Asymptomatic elevated parathyroid hormone level due to immunoassay interference.

Area of Science:

• Clinical Chemistry
• Immunology
• Endocrinology

Background:

• Immunoassays for parathyroid hormone (PTH) are susceptible to interference from various proteins, affecting up to 6% of tests.
• Common interferents include heterophile antibodies, human anti-animal antibodies, and rheumatoid factor.
• Persistent high PTH levels without clinical or biochemical abnormalities can indicate assay interference.

Observation:

• A 59-year-old woman presented with persistently high PTH levels (385-482 pg/ml) across two different assays.
• Despite elevated PTH, her calcium, phosphorus, vitamin D, and renal function remained normal, with no clinical signs of bone disease.

Findings:

• Serial dilution tests showed nonlinearity, and a polyethylene glycol precipitation test recovered minimal PTH, strongly suggesting interference.
• These results indicated the presence of heterophile antibodies causing falsely elevated PTH measurements.

Implications:

• Awareness of immunoassay interference is crucial for clinicians to avoid misinterpreting PTH levels.
• Proper diagnostic workup can prevent unnecessary investigations and inappropriate treatments for conditions like hyperparathyroidism.