Synovial and serous fluid analysis (overview)

Principle

• Analysis of joint (synovial) fluid and pleural, pericardial and peritoneal (serous) fluids to find the cause of an effusion.

Specimen

• Sterile tube for culture and Gram stain; sodium heparin or liquid EDTA tube for cell count and crystals; plain tube for chemistry.
• Avoid lithium heparin and powdered EDTA or oxalate for crystal studies, as they can form crystals.

Procedure

• Record appearance, color, clarity and (for synovial fluid) viscosity.
• Cell count with a hemocytometer or validated analyzer; dilute synovial fluid with saline, not acetic acid (which forms a mucin clot); hyaluronidase lowers viscosity.
• Differential on a cytocentrifuge slide; Gram stain and culture; cytology for malignancy.
• Crystals: wet preparation under compensated polarized light.

Interpretation

• Normal synovial fluid is clear, pale yellow and highly viscous, with WBC typically below 200/µL.
• Typical groups (local criteria apply): non-inflammatory about 200–2,000/µL; inflammatory about 2,000–50,000/µL; septic often above 50,000/µL with mostly neutrophils.
• Monosodium urate (gout): needle-shaped, strongly negatively birefringent (yellow when parallel to the compensator slow axis).
• Calcium pyrophosphate (pseudogout): rhomboid or rod-shaped, weakly positively birefringent (blue when parallel).
• Serous fluids: Light's criteria define an exudate if any one is met: fluid/serum protein ratio above 0.5, fluid/serum LDH ratio above 0.6, or fluid LDH above two-thirds of the upper limit of normal serum LDH.
• Serum–ascites albumin gradient (SAAG) ≥1.1 g/dL indicates portal hypertension.
• Chylous effusion: triglycerides above 110 mg/dL.

Quality control and pitfalls

• Process promptly; cells degrade and crystals can dissolve or form in vitro.
• Clean slides and coverslips to avoid mistaking debris or talc for crystals.
• Traumatic taps affect cell counts.

Clinical use

• Diagnosis of septic arthritis, gout, pseudogout and inflammatory arthritis; distinguishing transudates (e.g. heart failure, cirrhosis) from exudates (infection, malignancy, TB).