Case Report: Falsely elevated PTH level in a young woman caused by immunoassay interference resulting from macro-PTH

Area of Science:

• Endocrinology
• Clinical Chemistry
• Laboratory Medicine

Background:

• Elevated parathyroid hormone (PTH) levels typically indicate parathyroid disorders.
• Immunoassay interference, particularly macro-PTH, can cause falsely high PTH readings, leading to diagnostic errors.

Observation:

• A 27-year-old woman presented with persistently elevated PTH levels, normal serum calcium, phosphate, and vitamin D.
• Standard imaging (ultrasound, sestamibi, PET-CT) showed no parathyroid abnormalities.
• A polyethylene glycol (PEG) 6000 precipitation assay was performed to investigate immunoassay interference.

Findings:

• The PEG assay significantly reduced the patient's PTH levels from >1200 pg/mL to ~40 pg/mL (97% decrease), indicating macro-PTH.
• Control samples with known parathyroid pathologies showed smaller PTH reductions (65-72%) with PEG precipitation.
• This suggests PEG selectively targets macro-PTH, distinguishing it from intact PTH.

Implications:

• Unexpectedly high PTH with normal electrolytes and imaging may suggest immunoassay interference.
• PEG 6000 precipitation is a valuable tool for detecting macro-PTH.
• Improved immunoassay techniques are needed for more reliable PTH measurements in clinical practice.