S3251 A Diagnostic Challenge: CA 19-9 Results Interference From Multivitamin Use

Introduction: Carbohydrate antigen 19-9 (CA 19-9) has high sensitivity and specificity for pancreatic and biliary tract tumors; however, its interpretation can be complicated by non-malignant conditions and laboratory assay interference. Here, we present a case of an asymptomatic woman with a significantly elevated level of Ca 19-9 above 1500. Case Description/Methods: A 65-year-old woman was evaluated by gynecologist due to postmenopausal bleeding. The Ultrasound showed a large submucosal uterine fibroid and asymmetric bilateral ovarian cysts. Magnetic resonance imaging of the pelvis confirmed the findings. CA125 and CA 19-9 markers were drawn to rule out malignancy. While Ca 125 was normal, CA 19-9 returned elevated to 428 u/mL, and on the repeat, it increased to 1691 u/mL. Endometrial biopsy returned with no evidence of epithelial atypia or malignancy. Due to concern for pancreatic malignancy, she was referred to a gastroenterologist. On endoscopic ultrasound (EUS), the examined pancreatic parenchyma appeared unremarkable. The pancreatic and biliary ducts were nondilated. Gallbladder appeared unremarkable. A colonoscopy performed 2 years ago showed no significant findings. Given that no clear etiology was identified, she was recommended to undergo a positron emission tomography computed tomography scan to rule out any occult lesions, which resulted with normal findings. Due to high concern for laboratory error caused by outside medication, she was asked about the intake of any supplements or non-prescribed medications, where the patient confirmed taking vitamins with high biotin content. After she stopped taking them, the CA 19-9 level was repeated and decreased to 596 u/mL. Discussion: Many current immunoassays, including those for CA 19-9, utilize the biotin-streptavidin binding system to capture or detect antigens. High levels of circulating biotin, especially from supplements exceeding 1 mg per day, can disrupt this interaction, leading to falsely high or low results. In our case, the patient’s markedly elevated CA 19-9 prompted an extensive workup for gastrointestinal malignancy, including EUS, PET-computed tomography, and colonoscopy—all of which were normal. The resolution of CA 19-9 levels following discontinuation of a biotin-rich supplement strongly supports the likelihood of assay interference by biotin rather than actual pathology. Given the increased use of over-the-counter biotin-containing supplements, our case shows the importance of considering biotin supplementation as a potential confounder in Ca 19-9 test results.