Differential diagnosis between hypoparathyroidism and pseudo-hypoparathyroidism.
Patients with idiopathic hypoparathyroidism show a marked rise in urine concentrations of cAMP following infusion of parathyroid hormone (PTH). Patients with pseudohypoparathyroidism show little or no change in urinary cAMP in response to PTH.
The test should be carried out in the morning after an overnight fast from midnight, but ensure adequate hydration. Before starting the test, the patient must void any overnight urine, and a blood sample (5 mL) must be taken for PTH measurement. Ensure the patient takes 150 mL water, in sips, during each 30 min period of the test to ensure a good urine flow. Throughout the course of the test, collect a urine sample after each of six consecutive 30 min periods. After collection of the first two urine samples, infuse, via an indwelling catheter in an antecubital vein, PTH solution (see below) at 1 mL per min for 10 min. Collect the third urine sample at the end of the 30 min period and collect three more 30 min urine samples.
Prepare PTH (Parathar, synthetic human parathyroid hormone, 1-34) for infusion by reconstituting 1 vial of Parathar with 10 mL of diluent also supplied. In children, 3 years of age or older, the recommended dose is 3 units per kg of body weight, up to a maximum of 200 units. Reconstituted solution should be used within 4h and any unused discarded.
Parathar (200 units) is available from:
IDIS Ltd, 6/7 Canbury Business Park, Elm Crescent, Kingston-upon-Thames, Surrey KT2 6BR.
Tel: 020 8549 1355; Fax 020 8547 1372.
Transfer the blood (5 mL) to a plain tube and transfer to the laboratory on ice. Allow to clot for up to lh. Transfer the serum to a plastic tube (preferably 12 x 75 mm). Send a portion to the local laboratory for measurement of calcium. Store the remainder at -200C.
Send the frozen sample (2 mL) to the appropriate SAS laboratory. Ensure the sample remains frozen during transport.
Measure the volume of each 30 min urine collection and record this with the timing of each collection on the SAS request form. Send a portion (10 mL) of each collection to the local laboratory for measurement of calcium, phosphate and creatinine. Take a portion (5 mL) of each 30 min sample, freeze and send to the SAS laboratory for measurement of cAMP. Ensure the samples remain frozen during transport. Record on the SAS request form the values for calcium, phosphate and creatinine.
Urinary cAMP/creatinine ratio:
Adults: Approximately 0.5 nmol/mmol;
Following infusion of PTH, a rise of greater than sixfold should be observed.
Centre offering this assay:
Cardiff UHW Hormone and Steroid Laboratory.