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Myeloma & MGUS: Diagnostic Criteria for MRCP Part 1

TL;DR

Multiple myeloma and MGUS are high-yield plasma cell disorders frequently tested in MRCP Part 1, especially via diagnostic thresholds and lab patterns. Examiners expect you to distinguish MGUS, smouldering myeloma, and overt myeloma using exact cut-offs, CRAB features, and modern myeloma-defining events. This guide gives you the ultimate exam-ready list, plus a mini-MCQ and a practical checklist.


Why this matters

Plasma cell dyscrasias are a core haematology theme in the MRCP exams. Questions are rarely vague: they test whether you know when not to diagnose myeloma, recognise asymptomatic disease, and avoid overcalling malignancy based on paraprotein alone. If you confuse MGUS with smouldering myeloma—or forget the CRAB criteria—you will lose straightforward marks.

This article supports the MRCP Part 1 hub and is designed to be read alongside targeted practice from Crack Medicine’s question bank and mocks.


Scope and exam focus

You should be able to:

  • Define MGUS, smouldering myeloma, and multiple myeloma

  • Recall paraprotein and bone marrow plasma cell cut-offs

  • Apply CRAB criteria correctly

  • Recognise newer myeloma-defining events

  • Identify common MRCP traps

Core diagnostic framework (high-yield)

1. MGUS (Monoclonal Gammopathy of Undetermined Significance)

MGUS is a premalignant condition with a low but lifelong risk of progression (≈1% per year).

Diagnostic criteria (all must be met):

  • Serum monoclonal protein < 30 g/L

  • Bone marrow plasma cells < 10%

  • No end-organ damage attributable to a plasma cell disorder

MGUS is a diagnosis of exclusion. MRCP questions often describe an incidental paraprotein with entirely normal calcium, renal function, haemoglobin, and imaging—this is MGUS until proven otherwise.

2. Smouldering (asymptomatic) myeloma

Smouldering myeloma lies between MGUS and overt myeloma.

Key criteria:

  • Serum monoclonal protein ≥ 30 g/L and/or

  • Bone marrow plasma cells 10–59%

  • No CRAB features or myeloma-defining events

There are no symptoms requiring treatment, but the risk of progression is substantially higher than in MGUS.

3. Multiple myeloma

Multiple myeloma requires clonal plasma cells plus evidence of end-organ damage or specific biomarkers.

The CRAB criteria (classic MRCP list)

  • Calcium: > 2.75 mmol/L

  • Renal impairment: creatinine > 177 µmol/L

  • Anaemia: Hb < 100 g/L or ≥ 20 g/L below normal

  • Bone disease: lytic lesions or pathological fractures

CRAB features must be attributable to the plasma cell disorder, not explained by another condition.

4. Myeloma-defining events (IMWG criteria)

Even without CRAB features, myeloma can be diagnosed if any of the following are present:

  1. Bone marrow plasma cells ≥ 60%

  2. Serum free light chain ratio ≥ 100

  3. 1 focal lesion on MRI (≥ 5 mm)

These features predict near-inevitable progression and are examinable at MRCP level.


The one table you must memorise

Condition

Paraprotein

Bone marrow plasma cells

End-organ damage

MGUS

< 30 g/L

< 10%

None

Smouldering myeloma

≥ 30 g/L

10–59%

None

Multiple myeloma

Any level

≥ 10%

CRAB or defining event

This table alone answers several MRCP Part 1 questions every year.


MRCP Part 1 exam preparation with haematology notes and revision checklist

The five most tested subtopics

  1. Renal disease Light-chain cast nephropathy is most common; amyloidosis causes nephrotic syndrome.

  2. Anaemia Typically normocytic, normochromic due to marrow infiltration and reduced erythropoietin.

  3. Bone disease Lytic (not sclerotic) lesions from osteoclast activation.

  4. Infection risk Recurrent infections with encapsulated organisms due to hypogammaglobulinaemia.

  5. Laboratory clues Markedly raised ESR with a normal CRP is a classic exam stem.

Practical example: mini-MCQ

A 68-year-old man is found to have an IgG paraprotein of 22 g/L on routine blood tests. Bone marrow biopsy shows 6% plasma cells. Calcium, creatinine, haemoglobin, and skeletal survey are normal.

What is the most likely diagnosis?

Answer: MGUS

Explanation: Paraprotein < 30 g/L, plasma cells < 10%, and no CRAB features fulfil MGUS criteria. No treatment is indicated—only surveillance. This scenario is a classic MRCP trap.


Common pitfalls (examiner favourites)

  • Diagnosing myeloma based on paraprotein level alone

  • Forgetting that CRAB features must be caused by the plasma cell disorder

  • Confusing smouldering myeloma with MGUS

  • Missing light-chain-only myeloma (low or absent paraprotein)

  • Assuming a raised ESR always indicates infection or inflammation


Practical study-tip checklist

Before your next revision block:

  • ☐ Can I recall the exact paraprotein cut-offs?

  • ☐ Do I know the bone marrow plasma cell thresholds?

  • ☐ Can I list CRAB features without hesitation?

  • ☐ Do I recognise myeloma-defining biomarkers?

  • ☐ Have I practised mixed stems using MRCP-style MCQs and mocks?


FAQs

What is the main difference between MGUS and smouldering myeloma?Smouldering myeloma has a higher paraprotein level and/or higher marrow plasma cell percentage, but still no CRAB features.

Can multiple myeloma be diagnosed without CRAB features?

Yes. Certain biomarkers (≥60% plasma cells, FLC ratio ≥100, or focal MRI lesions) are sufficient.

Is Bence Jones protein diagnostic of myeloma?

No. It supports the diagnosis but is not diagnostic on its own.

How often should MGUS be monitored?

Typically every 6–12 months, depending on risk factors and paraprotein stability.


Ready to start?

To secure these easy haematology marks, revise this topic alongside targeted questions and timed practice. Explore the MRCP Part 1 overview (https://www.mrcpuk.org/mrcpuk-examinations/part-1), practise with Crack Medicine’s QBank (/qbank/), and test yourself under exam conditions using full MRCP mock tests (/mock-tests/).


Sources

 
 
 

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