Synovial Fluid Analysis

Synovial fluid is a viscous ultrafiltrate of plasma enriched with hyaluronan that lubricates and nourishes the joint. A normal knee has under 3.5 mL. An effusion forms when the joint is inflamed, infected, injured or affected by crystal deposition. Arthrocentesis (joint aspiration) with analysis of the fluid is the key test in acute monoarthritis, since it separates septic arthritis, crystal arthritis, inflammatory arthritis and mechanical or traumatic causes.

The test is used mainly to exclude or confirm septic arthritis (an emergency that destroys cartilage within days) and to identify urate or calcium pyrophosphate crystals. The joint is aspirated before antibiotics when infection is suspected, but antibiotics must not be delayed in a toxic patient.

Septic arthritis affects 2 to 10 per 100 000 people a year (up to 30 to 70 per 100 000 in rheumatoid arthritis or with prosthetic joints), and carries a mortality of 5 to 15% and permanent joint damage in up to 30%. Gout affects about 1 to 4% of adults, and calcium pyrophosphate disease (pseudogout) 4 to 7% of adults over 60 years.

Indications. Any acute monoarthritis, a hot swollen joint with fever, suspected septic arthritis (including a prosthetic joint), suspected crystal arthritis, unexplained effusion, haemarthrosis, and diagnostic uncertainty in inflammatory arthritis. A swollen joint over cellulitis should not be aspirated through infected skin if possible.

Specimen. Fluid is divided: a sterile syringe or tube (heparin free of crystals, ideally sodium heparin or EDTA) for cell count and crystal examination (examine within 6 hours, fresh), a plain sterile tube for Gram stain and culture (also inoculated into blood culture bottles at the bedside, increasing yield), and a tube for chemistry (glucose, protein, with a paired serum glucose). A fluoride tube is used for glucose. Powdered anticoagulants (lithium heparin, oxalate) contain crystals and interfere with microscopy.

Normal synovial fluid. Clear, colourless to pale yellow, high viscosity (forms a string over 4 cm), white cells under 200 per microL (under 25% neutrophils), no crystals, glucose within 0.5 mmol/L (10 mg/dL) of the fasting blood level, sterile.

Classification.

Interpretation points.

• White cell count over 50 000 per microL has a likelihood ratio of about 7 for septic arthritis, over 100 000 about 28. Counts under 25 000 make it unlikely but not impossible, particularly in prosthetic joints (a threshold over 1700 cells per microL with over 65% neutrophils defines infection in knee arthroplasty) and in gonococcal disease.
• Gram stain is positive in 50 to 75% of non-gonococcal septic arthritis and under 25% of gonococcal. Culture is positive in 75 to 95% in non-gonococcal disease. Staphylococcus aureus is the commonest pathogen (about 50 to 60%), then streptococci, Gram negative organisms (in injecting drug users, older adults, the immunocompromised), Neisseria gonorrhoeae (sexually active young adults) and Kingella kingae in young children.
• Nucleic acid tests add sensitivity for gonococcus, Lyme disease and culture negative infection.
• Glucose under 50% of blood and lactate over 10 mmol/L support infection, and are not specific.
• Crystals under polarised light: monosodium urate crystals are needle shaped with strong negative birefringence (yellow when parallel to the compensator axis). Calcium pyrophosphate crystals are rhomboid or rod shaped with weak positive birefringence (blue when parallel). Crystals may be intracellular in acute attacks and are seen even between attacks. Crystal presence does not exclude infection, and both can coexist.
• Other crystals: cholesterol (notched plates), calcium oxalate, hydroxyapatite (needs special stains), corticosteroid ester after injection (mimic urate), and Charcot-Leyden crystals.
• Haemorrhagic fluid with fat globules suggests intra-articular fracture. Haemarthrosis in a patient with a bleeding disorder needs factor replacement.
• Rheumatoid factor, anti-CCP and complement in synovial fluid are rarely helpful.

Critical values and findings needing immediate action.

• Turbid or purulent fluid, white cells over 50 000 per microL with over 75% neutrophils, or a positive Gram stain: septic arthritis until proven otherwise, with urgent joint washout or repeated aspiration and intravenous antibiotics.
• Any acute monoarthritis with fever, rigors or a prosthetic joint: aspirate urgently.
• Haemarthrosis in a haemophilic or anticoagulated patient: reversal or replacement.
• Gonococcal or meningococcal arthritis: with skin lesions, tenosynovitis and migratory polyarthralgia, testing for sexual health.
• Joint infection with bacteraemia or sepsis: blood cultures and resuscitation.

Interferences and pitfalls.

• A dry tap does not exclude infection. Fluid may be sent in a syringe, and imaging may be needed.
• Crystal analysis needs fresh fluid and a trained observer. Old fluid dissolves urate crystals and generates artefacts.
• Antibiotics before the tap reduce culture yield.
• Crystals do not exclude septic arthritis, and a positive culture may coexist with gout.
• A low white cell count does not exclude infection in immunosuppressed patients, prosthetic joints and early disease.
• Intra-articular steroid injection makes crystals that mimic urate, and causes a post-injection flare.
• Hyaluronidase added to the cell count tube affects viscosity but also cell counts, and automated analysers can mis count in viscous fluid.
• Cell counts on automated analysers can be wrong when debris and crystals are present, and manual counts are used.
• Aspirating through cellulitis can seed the joint, and a different site is used.

Special groups.

• Children: Kingella kingae and Staphylococcus aureus, with fever, refusal to bear weight and a raised CRP.
• Prosthetic joints: low grade infection with Staphylococcus epidermidis and Cutibacterium, with culture held for 14 days.
• Older adults: pseudogout and septic arthritis may coexist, and presentation is atypical.
• Immunosuppressed patients: fungal, mycobacterial and atypical infection.
• Regions: tuberculous arthritis, brucellosis and haematogenous infection are frequent in endemic regions, and delay is common.

Follow up tests. Blood cultures, full blood count, CRP and ESR, serum uric acid (may be normal in an acute gout attack), creatinine, plain radiograph, ultrasound or MRI of the joint, dual energy CT for urate, mycobacterial culture and Xpert, fungal culture, gonococcal and chlamydial PCR, Lyme and brucella serology, HIV and hepatitis tests, rheumatoid factor and anti-CCP antibodies, and synovial biopsy.