Ionized calcium and cyclic AMP in plasma and urine. Biochemical evaluation in calcium metabolic disease.

Measurement of ionized calcium and cAMP in plasma and urine are used as sensitive parameters for the evaluation of calcium disorders. Ionized calcium is accepted as the biologically active form of calcium in the extracellular fluid, while urine cAMP provides an in vivo receptor assay for the biologically active parathyroid hormone. When urine is included as part of the calcium metabolic investigation it usually requires 24 h urine collection with a variety of different laboratory tests. Ionized calcium and cAMP are described in the literature in terms of several derived quantities, nomenclatures, and units which are rather unsystematic. The author developed reliable techniques and proposed systematic names and symbols and reference values for these quantities. Due to the lack of guidelines for the collection of urines in calcium metabolic evaluation, the author presented a simplified protocol (4 h standardized urine collection). In clinical investigation plasma and urine cAMP have been used to differentiate idiopathic hypoparathyroidism from pseudohypoparathyroidism (PsHP) based on the results of i.v. injection of parathyroid hormone (PTH). Nephrogenous cAMP has also been used for the detection of primary and secondary hyperparathyroidism with a high nosographic sensitivity (90%) (Broadus). The author showed that measurement of cAMP after i.v. PTH was a reliable and sensitive test to establish the diagnosis of PsHP, and that the urinary cAMP was useful for the diagnosis of secondary hyperparathyroidism in patients with jejunoileal bypass, but could not confirm the high nosographic sensitivity for the diagnosis of primary hyperparathyroidism. Further data are needed for proper conclusion. Although pursued vigorously the research into idiopathic stone formation using different protocols has not prevented stone recurrence nor indicated where further progress might be made. For the evaluation of recurrent calcium disease, the author proposed a simplified 4 h standardized urine collection with plasma albumin, urinary pH, standardized excretion rate of calcium, plasma phosphate glomerular filtration rate, and nephrogenous cAMP as the most important parameters. In this way the author obtained a sensitivity of 93% and specificity of 95.6% for the diagnosis of recurrent stone former. The test may therefore be of value for predicting the risk of recurrent stone formation in the single stone former.