There are rare case reports of pseudohypercalcaemia secondary to calcium-binding paraproteins both from IgM and IgG. We present a case of a 76 year old male with asymptomatic PTH-dependent hypercalcaemia on a background of previous primary hyperparathyroidism treated with 3 gland parathyroidectomy 20 years prior, multiple myeloma under surveillance and bladder cancer awaiting resection.
Laboratory data showed corrected calcium 2.91mmol/L, PTH 3.6pmol/L, Vitamin D 80nmol/L, 24 urine calcium 3.1mmol/24hr with a FeCa - 0.0093 and phosphate 1.6mmol/L. PTHrp was normal at <1.0pmol/L and CASR testing showed no variant of clinical significance. Serum electrophoresis demonstrated elevated IgG and Kappa ratio. Bone marrow aspiration showed plasma cell myeloma with low level marrow infiltration (15%). BMD demonstrated a lowest T score of -1.1 at the femoral neck, bone scan did not reveal any metastatic bone disease and he had no history of renal calculi. Ionised calcium was at the lower limit of the normal range, and on retrospective review was below the reference range prior to his parathyroidectomy, raising the suspicion for pseudohypercalcaemia in the setting of paraproteinemia.
Our patient had long-standing MGUS prior to diagnosis of multiple myeloma and his Kappa IgM paraproteins were confirmed to be negatively charged, therefore plausibly falsely elevating serum total calcium levels. Further laboratory investigations are pending to confirm pseudohypercalcaemia secondary to paraproteinaemia. This case highlights the importance of excluding pseudohypercalcaemia prior to initiating treatment; particularly in the setting of paraproteinaemia and we suggest monitoring ionised calcium rather than corrected calcium.