Regarding the diagnostic criteria and laboratory findings in multiple myeloma, all of the following are true EXCEPT:

## Diagnostic Criteria for Multiple Myeloma **Key Point:** The CRAB criteria (Calcium, Renal, Anemia, Bone lesions) and the newer myeloma-defining events (MDEs) are used to diagnose symptomatic myeloma. The serum M-spike threshold of ≥3 g/dL is NOT part of CRAB; rather, it is part of the SLiM criteria (Sixty percent bone marrow plasma cells, Light chain ratio ≥100, MRI with ≥2 focal lesions) for myeloma-defining events. ### CRAB Criteria (Symptomatic Myeloma) | Criterion | Definition | | --- | --- | | **C**alcium | Serum calcium >11 mg/dL (corrected) or >2.75 mmol/L | | **R**enal | Creatinine >2 mg/dL or creatinine clearance <40 mL/min | | **A**nemia | Hemoglobin <10 g/dL or 2 g/dL below baseline | | **B**one | ≥1 osteolytic lesion on imaging | ### Myeloma-Defining Events (SLiM) | Event | Threshold | | --- | --- | | **S**ixty percent | Bone marrow clonal plasma cells ≥60% | | **L**ight chain ratio | Serum free light chain ratio (involved/uninvolved) ≥100 | | **M**RI | ≥2 focal lesions on whole-body MRI | **High-Yield:** The M-spike of ≥3 g/dL is used to define MGUS (monoclonal gammopathy of undetermined significance) and smoldering myeloma, but it is NOT a diagnostic criterion for symptomatic myeloma. The distinction between CRAB and SLiM is frequently tested. **Mnemonic:** **CRAB** = Calcium, Renal, Anemia, Bone lesions (symptomatic disease). **SLiM** = Sixty %, Light chain, MRI (myeloma-defining events). **Warning:** Do not confuse the M-spike threshold (≥3 g/dL for MGUS definition) with diagnostic criteria for myeloma. Myeloma diagnosis requires EITHER CRAB criteria OR SLiM criteria, not M-spike alone.