What are the key diagnostic criteria for multiple myeloma that I should consider in a patient presenting with unexplained bone pain?

Key diagnostic criteria for multiple myeloma in a patient presenting with unexplained bone pain include:

• Presence of a paraprotein detected by serum protein electrophoresis and confirmed by serum immunofixation, indicating possible myeloma or monoclonal gammopathy of undetermined significance (MGUS).
• Serum-free light-chain assay and serum-free light-chain ratio to assess for abnormal light chains and prognosis.
• Bone marrow aspirate and trephine biopsy showing increased plasma cell percentage and abnormal plasma cell phenotype confirmed by morphology and flow cytometry.
• Imaging evidence of myeloma-related bone disease, preferably by whole-body MRI as first-line imaging, or whole-body low-dose CT if MRI is unsuitable; skeletal survey only if these are not possible.
• Laboratory tests including full blood count, calcium levels, and markers of inflammation such as plasma viscosity or erythrocyte sedimentation rate.

These criteria together help confirm the diagnosis of multiple myeloma in the context of unexplained bone pain and guide further management.

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