Joint Aspirate Collection
Synovial fluid analysis is the definitive first step in any acute monoarthritis workup.
- Arthrocentesis: Procedure (needle aspiration of joint fluid)
- <1 hr: Sample handled (crystals degrade if delayed)
- Cloudy/turbid: Fluid appearance (inflammatory effusion)
- 2k–100k: WBC count (per µL, inflammatory range)
Why aspirate first
Crystal identification requires fresh joint fluid, not blood or imaging alone.
Ruling out infection
Gram stain and culture run in parallel to exclude septic arthritis.
Sample handling
Fluid is examined promptly since crystals can dissolve or form artifacts.
Crystal Shape & Birefringence Comparison
A red compensator plate reveals each crystal type's optical sign at a glance.
- Needle: MSU shape (tapered, pointed ends)
- Rhomboid: CPPD shape (blunt, rod or brick-like)
- Strong negative: MSU birefringence (yellow when parallel)
- Weak positive: CPPD birefringence (blue when parallel)
Reading the compensator
Crystal color flips with orientation relative to the slow-ray axis.
Needle vs rhomboid
Shape alone often distinguishes MSU from CPPD at a glance.
Common pitfalls
Weak CPPD birefringence is easy to miss without careful technique.
Joint Distribution Pattern Comparison
Where the joint flares up is itself a powerful diagnostic clue.
- Great toe: Gout hallmark joint (first MTP, podagra)
- Knee: Pseudogout hallmark joint (most common CPPD site)
- ~10%: Gout knee involvement (less typical than toe)
- ~25%: Pseudogout wrist involvement (second most common site)
Podagra
First MTP joint pain is the classic presenting sign of gout.
Large joint predominance
Pseudogout favors knees and wrists over small joints.
Overlap zones
Ankles can flare with either crystal, muddying the picture.
Treatment Response Comparison
Acute flares of both diseases share first-line therapy, but chronic management diverges.
- Both effective: Colchicine response (gout and pseudogout)
- Both effective: NSAID response (first-line acute therapy)
- Gout only: Urate-lowering therapy (allopurinol, febuxostat)
- None: CPPD-specific cure (no analogous chronic agent)
Shared acute therapy
Colchicine, NSAIDs, and steroids calm both types of flare.
Diverging chronic therapy
Only gout has a drug that dissolves the underlying crystal.
Why the difference matters
A trial of urate-lowering therapy that fails hints at pseudogout.
Differential Diagnosis Output
Crystal shape, birefringence, joint site, and treatment response converge on one diagnosis.
- Crystal ID: Definitive test (polarized microscopy of aspirate)
- Joint site: Supportive clue (toe vs knee/wrist)
- Drug response: Confirmatory clue (urate-lowering efficacy)
- Chondrocalcinosis: Imaging clue (CPPD only, on X-ray)
Weighing the evidence
No single feature is perfect; the combination clinches the call.
Chondrocalcinosis on imaging
Cartilage calcification on X-ray supports CPPD, not gout.
Final confidence score
Slider-driven confidence reflects how clean the differential signal is.
Gout vs pseudogout at a glance