Arthrocentesis

Definition: Bacterial infection of a joint space and its synovial fluid

Causes

  • Hematogenous spread of bacteria to the joint capsule
    • Most common mechanism of infection
    • 60-90% of cases are Gram positive (S. aureus = 50-70%)
    • 5-25% of cases are Gram negative
  • Direct inoculation (e.g. recent arthrocentesis, prior joint surgery)

Epidemiology

  • Bimodal distribution of cases, peaking for young children and adults >55 years old
  • Nongonococcal:
    • Intravenous drug use (IVDU)
    • Prosthetic joint
    • Diabetes
    • Immunocompromised states
    • Elderly
    • Indwelling IV catheters
  • Gonococcal
    • Sexually active population
    • 4:1 female-to-male predominance
  • Consider Gram negative, fungal, or mycobacterial arthritis in immunocompromised patients and elderly

Complications

  • Cartilage destruction requiring joint replacement
  • Osteomyelitis
  • Sepsis
  • Immunocompromised patients may decompensate rapidly

Differential Diagnosis for monoarticular arthritis

  • Gout / pseudogout
  • Trauma
  • Cellulitis of overlying joint
  • Bursitis / tendonitis
  • Reactive arthritis
  • Lyme arthritis
  • Autoimmune disease (SLE, RA, seronegative spondyloarthropathy, etc.)
  • Hemarthrosis secondary to coagulopathy
  • Osteoarthritis

History

  • Risk group from Epidemiology section above
  • Large joints more commonly affected (knee most common)
  • >80% monoarticular
  • Fever (44-97% sensitive) (Carpenter 2011)
  • Chronic joint disease (e.g. gout) does not rule out concomitant septic arthritis
  • Sexual history should be obtained
    • Gonococcal – usually oligoarticular arthritis (generally with rash, migratory arthralgias, tenosynovitis)

Physical Exam

  • Joint pain and effusion
  • Erythema and warmth
  • Tenderness on palpation
  • Limited in both active and passive range of movement
  • Immunocompromised patients
    • Often do not present typically
    • May be well-appearing or afebrile
    • More likely to have a polyarticular involvement

Diagnostic Testing

  • Laboratory Analysis
    • Serum lab tests cannot be used to rule in or out the diagnosis
    • Suspicion for Gonococcal disease: obtain genital/oral/rectal Gonorrhea and Chlamydia specimens (PCR, Culture), screen for concomitant STIs and HIV

Ranges of Serum Lab Value Probabilities (Margaretten 2007, Horowitz 2011)

+ LR – LR Sensitivity (%) Specificity (%)
WBC >10,000 cells/mm3 1.4 (1.1-1.8) 0.28 90 36
ESR >30 mm/h 1.3 (1.1-1.8) 0.17 95 29
CRP >100 mg/L 1.6 (1.1-2.5) 0.44 (0.24-0.82) 77 53
Blood culture

(non-gonococcal)

 50% positive *
Blood culture

(gonococcal)

 Seldom positive *
  • Imaging
    • The role of X-ray is limited. It can assess for:
      • Joint/bony destruction
      • Trauma or foreign body
    • Not sensitive for osteomyelitis or other complications
    • Findings suggestive of other arthropathies (e.g. osteophytes) do not rule out septic arthritis

  • Knee Arthrocentesis (medpagetoday.com)

    Arthrocentesis

    • Relative contraindications:
      • Overlying cellulitis: if septic joint is suspected, arthrocentesis should still be performed through cellulitic-appearing skin
      • Coagulopathy: if septic joint is suspected, arthrocentesis should still be performed (Thumboo 1998)
      • Prosthetic joint: if no orthopedic surgeon is available and septic joint is suspected, arthrocentesis should still be performed by the ED provider
      • Do not need to wait for serum lab study results prior to arthrocentesis
    • Most important synovial fluid studies: Cell count, Gram stain, aerobic and anaerobic cultures

Synovial Fluid Interpretation

Normal Inflammatory Crystal-Induced Septic
Color Clear/yellow Yellow/white White/turbid Cloudy/opaque
Viscosity Thick, sticky N/A N/A Thin, water-like
Synovial WBC <25,000/mm3 <100,000/mm3 >50,000/mm3 >50,000/mm3
Synovial PMN <30% >50% <90% >90%
Other Crystals seen Culture positive

Pooled Joint Aspirate WBC Probabilities (Carpenter 2011, Margaretten 2007)

+ LR – LR Sensitivity (%) Specificity (%) Study
WBC >25,000 cells/mm3 3.2 (2.3-4.4) 0.35 (0.23 – 0.50) 73 (64-81) 77 (73-81) Carpenter 2011
2.9 (2.5-3.4) 0.32 77 73 Margaretten 2007
WBC >50,000 cells/mm3 4.7 (2.5-8.5) 0.52 (0.38-.72) 56 (49-63) 90 (88-92) Carpenter 2011
7.7 (5.7-11.0) 0.41 62 92 Margaretten 2007
WBC >100,000 cells/mm3 13.2 (3.6-51.1) 0.83 (0.80 – 0.89) 19 (14-20) 99 (96-100) Carpenter 2011
28.0 (12.0-66.0) 0.72 29 99 Margaretten 2007
>90% PMN 2.7 (2.1-3.5) 0.49 60 (51-68) 78 (75-80) Carpenter 2011
3.4 (2.8-4.2) 0.34 73 79 Margaretten 2007
Lactate >10 mmol/L Infinity 0.14 86 100 Carpenter 2011
WBC > 1,100 cells/m3 (prosthetic joint) 7.6 0.10 91 88 Ghanem 2008
WBC PMN > 64% (prosthetic joint) 17.9 0.53 95 95 Ghanem 2008
  • Prosthetic joints have lower diagnostic threshold (Ghanem 2008, Tande 2014)
    • WBC > 1,100 cells/mm3 (+LR 7.6) OR
    • >64% PMNs (+LR 17.9)
  • Gram stain is only positive in 50-60% of infected joints
  • Synovial culture is positive in >90% of nongonoccal septic arthritis but only positive in 25-70% of gonococcal septic arthritis (Horowitz 2011)

Treatment Thresholds

  • There is no absolute serum or synovial laboratory value to rule in or out septic arthritis
  • Decision to initiate treatment is based on overall clinical suspicion derived from
    • Patient risk factors
    • Clinical findings
    • Synovial fluid results
  • Low threshold for workup and treatment: (Carpenter 2011)
    • If clinical suspicion is 5%, further testing should be pursued
    • If probability of septic arthritis is above 39%, treatment should be initiated
  • Dry or non-diagnostic tap
    • If suspicion for septic joint, start antibiotics and admit
    • Presence of crystals in synovial fluid does not rule out septic arthritis
  • Suspect septic joint in gout patient with: (Abelson 2010)
    • Fever
    • Involvement of an new/atypical joint
    • New/worsening synovitis despite adherence to outpatient treatment
  • Suspect septic joint in HIV+ patient with: (Takhar 2010)
    • IVDU – likely MRSA
    • Risky sexual behaviors – likely gonococcal
    • CD4 <200 – possibly fungal or mycobacterial
    • Abnormally low synovial cell counts are possible

ED Management

  • Provide analgesia
  • Investigate for primary source if hematogenous spread suspected
  • Antibiotic management
    • Based on suspicion for particular organism
    • Gram positive:
      • Vancomycin 15 mg/kg IV q12h OR
      • Linezolid 600mg q12h
    • Gram negative/Gonococcal:
      • Ceftriaxone 2g IV q24h OR Cefotaxime 2g IV q8h OR Ceftazidime 1g IV q8h PLUS
      • Gentamycin 5mg/kg IV q8h for Pseudomonas coverage (e.g. IVDU)
  • Orthopedic surgery consultation for possible operative debridement or hardware removal
  • Gonococcal infection often managed non-operatively

Take Home Points

  • Septic arthritis is a critical diagnosis that should be considered in all patients with mono- or oligoarticular arthritis
  • Be aware that immunocompromised patients often present atypically with septic arthritis. Have a low threshold to obtain a joint aspirate
  • Serum laboratory testing and imaging cannot be used to rule in or out the diagnosis. Do not skip the arthrocentesis because these tests are unremarkable
  • The decision to start treatment in the ED for septic arthritis is based on overall clinical suspicion derived from patient’s risk factors, clinical picture, and synovial fluid cell count
  • If suspicion is still high after equivocal or dry tap, admit the patient and initiate empiric IV antibiotics while the synovial culture results

Read More

Taming the SRU: Joint Fluid Analysis in the ED

EM Lyceum: Monoarticular Arthritis

ALiEM: Paucis Verbis Card – Septic Arthritis

References

  • Liang SY et al.  Orthopedic infections and other complications.  Orthopedic Emergencies.  Cambridge University Press 2013.  178-180.
  • Matteson EL, Osmon DR.  Infections of Burae, Joints, and Bones.  Goldman’s Cecil Medicine, 24th Edition.  Elsevier Saunders 2012.  1731-1736.
  • Genes N, Chisolm-Straker M.  Monoarticular Arthritis Update: Current Evidence for Diagnosis and Treatment in the Emergency Department.  Emergency Medicine Practice.  EBMedicine.net. 2012; 14(5): 1-20.
  • Carpenter CR et al. Evidence based diagnostics: adult septic arthritis. Acad Emerg Med. 2011; 18(8): 781-796.  PMID 21843213.
  • Margaretten ME et al. Does this patient have septic arthritis? JAMA. 2007; 297:1478-88. PMID 17405973.
  • Horowitz DL et al.  Approach to Septic Arthritis.  Am Fam Physician. 2011; 84(6): 653-660.  PMID 21916390.
  • Visser S.  Septic until proven otherwise.  Can Fam Physician.  2009; 55(4): 374–375.  PMCID PMC2669005.