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This guide provides a structured approach to joint fluid interpretation, including typical joint fluid aspiration results for specific disease processes. Reference ranges vary between labs, so always consult your local medical school or hospital guidelines.


Normal joint fluid results (adults)

  • Colour: Colourless
  • Clarity: Transparent
  • Viscosity: Normal
  • WBC: < 200 cells/mm3
  • Neutrophils (% of total WCC): <25 %
  • Gram stain: Negative
  • Crystals: Negative

Overview of joint fluid findings

This table summarises the typical findings in joint fluid analysis for various pathologies.

Joint fluid analysis


Non-inflammatory joint effusion

Causes

  • Osteoarthritis (OA)
  • Trauma

 

Symptoms

  • Localised joint pain
  • Gradual onset of symptoms
  • Pain on movement
  • Crepitus
  • Worse at the end of the day

 

Typical joint fluid findings

  • Colour: Straw like
  • Clarity: Translucent
  • Viscosity: Increased
  • WBC: 200 – 2000 cells/mm3
  • Neutrophils: <25 %
  • Gram stain: Negative
  • Crystals: Negative

 

 

Further investigations

  • Bloods – WCC/CRP would typically be normal
  • X-Ray – may reveal fractures or changes consistent with OA
  • MRI – useful for assessing the ligaments and menisci for injury

Inflammatory joint effusion

Causes

  • Rheumatoid arthritis
  • Reactive arthritis
  • Psoriatic arthritis
  • Acute gout or pseudogout

 

Symptoms

Rheumatoid arthritis:

  • Symmetrical swollen, warm, erythematous and painful joints
  • Usually the small joints of the hands and feet are affected
  • Morning stiffness greater than one hour
  • May be associated with systemic features (fever, weight loss)

 

Psoriatic arthritis:

  • Affected joints are generally red or warm to the touch
  • Small joints of the hand and wrist are commonly affected (most commonly the distal interphalangeal joints)
  • Typically asymmetrical in presentation
  • Associated changes to the nails, such as onycholysis and hyperkeratosis

 

Acute gout:

  • Typically presents with a single hot, swollen and tender joint (most commonly the MTP joint at the base of the big toe)

 

Pseudogout:

  • Typically presents with a single hot, swollen and tender joint (most commonly the knee joint)

 

Typical joint fluid findings

  • Colour: Yellow
  • Clarity: Cloudy
  • Viscosity: Decreased
  • WBC: 2000–50,000 cells/mm3
  • Neutrophils: >50 %
  • Gram stain: Negative
  • Crystals: Positive
    • Gout – needle negative birefringent crystals
    • Pseudogout – rhomboid positively birefringent crystals

 

Further investigations

  • Bloods:
    • Full blood count
    • CRP/ESR
    • Urate (raised in gout)
    • Antibodies (e.g. anti–CCP, RhF)
  • X-ray of the joint

Septic joint effusion

Causes

  • Staphylococcus aureus
  • Streptococci
  • Neisseria gonorrhoeae (young sexually active adults)
  • Escherichia coli (elderly, IV drug users)

 

Symptoms

  • Painful, swollen and warm
  • Usually a single joint affected

 

Typical joint fluid findings

  • Colour: Yellow/green
  • Clarity: Cloudy/opaque
  • Viscosity: Decreased
  • WBC: >50 000 cells/mm3
  • Neutrophils: >75 %
  • Gram stain: Often positive
  • Crystals: Negative

 

Further investigations

  • Bloods:
    • Full blood count – WCC raised
    • CRP – raised
  • Blood cultures
  • Fluid cultures
  • X-Ray of the joint

Haemorrhagic joint effusion

Causes

  • Trauma
  • Tumours
  • Bleeding disorders

 

Symptoms

  • Painful, swollen and warm
  • Restricted range of movement
  • Excessive bruising surrounding affected joint

 

Typical joint fluid findings

  • Colour: Red/xanthochromic
  • Clarity: Bloody
  • Viscosity: Variable
  • WBC: 200-2000 mm³
  • Neutrophils: 50-75%
  • Gram stain: Negative
  • Crystals: Negative

 

Further investigations

  • Bloods:
    • Full blood count – may note drop in haemoglobin
    • Coagulation studies
  • X-Ray of the affected joint – to identify associated fractures

Worked examples

CASE 1

A 23-year-old female presents with a swollen and warm knee joint that is very painful on flexion. On systemic enquiry, she also mentions she has been suffering from purulent vaginal discharge for several weeks. She is sexually active with multiple partners and uses condoms inconsistently. Her knee joint is aspirated, with the results shown below.

 

Results

  • Colour: Yellow
  • Clarity: Opaque
  • Viscosity: Decreased
  • WBC: 60,000 mm³
  • Neutrophils: 90 %
  • Gram stain: negative

Answer

Septic arthritis (likely organism Neisseria gonorrhoeae)

 

CASE 2

A 25-year-old male presents with pain in the metacarpophalangeal (MCP)  joints of both hands. He has morning joint stiffness that lasts for more than an hour. On examination, you note multiple warm, swollen MTP joints, with associated erythema. The largest joint effusion is aspirated, with the results shown below.

 

Results

  • Colour:  Yellow
  • Clarity: Cloudy
  • Viscosity: Decreased
  • WBC: 40,000 mm³
  • Neutrophils: 60 %
  • Gram stain: Negative

Answer

Inflammatory joint effusion (secondary to rheumatoid arthritis)

 

 

CASE 3

A 67-year-old female presents with left knee stiffness and pain that worsens with movement. Her symptoms have progressively worsened over the last 8 years and she is now struggling to mobilise outside. On examination, there is left knee swelling, with significant crepitus noted on passive flexion and extension. There is no associated erythema or warmth. There is a positive sweep test on assessment and the effusion is aspirated, with the results shown below.

 

Results

  • Colour: Colourless
  • Clarity: Translucent
  • Viscosity: Increased
  • WBC: 1500 mm³
  • Neutrophils: 15 %
  • Gram stain: Negative

Answer

Non-inflammatory joint effusion (secondary to osteoarthritis)



References

1. Abdullah S, Young‐Min SA, Hudson SJ, Kelly CA, Heycock CR, Hamilton JD. Gross synovial fluid analysis in the differential diagnosis of joint effusion. Journal of Clinical Pathology. 2007;60(10):1144-1147. doi:10.1136/jcp.2006.043950.

2. Goldenberg DL. Bacterial arthritis. In: Ruddy S, Harris ED, Sledge CB, Kelley WN, eds. Kelley’s Textbook of Rheumatology. 6th ed. Philadelphia, Pa.: Saunders; 2001:1469–1483.

3. Schumacher HR Jr. Synovial fluid analysis and synovial biopsy. In: Ruddy S, Harris ED, Sledge CB, Kelley WN, eds. Kelley’s Textbook of Rheumatology. 6th ed. Philadelphia, Pa.: Saunders; 2001:605–619.