Analytically accurate, clinically inaccurate: A diagnostic case of Pseudohyperparathyroidism.
Chelsea B Swartchick, Jodi L Boysen, Ryan W Pearce et al.
✦ AI-curated · Sources linked
Case in brief
A 33-year-old kidney transplant recipient with prior secondary hyperparathyroidism treated by parathyroidectomy had a PTH result above the assay’s analytical measurement range, despite normal serum calcium and phosphorus. Serial dilution showed proportional recovery, while polyethylene glycol precipitation and heterophile-antibody blocking did not bring the result into range, making significant immunoassay interference unlikely. Investigation instead identified a collection-site-dependent elevation, not systemic PTH hypersecretion; repeat sampling from another site confirmed the discrepancy. The finding was relevant given prior parathyroid autotransplantation and limited vascular access.
What made this case unusual
The PTH elevation depended on the blood collection site despite analytically consistent assay results, creating a clinically misleading result in a patient with prior parathyroid autotransplantation.
Diagnostic / clinical pearl
When PTH is markedly discordant with calcium and phosphorus, consider preanalytical factors, including collection site, even after common immunoassay interference has been investigated.
Why it matters
Analytical validity does not guarantee clinical validity; collaboration between clinicians and the laboratory can help identify specimen-related causes of discordant results and avoid misinterpretation.