False Elevation of Parathyroid Hormone in a Patient With Lung Metastasis of Rectal Cancer After Immunotherapy: A Case Report and Literature Review

Background Immunoassays are commonly used in clinical laboratories to measure a variety of analytes, including hormones and tumor markers. Interference caused by rheumatoid factor (RF), heterophile antibodies, and human anti‐animal antibodies (HAAA) has been reported but is rarely identified in daily practice. Here, we report a case of falsely elevated parathyroid hormone (PTH) due to immunoassay interference and review the literature. Case Presentation A 57‐year‐old man who recovered well from lung metastasis of rectal cancer treated with bevacizumab and sintilimab for 1 year, presented to Peking University People’s Hospital with persistently high PTH levels (>1200 ng/L) measured by a Roche Elecsys assay. He had hypoadrenocorticism induced by anti‐programmed cell death 1 (PD‐1), normal renal function, normal total calcium level, and normal 25‐OH vitamin D concentration. The Beckman Coulter UniCel DxI 800 and Siemens Immulite 2000 platforms measured PTH levels of 18.3 ng/L and 8.7 ng/L, respectively. After the patient’s serum was treated with polyethylene glycol (PEG) precipitation or mouse serum, the PTH levels determined by the Roche immunoassay decreased to 56.5 ng/L and 265.3 ng/L, respectively. Conclusion Interference due to human anti‐mouse antibodies (HAMA) could be the cause of falsely elevated PTH in the patient. Physicians should realize that immunoassay interference can lead to false results and closely communicate with the laboratory to avoid misdiagnosis and inappropriate therapies.