CRAB: Calcium raised, Renal failure, Anaemia, Bone lesions
Expansion
CRAB:
- C - Calcium raised, from osteoclast activation
- R - Renal impairment, from light chain cast nephropathy, hypercalcaemia, amyloid and hyperuricaemia
- A - Anaemia, from marrow infiltration
- B - Bone lesions: lytic, causing pain and pathological fracture
Pathology: a clonal proliferation of plasma cells producing a monoclonal paraprotein, usually IgG or IgA, with free light chains appearing in urine as Bence Jones protein.
Investigations: serum and urine electrophoresis with immunofixation, serum free light chain ratio, marrow aspirate showing over 10 per cent plasma cells, and skeletal survey or whole body MRI showing lytic lesions and the pepper pot skull. The film shows rouleaux and the ESR is markedly raised.
Note that alkaline phosphatase and isotope bone scans are typically normal, because the lesions are purely lytic with no osteoblastic reaction, which distinguishes myeloma from metastatic carcinoma.
MGUS is the asymptomatic precursor, with a paraprotein but no CRAB features, and progresses at about 1 per cent a year.