Elevated Vitamin D leading to an Incidental Diagnosis of Multiple Myeloma.

A case involving the incidental diagnosis of multiple myeloma (MM) due to interference in the 25-hydroxy-vitamin D (25(OH) vitamin D) immunoassay is presented. The patient, under the care of rheumatology and receiving treatment with alendronic acid and vitamin D supplements, was referred to endocrinology for investigation of acromegaly. Acromegaly was subsequently ruled out; however, during the investigations, consistently elevated levels of 25(OH) vitamin D were noted, raising suspicion of vitamin D resistance syndrome. The laboratory and endocrinology teams engaged in discussions, and following the cessation of medication, repeated analyses for 25(OH) vitamin D and a single analysis of 1,25-dihydroxy-vitamin D levels were requested, yielding high and normal results, respectively. The laboratory conducted a three-step interference investigation, ultimately identifying a high molecular weight molecule responsible for the initially elevated 25(OH) vitamin D levels. Due to the clinical presentation of back pain, a proteinogram was requested, revealing a monoclonal band of 36 g/L. Subsequent free light chain analysis indicated an elevated ratio. With three risk factors identified, this was classified as an established MM and urgently referred to haematology for correct management. Laboratory assay interferences have the potential to disrupt the accurate diagnostic workup of patients. Collaborative discussions between laboratory and clinical teams regarding such cases aid in directing the diagnostic pathway appropriately, facilitating prompt and proper diagnosis and management.