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Diagnostic Tests – Arthrocentesis
1. What is this?
Arthrocentesis is a procedure in which synovial fluid is aspirated from a joint using a needle for laboratory and microscopic analysis. It is the most accurate immediate test to distinguish between:
2. When do you answer arthrocentesis?
You should answer arthrocentesis when a patient presents with:
This is especially critical when infection must be ruled out. In cases of suspected gout, serum uric acid may be elevated, but arthrocentesis is still required to confirm the diagnosis and exclude infection.
Exam pearl:
Any new, hot, swollen joint = arthrocentesis first
3. What findings are evident in the two synovial fluid samples?
Microscopic examination under polarized light reveals characteristic crystals:
1. What is this?
Arthrocentesis is a procedure in which synovial fluid is aspirated from a joint using a needle for laboratory and microscopic analysis. It is the most accurate immediate test to distinguish between:
- Normal joint fluid
- Inflammatory arthritis
- Septic (infectious) arthritis
- 0–2,000 cells/µL → Normal
- 2,000–20,000 cells/µL → Inflammatory (e.g., gout, rheumatoid arthritis)
- 20,000–50,000 cells/µL → Indeterminate
- >50,000 cells/µL → Septic arthritis
2. When do you answer arthrocentesis?
You should answer arthrocentesis when a patient presents with:
- Acute onset of a swollen joint
- Warmth, pain, and tenderness
- Joint effusion with decreased range of motion
This is especially critical when infection must be ruled out. In cases of suspected gout, serum uric acid may be elevated, but arthrocentesis is still required to confirm the diagnosis and exclude infection.
Exam pearl:
Any new, hot, swollen joint = arthrocentesis first
3. What findings are evident in the two synovial fluid samples?
Microscopic examination under polarized light reveals characteristic crystals:
- Gout
- Negatively birefringent
- Needle-shaped crystals
- Composed of monosodium urate
- Pseudogout
- Positively birefringent
- Rhomboid-shaped crystals
- Composed of calcium pyrophosphate
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