From the Guidelines
For arthrocentesis in the Emergency Room, the standard body fluid orders should include cell count with differential, crystal analysis, Gram stain, and culture for aerobic and anaerobic organisms, as recommended by the Infectious Diseases Society of America 1. These tests provide crucial diagnostic information to differentiate between infectious, inflammatory, and crystal-induced arthropathies. The cell count with differential helps identify infection (typically >50,000 WBC/mm³ with >75% neutrophils suggests septic arthritis) versus inflammation. Crystal analysis detects gout (monosodium urate crystals) or pseudogout (calcium pyrophosphate crystals). Gram stain and culture identify potential pathogens and guide antibiotic therapy if infection is present. In certain cases, additional tests may be ordered based on clinical suspicion, such as lactate (elevated in septic arthritis), PCR for specific pathogens, or specialized tests for Lyme disease or tuberculosis. Proper specimen handling is essential - fluid should be collected in appropriate tubes (sterile, EDTA, and heparinized) and transported promptly to the laboratory to ensure accurate results, as outlined in the guidelines for diagnosis and management of prosthetic joint infection 1. It is also important to note that blood cultures should be obtained if fever is present, there is an acute onset of symptoms, or if the patient has a condition or suspected condition or concomitant infection or pathogen that would make the presence of a bloodstream infection more likely, as recommended by the Infectious Diseases Society of America 1. Overall, the selection of body fluid orders for arthrocentesis in the Emergency Room should be guided by the clinical presentation and suspicion of infection, and should include a combination of tests to provide a comprehensive diagnosis and guide treatment.
From the Research
Body Fluid Orders for Arthrocentesis in the ER
The following body fluid orders should be considered for an arthrocentesis in the Emergency Room (ER):
- Synovial fluid analysis for cell count with differential 2, 3, 4
- Crystal analysis to detect the presence of crystals such as those found in gout or pseudogout 2, 5, 3
- Microbiological analysis, including Gram-stain and culture, to diagnose septic arthritis 2, 5, 6
- Viscosity test and cellularity assessment with particular reference to the neutrophilic granulocyte fraction 4
- Examination of the native preparation in polarized light to detect crystals or other abnormalities 4
Laboratory Tests
Laboratory tests that may be ordered in conjunction with arthrocentesis include:
- Complete blood count (CBC) to evaluate for signs of infection or inflammation
- Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) to evaluate for signs of inflammation
- Blood cultures to diagnose septic arthritis or other infections
Imaging Studies
Imaging studies that may be ordered in conjunction with arthrocentesis include:
- X-rays to evaluate for signs of trauma, osteoarthritis, or other joint abnormalities
- Ultrasound to guide the arthrocentesis procedure and evaluate for signs of joint effusion or other abnormalities 2, 5
- Magnetic resonance imaging (MRI) or computed tomography (CT) scans to evaluate for signs of internal derangement or other joint abnormalities 6