PILOT STUDY OF THE NEED TO ACIDIFY URINE FOR THE MEASUREMENT OF 24-HOUR CALCIUM EXCRETION

1. PILOT STUDY OF THE NEED TO ACIDIFY URINE FOR THE MEASUREMENT OF 24-HOUR CALCIUM EXCRETION D Masters1, G Streitberg1, N Wijeratne1,2, JCG Doery1,2, ZX Lu 1,2 1 Monash Medical Centre and 2Monash University, Clayton, Victoria daniel.masters@monashhealth.org.au RESULTS No significant difference in urinary calcium concentrations was found for untreated urine kept at room temperature, untreated urine at 4C and acidified urine at 4C (all p>0.9 compared to baseline). The maximumchanges in mean calcium concentrations over the duration of the study were <5%, below the defined significant analytical changes. On the other hand, urine calcium decreased significantly within 1-2 hours of collection in alkalinised condition and the values became extremely low or unmeasurable from day2. INTRODUCTION Hypercalciuriacan lead to formation of calcium based urinary calculi (Fig. 1a&b). Traditionally, acidified 24h urine has been used for investigation of calcium excretion. The use of acid aims to prevent calcium precipitation in the urine, however, it may be hazardous to the patient. It may also prevent the use of the same specimen for the measurements of other analytes, such as urinary protein. In addition, recent studies have indicated that acidification may not be necessary.1&2 In this study, the stability of calcium with and without acidification was assessed over six days using freshly collected spot urine. Spot urine was used to avoid potential calcium precipitation during 24h urine collection prior to the experiment. * ** ** Figure 1a Calcium based urinary calculi Figure 1b Electron microscopic photo of a calcium calculi METHOD Spot urine specimens were collected from 11 healthy adults into plain urine containers. Within 1-hour of collection, aliquots were assayed in duplicate for calcium concentration (baseline) using a Beckman Coulter DxC800. The specimens were then divided into four 10-mL tubes stored under the following conditions: (1) Untreated and at room temperature (2) Untreated and at 4C (3) Acidified (using 1 mol/L HCl to pH 3-4) and at 4C (4) Alkalinised (1 mol/L NaOH to pH >9) and at 4C Each tube was then analysed for calcium for up to 6days. At the time of analysis, each tube was mixed thoroughly and after having taken an aliquot for testing returned to appropriate storage. Significant analytical change was defined as initial value ±2.77CVa (urine calcium CVa =2.0%). 1 1.1 2 3 6 CONCLUSION This pilot study suggests that acidification during 24h collection for urinary calcium excretion may not be necessary. However alkalinised urine is unsuitable. Further study is required to confirm this finding in people with hypercalciuria, as all the volunteers in this study were taken from a healthy population with normal calcium excretion. • REFERENCES: • McConnell W and Vasikaran SD. Spot urine analysis: acidification does not increase calcium recovery. Ann ClinBiochem 2002; 39: 64- 65. • SodiR et al. Acidification and urine calcium: is it a preanalytical necessity? Ann ClinBiochem 2009; 46: 484–487.