No, Not All Four CRAB Criteria Are Required for Multiple Myeloma Diagnosis
The diagnosis of active multiple myeloma requires only ONE or more CRAB features (or myeloma-defining biomarkers), not all four. This is a critical distinction that prevents delays in diagnosis and treatment initiation 1.
Diagnostic Framework
The modern diagnostic criteria for symptomatic multiple myeloma require:
- ≥10% clonal bone marrow plasma cells (or biopsy-proven plasmacytoma), PLUS
- At least ONE of the following:
- Any single CRAB feature (hypercalcemia, renal insufficiency, anemia, OR bone lesions)
- Any myeloma-defining biomarker (SLiM criteria)
CRAB Criteria Specifics
You need only one of these four features 2, 1:
- Hypercalcemia: Serum calcium ≥11.5 mg/dL
- Renal insufficiency: Serum creatinine ≥2 mg/dL or creatinine clearance <40 mL/min
- Anemia: Hemoglobin <10 g/dL or ≥2 g/dL below normal
- Bone lesions: Lytic lesions on skeletal radiography, whole-body MRI, or FDG PET/CT
Expanded Myeloma-Defining Events (SLiM Criteria)
Since 2014, the International Myeloma Working Group added biomarkers that allow diagnosis before traditional end-organ damage occurs 1, 3:
- ≥60% clonal plasma cells in bone marrow
- Involved/uninvolved free light chain ratio ≥100 (with involved FLC ≥100 mg/L)
- >1 focal lesion on MRI (non-osteolytic)
Clinical Implications
Common Pitfall to Avoid
Do not wait for multiple CRAB features to develop before diagnosing myeloma. This outdated approach leads to unnecessary end-organ damage and increased morbidity 4, 5. The philosophy shifted from requiring overt end-organ damage to recognizing subclinical risk factors 3.
Real-World Presentation
Patients frequently present with only one or two CRAB features 6, 7. For example, a patient may have only anemia and bone lesions without hypercalcemia or renal failure and still meet diagnostic criteria for active myeloma requiring treatment 6.
Contrast with Smoldering Myeloma
Smoldering (asymptomatic) myeloma is defined by:
- Serum M-protein ≥30 g/L and/or 10-59% clonal bone marrow plasma cells
- Absence of ALL CRAB features AND myeloma-defining biomarkers 1
This distinction is crucial: smoldering myeloma requires the absence of all CRAB criteria, while active myeloma requires the presence of just one.
Treatment Initiation
Once any single CRAB feature or myeloma-defining biomarker is identified, treatment should begin promptly 1, 4, 5. The presence of even one criterion indicates active disease requiring intervention to prevent further organ damage and improve outcomes.