Synovial Fluid Analysis: Gout vs Sepsis — MRCP Part 1
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Synovial Fluid Analysis: Gout vs Sepsis — MRCP Part 1

TL;DR: 

In acute monoarthritis, synovial fluid analysis is the quickest way to distinguish gout from septic arthritis—an essential MRCP Part 1 skill. Identify crystals with polarised microscopy, interpret white cell counts cautiously, and remember that crystals do not exclude infection. When in doubt, manage as sepsis after aspiration.


Why this topic matters for MRCP Part 1

Acute monoarthritis appears frequently in MRCP Part 1 questions because it tests prioritisation, pattern recognition, and safe clinical decision-making. Septic arthritis is limb- and life-threatening; gout is common and dramatic. Synovial fluid analysis sits at the centre of this distinction and is repeatedly examined with subtle traps.

This article supports your core revision within the MRCP Part 1 syllabus and complements structured preparation via the Crack Medicine hub:


Scope: what examiners expect you to know

Expect integrated questions that combine history (risk factors), examination, synovial fluid results, and management steps. The following 10 high-yield points cover the core examinable scope:

  1. Joint aspiration is mandatory in acute monoarthritis unless contraindicated.

  2. Crystals do not rule out sepsis—co-existence is well described.

  3. Synovial WCC overlaps between inflammatory and septic arthritis.

  4. Neutrophil predominance suggests inflammation or infection, not cause.

  5. Gram stain is insensitive; a negative result is not reassuring.

  6. Culture is the gold standard but results are delayed.

  7. Polarised microscopy distinguishes urate from CPPD.

  8. Serum urate may be normal during an acute gout flare.

  9. Risk factors (prosthetic joint, diabetes, immunosuppression) raise pre-test probability of sepsis.

  10. Antibiotics follow aspiration, unless the patient is unstable.


Synovial fluid analysis: gout vs septic arthritis

Key differences at a glance

Feature

Gout

Septic arthritis

Appearance

Cloudy; sometimes milky

Turbid or frankly purulent

WCC (/µL)

2,000–50,000 (often 10–30k)

Often >50,000 (overlap common)

Differential

Neutrophil-predominant

Neutrophil-predominant

Crystals

Monosodium urate (MSU)

None (may coexist)

Polarisation

Strong negative birefringence

Not applicable

Gram stain

Negative

Positive in ~50%

Culture

Negative

Positive (diagnostic)

Serum urate

May be normal

Not diagnostic

Exam pearl: Very high synovial WCC supports sepsis but is not diagnostic on its own. Always integrate clinical context.


Five most tested subtopics

1) Crystal identification (polarised light microscopy)

  • MSU (gout): needle-shaped, strongly negatively birefringent (yellow when parallel, blue when perpendicular).

  • CPPD (pseudogout): rhomboid, weakly positively birefringent.Why tested: Visual confirmation is definitive, but examiners often add infection risk to test judgement.

2) Synovial white cell count

  • Inflammatory arthritis and sepsis overlap significantly.

  • No single cut-off rules in or out septic arthritis.Why tested: Over-reliance on numbers leads to incorrect answers.

3) Gram stain and culture

  • Gram stain: low sensitivity, high specificity.

  • Culture: definitive but slow; guides targeted therapy.Why tested: Negative Gram stain is a classic false reassurance trap.

4) Timing of antibiotics

  • Aspirate before antibiotics if the patient is stable.

  • Do not delay antibiotics in haemodynamic instability or sepsis.Why tested: Process errors are common MCQ distractors.

5) Serum urate interpretation

  • Can be normal during acute gout.

  • Hyperuricaemia alone does not diagnose gout.Why tested: Candidates misuse serum urate as a rule-out test.


MRCP Part 1 study setup with medical notes and practice questions on synovial fluid analysis.

Mini-case (MRCP style)

A 62-year-old man presents with a hot, swollen knee and fever. He has type 2 diabetes. Synovial aspirate shows cloudy fluid, WCC 68,000/µL (92% neutrophils), and needle-shaped negatively birefringent crystals. Gram stain is negative.

What is the most appropriate next step?

Answer: Treat as septic arthritis—send cultures and start empiric intravenous antibiotics after aspiration.

Explanation: MSU crystals confirm gout but do not exclude infection. Fever, diabetes, and very high WCC increase the likelihood of sepsis. A negative Gram stain does not rule it out.


Common pitfalls (5 exam traps)

  • Assuming the presence of crystals excludes infection.

  • Using a single WCC cut-off to diagnose sepsis.

  • Trusting a negative Gram stain.

  • Relying on serum urate during an acute attack.

  • Delaying antibiotics in a clinically septic patient.


Practical study-tip checklist

Use this before practice questions and on exam day:

  • â–¡ Identify sepsis risk factors first.

  • â–¡ Interpret crystals and culture together.

  • â–¡ Treat uncertainty as sepsis if unstable.

  • â–¡ Ignore serum urate in acute decision-making.

  • â–¡ Practise mixed-feature questions in the Qbank and mocks.


FAQs

Can gout and septic arthritis occur together?

Yes. Co-existent crystal arthritis and infection are well recognised and commonly tested.

What synovial WCC confirms septic arthritis?

There is no absolute cut-off. Counts >50,000/µL support sepsis but overlap with inflammatory disease.

Is Gram stain reliable in septic arthritis?

It is specific when positive but insensitive. A negative result does not exclude infection.

Should antibiotics be given before joint aspiration?

Aspirate first if the patient is stable. Do not delay antibiotics in unstable or septic patients.


Ready to start?

Strengthen your MRCP Part 1 performance by practising high-yield acute monoarthritis questions and exam-level scenarios:


Sources

 
 
 
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