Calcium (Urine)

Diagnostic Use

For investigation of renal calculi. Diagnosis of familial hypocalciuric hypercalcaemia – 24 h urine calcium/creatinine clearance ratio <0.01.

Renal calculi: Urine calcium should be measured in patients with renal stones to determine whether hypercalciuria is present. A 24 h urine provides the best diagnostic information. Spot urine samples (calcium/creatinine ratio) are less definitive, but can also give a useful indication as to whether hyper- or hypocalciuria is present.

Other indications : Urine calcium is usually high in hypercalcaemic states, and low in hypocalcaemia. Rare exceptions to this include mutations of the calcium-sensing receptor causing familial hypocalciuric hypercalciemia (FHH).

Calcium/creatinine clearance ratio (UCCR, fractional excretion of calcium): the UCCR may be useful in distinguishing FHH from primary hyperparathyroidism. This it not the same as the urine calcium/creatinine ratio. It requires both plasma and urine concentrations of calcium and creatinine measured together. The calcium/creatinine ratio (which requires only calcium and creatinine on a urine sample) is not the appropriate test. FHH is an uncommon condition, usually associated with mild hypercalcaemia with upper normal or sometimes slightly increased PTH levels. Patients typically have a family history of hypercalcaemia from a young age. Confirmation can be made by sequencing of the calcium sensing receptor.

UCCR = (urine Ca x plasma creatinine) / (plasma Ca x urine creatinine x 1000)

where plasma creatinine is in umol/L and all other quantities are in mmol/L. The corrected plasma calcium value should be used.

The UCCR is typically <0.01 in FHH and >0.01 in primary hyperparathyroidism. Reported sensitivity is 85% and specificity 88% (there is a modest overlap). Ideally the analysis should be performed on a 24hr urine sample and paired serum sample for calcium and creatinine. It has not formally been validated on spot urine sample collects but is often used in this way. It is important to consider other causes of hypercalcaemia with relative hypocalciuria, e.g. thiazides or lithium treatment.