Definitive diagnosis is by biopsy of marrow or plasmacytoma
CBC
Peripheral smear- may see Rouleaux formation of RBCs, atypical plasma cells
BMP, ionized Ca, Mg, phos
Paraproteins bind calcium, so non-ionized Ca levels could be falsely elevated
Pseudohyponatremia from paraproteinemia
Hyperphosphatemia from increased bone turnover
Serum and urine testing for monoclonal protein (to aid in initial diagnosis if patient not already known to have<unk> 
SPEP and UPEP electrophoresis
Urine analysis will show large, waxy, laminated casts due to monoclonal proteins known as Bence Jones proteinuria
Urine dipstick will be negative for protein as it detects albumin rather than monoclonal proteins
Most common monoclonal proteins in order are as follows: IgG, IgA, Kappa > lambda light chains
Other work up contingent on presentation:
MRI spine: low threshold if any clinical evidence of cord compression
Plain films of suspected pathologic fractures
Infectious work up
ECG can show shortened QT interval secondary to hypercalcemia
Paraproteins bind calcium, so non-ionized Ca levels could be falsely elevated
Pseudohyponatremia from paraproteinemia
Hyperphosphatemia from increased bone turnover
SPEP and UPEP electrophoresis
Urine analysis will show large, waxy, laminated casts due to monoclonal proteins known as Bence Jones proteinuria
Urine dipstick will be negative for protein as it detects albumin rather than monoclonal proteins
Urine dipstick will be negative for protein as it detects albumin rather than monoclonal proteins
MRI spine: low threshold if any clinical evidence of cord compression
Plain films of suspected pathologic fractures
Infectious work up
ECG can show shortened QT interval secondary to hypercalcemia
Multiple myeloma
Hemorrhage
Myelofibrosis
Hereditary
Non-Hodgkin's lymphoma
Acquired
Chronic lymphocytic leukemia
Microangiopathic Hemolytic Anemia 