Endocrine Abstracts

Patients with primary hyperparathyroidism (PHP) should have assessment of urinary calcium excretion as part of routine work up. This helps in the differential diagnosis of PHP in that urine calcium is low in familial benign hypocalciuric hypercalcaemia. Possible measures of calcium excretion include 24 h urine collection, spot sample for urine for urine calcium concentration and calcium/creatinine ratio (UCa/creat), and fractional excretion of calcium (FrExCa=urine calcium×serum creatinine/serum calcium×urine creatinine). 24 h urine collections, although perceived as gold-standard, are notoriously difficult to carry out reliably while FrExCa requires simultaneous blood and urine testing and a further calculation based these results. Spot urine for urine calcium concentration and UCa/creat on the other hand is easy to collect and requires no further calculation. It is not established whether any of the three measures of urinary calcium excretion (urine calcium concentration, UCa/creat and FrExCa) correlate with one another. Close correlation between any two would suggest that there was little point in measuring both.

We used a database of 51 confirmed cases of PHP to collect simultaneous urine and blood samples and measured urine calcium concentration, UCa/creat and FrExCa for the 51 datasets. Using Excel software we estimated correlation coefficients between the three.

The following correlation coefficients were obtained. Urine calcium concentration and UCa/creat r=0.431, Urine calcium concentration and FrExCa r=0.385, and UCa/creat and FrExCa r=0.961. UCa/creat and FrExCa were very closely correlated with each other, but both measures were weakly correlated with urine calcium concentration. Our long term objective will be to test whether any of these three measurements can be used to reliably differentiate between PHP patients and the normal population. With this in mind, since UCa/creat and FrExCa are so closely correlated there would seem little point in measuring both, and because FrExCa requires both a blood sample and a further calculation then UCa/creat would be the preferred option. We intend therefore to test whether urine calcium concentration and/or UCa/creat can be used to differentiate between PHP patients and the normal population.

DOI: 10.1530/endoabs.31.P19