Acidification and urine calcium: is it a preanalytical necessity?
BACKGROUND
It has been suggested that for the accurate measurement of calcium in urine, samples must be collected into bottles containing acid. Acidification poses risks to both patients and laboratory staff. Here we reappraise whether acidification is a preanalytical necessity.


METHODS
Twenty-four-hour urine samples were collected from 133 patients into bottles without acid or preservatives. In a subset of 29 patients, 10 mL aliquots were prepared to test the effect on urine calcium of 0.1, 1.0 and 5.0 mol/L hydrochloric acid (HCl). Calcium was then measured immediately after acidification, after 12 h and seven days storage at 4 degrees C. In a separate study, urine calcium concentrations in paired control (non-acidified) and acidified (with 5 mol/L HCl) samples were compared in 133 patients. When available, we recorded the time from start of urine collection to time of analysis. Calcium was measured using the cresolphthalein complexone colorimetric endpoint assay on the 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 between basal urine pH and urine calcium at all time points studied.


CONCLUSIONS
Our results suggest that the acidification of urine samples is not a preanalytical necessity for the measurement of urine calcium.
Sodi
R
R
Department of Clinical Biochemistry & Metabolic Medicine, Royal Liverpool & Broadgreen University Hospital, UK. ravsodi@yahoo.com
Bailey
L B
LB
Glaysher
J
J
Allars
L
L
Roberts
N B
NB
Marks
E M
EM
Fraser
W D
WD
D000143:Acids / Q000652:urine*; D002118:Calcium / Q000652:urine*; D002624:Chemistry, Clinical / Q000379:methods / Q000592:standards; D004305:Dose-Response Relationship, Drug; D006801:Humans; D006851:Hydrochloric Acid / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D013696:Temperature; D013997:Time Factors; D016482:Urinalysis / Q000379:methods*
D000143:Acids; D006851:Hydrochloric Acid; D002118:Calcium