PTH-Related Protein
Label Mnemonic: PTHRP
Epic Lab Code: LAB8205
Downtime Form: A-1a Miscellaneous Request
Commercial Mail-out Laboratory
5231 RCP
356-8593
Specimen(s):
Plasma
Specimen Instructions:
Collect fasting sample in pre-chilled 3 mL lavender EDTA tube.
Collection Medium:
Lavender top tube 3 mL (EDTA)
Minimum:
Preferred Minimum: 0.7 mL
Absolute Minimum: 0.25 mL
Turn Around Time:
2 days upon receipt at reference laboratory
Reference Range:
<2.0 pmol/L
Interpretive Data:
Depending on the patient population, up to 80% of patients with malignant tumors and hypercalcemia will be suffering from humoral hypercalcemia of malignancy (HHM). Of these, 50% to 70% might have an elevated parathyroid hormone-related peptide (PTHrP) level. These patients will also usually show typical biochemical changes of excess parathyroid hormone (PTH)-receptor activation, namely, besides the hypercalcemia, the might have hypophosphatemia, hypercalcuria, hyperphosphaturia and elevated serum alkaline phosphatase. Their PTH levels will typically be less than 30 pg/mL or undetectable.

In patients with biochemical findings that suggest but do not prove primary hyperparathyroidism (eg, hypercalcemia, but normal or near normal serum phosphate and a PTH level that is within the population reference range, but above 30 pg/mL), HMM should be considered as a diagnostic possibility, particularly if the patient is elderly, has a history of malignancy or risk factors for malignancy. An elevated PTHrP level in such a patient is highly suggestive of HHM as the cause for the hypercalcemia.
Comments:
Useful For:
• Diagnostic work-up of patients with suspected hypercalcemia of malignancy

• Diagnostic work-up of patients with hypercalcemia of unknown origin

Cautions:
The test should not be used to exclude cancer or screen tumor patients for humoral hypercalcemia of malignancy.

Parathyroid hormone-related peptide (PTHrP) can be elevated in pregnant and lactating women and in newborn infants. Nonmalignant conditions that have been described in association with elevated plasma PTHrP levels include systemic lupus erythematosus, HIV-associated lymphadenopathy, lymphedema of chest or pleural cavities, and with benign tumors of the ovary, kidney and the neuroendocrine system.

Because of the complexity of PTHrP isoforms, the differences between various PTHrP assays and the lack of a common calibration standard, PTHrP measurements performed with different assays can not be compared easily.

The complex isoform mixture of PTHrP can occasionally lead to pronounced nonlinearity on dilution of patient specimens. In these situations an accurate measurement of PTHrP concentrations might be impossible.

Like all immunometric assays, PTHrP assays are susceptible to false low results at extremely high analyte concentrations ("hooking") and to rare false positive results due to heterophile antibody interference. Therefore, if test results are incongruent with the clinical picture, the laboratory should be contacted.
Methodology:
Immunochemiluminometric Assay (ICMA)
CPT Code:
82397