PTH immunoassay interference: differential diagnosis with normocalcemic primary hyperparathyroidism?

Area of Science:

• Endocrinology
• Clinical Chemistry
• Laboratory Medicine

Background:

• Differentiating normocalcemic hyperparathyroidism from secondary causes of elevated parathyroid hormone (PTH) is a diagnostic challenge.
• Immunoassays for PTH, while sensitive, can be subject to analytical interferences, impacting accuracy.
• Heterophilic antibodies are a known cause of interference in immunometric assays.

Observation:

• Two patients presented with normocalcemia and elevated PTH levels by a second-generation Roche electrochemiluminescence assay (ECLIA).
• Subsequent testing on a different platform (third-generation Roche Elecsys ECLIA) revealed normal PTH levels.
• Serial dilutions with normal mouse serum in the second-generation ECLIA showed a lack of linearity, confirming assay interference.

Findings:

• The study demonstrates PTH immunoassay interference in two patients, leading to falsely elevated PTH results.
• Interference was identified using a second-generation ECLIA, while a third-generation assay provided accurate results.
• Lack of serial dilution linearity confirmed the presence of assay interference.

Implications:

• Immunoassay interference can lead to misinterpretation of laboratory results, potentially causing incorrect diagnoses.
• Physicians must consider assay interference when laboratory findings do not align with clinical presentation.
• Recognizing and addressing PTH immunoassay interference is crucial to prevent erroneous diagnoses and unnecessary medical interventions.