7337 High Parathyroid Hormone (PTH) Due To Immunoassay Interference

Abstract Disclosure: V.A. Mendpara: None. A.J. McShane: None. L.Z. Khan: None. Background: PTH levels are not always accurate. This case illustrates immunoassay interference in a female with markedly abnormal PTH values. Understanding immunoassay interference in diverse hormonal assessments can prevent diagnostic inaccuracies. Clinical Case: A 63-year-old female with osteopenia underwent serum parathyroid hormone (PTH) testing as part of a thorough bone evaluation. The initial PTH level revealed an abnormal level of 4008 pg/mL (normal range: 15 to 65 pg/mL). The follow-up PTH level with the same assay continued to result in elevated values of >5000 pg/mL. The medication list was negative for biotin use, which is a well-known interfering substance in the PTH assay. Clinical evaluation was negative for hyperparathyroidism symptoms, and thyroid/parathyroid ultrasound was normal, as were serum levels of phosphate, vitamin D, and calcium. No PTH values had been checked for patients in the past after an extensive review of prior records. The initial PTH testing used the Roche Elecsys PTH STAT assay. To investigate a potential immunoassay interference, the following studies were performed: First, the sample was diluted with polyethylene glycol (PEG) to precipitate large molecular weight proteins; second, the sample was treated with a commercial heterophilic blocking reagent (Scantibodies Laboratory); and lastly, the sample was assayed utilizing a different PTH vendor (Siemens Atellica, chemiluminescent immunoassay). The PEG and heterophilic blocking treatments resulted in significantly lower PTH concentrations of 895.6 and 40.7 pg/mL, respectively. We do not anticipate a change in PTH levels in samples supplemented with blocking reagents or PEG without interference. The result from Siemens’ assay was markedly lower at 43.4 pg/mL. Both the Siemens and Roche assays employ mouse antibodies; however, the antibodies are proprietary between the vendors. This results in potentially different anti-mouse antibody epitope targets. Due to a history of advanced ovarian cancer, the patient was taking Oregovomab, a murine monoclonal antibody, as therapy for her advanced disease. It is unclear if this medication resulted in assay interference, but it is possible. Discussion: In summary, the discrepancy in PTH assay results highlights the need to consider immunoassay interference, particularly in patients with a history of cancer on immunotherapy agents. Our case highlights the importance of the clinician’s review of medications when assessing patients. While prior PTH assays were not conducted before the patient started Oregovomab, the potential development of interfering antibodies warrants consideration. Clinicians should be vigilant for atypical results and collaborate closely with laboratory professionals, as detecting interfering substances can result in misdiagnosis. Presentation: 6/2/2024