Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1101 page)

Decreased In

   Autoimmune hypoparathyroidism

   Sarcoidosis

   Nonparathyroid hypercalcemia in the absence of renal failure

   Hyperthyroidism

   Hypomagnesemia

   Transient neonatal hypocalcemia

   DiGeorge syndrome

   Limitations

   At this time, there are significant intermethod variations in PTH results for different manufacturer assays. This is mainly due to cross-reactivity with various PTH fragments of the assay.

   The finding of a persistently high-normal calcium accompanied by a high-normal PTH (alternatively, a low-normal calcium accompanied by a low-normal PTH) warrants further investigation; for the PTH, although itself within normal limits, may still be inappropriately high (or inappropriately low) relative to the circulating calcium level.

   Because of a pronounced nocturnal rise in intact PTH levels observed in a small experimental male population, sampling after 10
AM
for optimum discrimination between normals and those with mild primary hyperparathyroidism has been suggested.

   Sedative–hypnotic drug propofol (Diprivan) may give falsely low PTH values.

   High concentrations of hemolysis, lipemia, and bilirubin should be avoided.