Endocrine Abstracts

Aims: To assess the short-term (one day) and longer-term (10–113 days) test-retest reliability of calcium:creatinine (Ca:Cr) ratio calculated from 24-hour urine collection and its surrogate fractional excretion of calcium (FE_Ca) from urine aliquots.

Methods: On the first visit, eight subjects (aged 22–55 years) collected two fasting urine aliquots (5 ml) at the start and end of their non-fasting 24-hour urine collection for FE_Ca and Ca:Cr ratio calculations respectively. The procedure was repeated on a second visit 10–113 days later. Test-retest reliability was assessed using bivariate Pearson correlations.

Results: The correlations between single FE_Ca values repeated over short-term or longer-term were relatively weak, but strengthened when the average of two consecutive FE_Ca values from the first visit was compared with the corresponding average from the second visit (r=0.94, P=0.002; Table). There was a significant correlation between Ca:Cr ratios obtained between visits (r=0.86, P=0.007). Paired t-tests showed no significant differences in FECa or Ca:Cr ratio obtained between visits.

Conclusions: These findings suggest that two consecutive 5ml of fasting urine aliquots are required for reliable computation of FE_Ca up to 16 weeks apart, and appear to be more repeatable than Ca:Cr ratio calculated from non-fasting 24-hour urine.