MON-507 Vitamin D Intoxication in Multiple Myeloma: Is It Something Else?

Introduction Hypercalcemia is a common finding of multiple myeloma (MM) due to various pathophysiology including osteolysis. However, other treatable causes of hypercalcemia should also be investigated and correctly interpreted in patients with MM who present with hypercalcemia. We herein report two cases of falsely elevated 25-OH Vitamin D in MM patients who presented with symptomatic hypercalcemia. Case report Case I A 63 year-old female who recently diagnosed with IgG lambda MM with extensive osseous metastasis presented with abdominal pain, progressively weakness and altered mental status. Initial laboratory tests showed serum Ca16.6 (corrected Ca 18.0) mg/dL. She received calcitonin, zoledronic acid, aggressive intravenous fluid hydration, dexamethasone, as well as furosemide. Her symptoms were gradually improved along with down trended serum Ca to 7.6 (9.5) mg/dL. Further investigation revealed serum parathyroid hormone (PTH) 8.0 pg/mL (9-73), 25-OH vitamin D via radioimmunoassay (RIA) > 96.0 ng/mL (30-50) and 1,25 OH vitamin D 12 pg/mL (18-72). After meticulous review of patient history including medication, no vitamin D supplement was found. We suspected laboratory errors. The repeated 25-OH vitamin D level was sent using liquid chromatography/mass spectrometry (LCMS) method. The result was within normal limit, 46 ng/mL (30-100). Case II A 72 year-old male with known history of stage III IgA Kappa MM treated with palliative melphalan presented with altered mental status and auditory hallucination secondary to hypercalcemia. His serum Ca on presentation was 14.9 (16.1) mg/dL. His serum 25-OH vitamin D (via RIA) was >96 ng/mL and PTH was 16.2 pg/mL. There was a high suspicious of interference. Serum 25-OH vitamin D via LCMS was done and again, resulted within normal limit, 68 ng/mL. His hypercalcemia was treated by intravenous fluid hydration, calcitonin, zoledronic acid, furosemide and dexamethasone. His serum Ca subsequently improved to 8.0 (10.3) mg/dL in conjunction with regain consciousness. Conclusion Presence of serum monoclonal immunoglobulin in MM can cause assay interference. These two cases are examples of misleading high 25-OH vitamin D level via RIA method by endogenous antibody that can bridge the capture and detection of the assay. LCMS is an alternative platform that was used to confirm 25-OH vitamin D level. Clinician must be aware of the limitations of laboratory method. Collaboration between clinical picture and laboratory result is crucial to avoid inappropriate treatment.