What are the CRAB criteria for symptomatic multiple myeloma and how should each manifestation be managed?

CRAB Criteria for Multiple Myeloma: Definition and Management

What CRAB Stands For

CRAB criteria define the end-organ damage that distinguishes symptomatic multiple myeloma from asymptomatic disease and mandates immediate treatment. 1

The acronym represents four specific manifestations:

• C = Hypercalcemia: Serum calcium >11.5 mg/dL 2, 1
• R = Renal insufficiency: Serum creatinine >2 mg/dL (>173 µmol/L) OR creatinine clearance <40 mL/min 2, 1
• A = Anemia: Hemoglobin <10 g/dL OR ≥2 g/dL below the lower limit of normal (normochromic, normocytic pattern) 2, 1
• B = Bone disease: Lytic lesions on skeletal survey, severe osteopenia, OR pathologic fractures 2, 1

Diagnostic Requirements

Multiple myeloma requires BOTH ≥10% clonal plasma cells on bone marrow examination (or biopsy-proven plasmacytoma) AND at least one CRAB criterion attributable to the plasma cell disorder. 2, 1

Essential confirmatory workup includes:

• Serum and urine protein electrophoresis with immunofixation to identify monoclonal protein 1
• Nephelometric quantification of IgG, IgA, and IgM 1
• Serum free light chain assay with kappa/lambda ratio 1
• Bone marrow aspirate and biopsy with CD138 staining to quantify plasma cells 1
• Cytogenetic/FISH studies for risk stratification (del(17p), t(4;14), t(14;16), del(13q)) 1
• Skeletal survey or preferably whole-body low-dose CT or PET/CT for bone lesions 1

Management of Each CRAB Manifestation

Hypercalcemia (C)

Aggressive hydration with normal saline (>3 L/24h) is first-line management, followed by bisphosphonates. 3, 4

• Administer zoledronic acid or pamidronate intravenously 3
• Hydration serves dual purpose: treats hypercalcemia and protects renal function 3
• Avoid NSAIDs which can worsen renal function 3

Renal Insufficiency (R)

Bortezomib-based regimens are preferred because they can be safely administered without dose adjustment in renal impairment. 3, 4

• Maintain aggressive hydration with normal saline 3
• Avoid nephrotoxic agents including NSAIDs 3
• Bortezomib does not require dose reduction even with severe renal dysfunction 3, 4

Anemia (A)

Anemia typically improves with effective anti-myeloma therapy; transfusion support may be needed initially. 2

• Red blood cell transfusions for symptomatic anemia while initiating treatment 2
• Monitor hemoglobin response to myeloma-directed therapy 2

Bone Disease (B)

Long-term bisphosphonates (zoledronic acid or pamidronate) reduce skeletal-related events and should be initiated immediately. 3, 4

• Administer monthly bisphosphonates to all patients with bone lesions 3, 4
• Consider radiation therapy for impending fractures or spinal cord compression 1
• Orthopedic consultation for pathologic fractures or structural instability 1

Treatment Initiation Algorithm

Treatment must be started immediately in all patients meeting CRAB criteria; delaying therapy increases morbidity and mortality. 3, 4

For Transplant-Eligible Patients (<65 years, fit):

• Induction: Bortezomib, lenalidomide, dexamethasone (VRd) for 3-4 cycles 4
• Consolidation: High-dose melphalan 200 mg/m² IV followed by autologous stem cell transplantation 4
• Maintenance: Continue therapy post-transplant 4

For Transplant-Ineligible Patients (≥65 years or unfit):

• First choice: Bortezomib, melphalan, prednisone (VMP) for 8-12 cycles 2, 3, 4
• Alternative: Melphalan, prednisone, thalidomide (MPT) 2, 3
• Avoid: Classical VAD regimen (vincristine, adriamycin, dexamethasone) as it is inferior and damages stem cells 4

Special Considerations for Renal Impairment:

Prioritize bortezomib-based regimens (VMP or bortezomib-dexamethasone) as they do not require dose adjustment. 3, 4

Critical Distinctions to Avoid Misdiagnosis

CRAB criteria are not pathognomonic for multiple myeloma; lymphoma can present identically. 5, 6

• If serum/urine protein electrophoresis and bone marrow are negative for plasma cells, consider lymphoma 5, 6
• Diffuse large B-cell lymphoma can cause hypercalcemia via PTHrP secretion, osteolytic lesions, anemia, and renal dysfunction 5
• Always confirm ≥10% clonal plasma cells before diagnosing multiple myeloma 1, 3

Distinguish from Smoldering Multiple Myeloma:

• Smoldering MM: M-protein ≥3 g/dL and/or bone marrow plasma cells ≥10% WITHOUT any CRAB features 2, 1
• Smoldering MM does NOT require immediate treatment and should be monitored closely 2, 1
• Progression rate: 10% per year for first 5 years in smoldering MM versus 1% per year in MGUS 1

Prognostic Implications

Among CRAB features, bone disease and hypercalcemia confer the worst prognosis in the novel agent era, while renal failure shows improved outcomes with modern therapy. 7

• Bone disease is the most common CRAB feature and has the strongest negative prognostic impact 7
• Hypercalcemia significantly worsens survival 7
• Novel agents have improved outcomes for patients with renal failure compared to conventional therapy 7
• Anemia alone does not significantly impact survival in the modern treatment era 7