Endocrine Abstracts (2016) 44 CC4 | DOI: 10.1530/endoabs.44.CC4

Asymptomatic elevated PTH level due to immunoassay interference resulting from Macro-PTH: a case report

Paul Prodan, Eka Nandoshvili, Craig Webster & Jayadave Shakher

Heart of England NHS Foundation Trust, Birmingham, UK.

Introduction: Immunoassays are important tools in the diagnosis and management of disease; however, they are not free from interference by cross-reacting substances. Discordant clinical evidence and laboratory results raised suspicion of interference in a patient with persistently raised parathyroid hormone.

Case Presentation: Case description: A 56-year-old Caucasian female previously diagnosed with hypothyroidism consistently demonstrated elevated PTH levels with normal renal function, calcium and vitamin D concentrations. There was no clinical evidence of hyperparathyroidism and imaging of thyroid and parathyroid showed no evidence of pathology.

Results: The PTH levels were persistently elevated as measured by 2 different methods. (Roche and Abbott). PTH concentrations increased on dilution (base sample 28.5 pmol/l, 1:2 dilution 47 pmol/l). After treatment of the sample with polyethylene glycol (PEG) the PTH concentration decreased to 10.2 pmol/l. On dilution of the PEG treated sample, the diluted concentration was 12.1 pmol/l, demonstrating an appropriate response to dilution. These results indicated the presence of a Macro-PTH.

Reference range: (1.6 – 7.2 pmol/l)
Reference range: (1.6 – 7.2 pmol/l)
Oct 2012122.5 pmol/lJan 2016126.9 pmol/l
Dec 2012195.8 pmol/l
Feb 2013143.7 pmol/l
July 2013165.6 pmol/l

Conclusion: Macro-PTH is PTH bound to an immunoglobulin molecule, which prevents its clearance by the kidneys and thus increased half-life. These molecules are usually not biologically active. Interference should be sought when there is lack of clinical correlation with immunoassay hormone levels.

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