcium.


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