Endocrine Abstracts

Aims: To determine the relationships between calcium:creatinine (CaCr) clearance ratio and fractional excretion of calcium (FECa).

Design: Ten-week study of 15 patients aged 25–93 years undergoing investigations for parathyroid hormone (PTH)-related hypercalcaemia and 39 healthy volunteers aged 20–61 years.

Outcome measures: CaCr clearance ratio calculated from 24-hour urine collection and FECa from 5 ml fasting urine aliquots extracted from the first and the last samples of the 24-hour urine collection.

Results: Linear regression analysis was performed using each of the two FECa and their average to estimate CaCr clearance ratios (CaCr clearance ratio=0.925×Average FECa+0.5). The precision of estimate (R²) of CaCr clearance ratio was 47.5 and 50.0% by the first and last FECa. Using the average FECa to estimate CaCr clearance ratio resulted in further improvement of R² (61.9%) with less bias as indicated by regression slope of 0.925 that approaches unity. The resultant regression equation was then applied to FECa to obtain estimated CaCr clearance ratios. The measured and estimated CaCr clearance ratios were compared by Bland–Altman method: The mean error of estimate of CaCr clearance ratio using average FECa was 0.0 (95% confidence limits of bias=−0.15 to +0.15 and agreement =−1.1 to +1.1). 2×2 table analysis showed that all PTH-related hypercalcaemic patients had measured as well as estimated CaCr clearance ratios greater than the cut-off ratio of 0.01 for discriminating PHPT from familial hypocalciuric hypercalcaemia (FHH), i.e. applying the estimated CaCr clearance ratio did not result in false negatives and the positive prediction and sensitivity values were 100% (specificity was not calculated in the present study as there were no subjects with CaCr clearance ratio below 0.01).

Conclusions: FECa calculated from fasting urine aliquots may be considered for estimating CaCr clearance ratio, providing a practical method for the differential diagnosis of PTH-related hypercalcaemia dispensing with the need for 24-hour urine collections and computation of CaCr clearance ratios. Cross-validation study of separate population including FHH patients is required.