Intact PTH Assay

Intact PTH Assay

MuirLab is pleased to announce that we will be offering the intact PTH assay as an in-house test. PTH is one of the primary hormones for calcium metabolism and maintenance of optimal calcium ion concentrations, and works directly on bone and kidney. Therefore, measurement of PTH is the most important test for the differential diagnosis of hypercalcemia.

Special Specimen Requirements

The test requires 1-2 ml of serum only, drawn in an SST tube. Because of the nocturnal rise in intact PTH levels, samples should preferably be collected in the morning, after 7 AM, but specimens drawn throughout the day will be accepted. Once the specimen is drawn and is allowed to clot for 30 - 60 minutes (or at most 2 hours) at room temperature, it will be necessary to centrifuge and immediately place the specimen on ice. The specimen will then be handled as a Critical Transport and should be transported on wet ice to INPATIENT processing at John Muir Medical Center (Walnut Creek) within 5 hours of the collection time. The specimen will need to be brought directly to Special Chemistry on ice or in a cold block.

Feedback Mechanism for Calcium Regulation

In healthy individuals, PTH is secreted according to a negative feedback mechanism. A lowered circulating calcium level, triggers a pronounced increase in PTH secretion. Higher than normal calcium levels inhibit PTH secretion. PTH is synthesized and secreted by the parathyroid glands located near the thyroid glands. PTH raises serum ionized calcium levels by increasing the rate of calcium ion flow from bone to the extracellular fluid, and increases both the renal tubular readsorption of ionized calcium and the renal excretion of phosphate. Long-term regulation of total body calcium by PTH occurs in combination with its stimulation of vitamin D metabolism, resulting in enhanced intestinal adsorption of ionized calcium.

PTH Fragments

Intact PTH contains 84 amino acids and has a molecular weight of 9425. PTH undergoes proteolysis to a lesser extent in the parathyroid glands but mostly peripherally- especially in the liver but also in the kidneys and bone- to yield N-terminal fragments and longer lived C-terminal and midregion fragments. The N-terminal fragment contains the region that confers bioactivity. C-terminal and N-terminal fragments are initially generated in equivalent amounts, but the N-terminal fragments disappear rapidly. The C-terminal fragment has a half-life of several hours. In renal failure, C-terminal fragment clearance is impaired, so that high levels are found. C-terminal assays, as well as midregion assays, are especially unreliable in chronic renal failure, where increased PTH is typically just a reflection of impaired renal clearance. In normal renal function, intact PTH is the greatest part of circulating PTH-like bioactivity.

Clinical Significance

In hypercalcemia due to primary hyperparathyroidism production, the majority of patients have elevated PTH levels. By contrast, in hypercalcemia due to malignancy or other causes, the PTH levels are typically low or within normal ranges. A finding of increased PTH in patients with hypercalcemia and malignancy suggests coexisting hyperparathyroidism and malignancy, since ectopic PTH production appears to be extremely rare. PTH levels are also usually high in secondary hyperparathyroidism- usually associated with renal failure- as a result of constant stimulation of the parathyroid gland by low calcium levels. Hypocalcemia together with a low PTH level, on the other hand is to be expected in hypoparathyroidism, either postsurgical or idiopathic.

Types of Assays

Immunoassays for various specific PTH fragments have been developed, by relying on antisera specific for a discrete region such as the C-terminal, the N-terminal, or midmolecule. The antisera employed in these assays recognize not only the specific region in the intact molecule, but similar fragments as well. Recent assays for intact PTH have the necessary sensitivity for detecting circulating intact PTH (amino acid sequence 1-84) in normals and for discriminating between normals and those with primary hyperparathyroidism. These intact assays also appear to discriminate better between primary hyperparathyroidism and hypercalcemia of malignancy compared with previous assays, and do so without significant overlap between the groups.

The intact PTH assay at MuirLab is a solid-phase, two-site chemiluminescent enzyme-labeled immunometric assay.

References:

  1. IMMULITE 2000 Intact PTH Package Insert (PIL2KPP-14, 2005-04-05)
  2. Burtis C, Ashwood E, Bruns D. Tietz, Textbook of Clinical Chemistry and Molecular Diagnostics 2006: 913-919