Septic Arthritis


Indications for Testing

  • Acute mono- or oligoarticular arthritis
  • Loosening of prosthesis

Laboratory Testing

  • CBC with differential – expect mild to moderate leukocytosis and left shift of cell composition (immature band forms)
    • Results that increase likelihood ratio (LR) for septic arthritis
      • White blood cell count (WBC) >10,000/µL = LR 1.4
      • Neutrophils >90% = LR 3.4
  • Joint aspiration (arthrocentesis) with synovial specimen (when possible) – cornerstone of diagnosis and should be performed prior to antibiotic administration
    • Aspiration should not be performed through overlying cellulitis
    • Macroscopic assessment – viscosity, color, clarity
      • Inflammatory fluid
        • Color – ranges from yellow to greenish
        • Consistency – turbid
    • WBC count with differential
      • Usually >50,000/µL with predominance of neutrophils
        • Same degree of leukocytosis may be noted in gout and pseudogout
      • WBC count >50,000/µL increases likelihood of septic arthritis (LR 7.7 positive; LR 0.42 negative)
        • At least 90% leukocytes (LR 3.4 positive; LR 0.34 negative)
        • <50,000/µL does not rule out septic arthritis
        • Low WBC count common in immunosuppressed patients
      • Prosthetic joints – cell count cutoffs are much lower      
        • Knee – WBC >1,700/µL or differential >65% neutrophils
        • Any other joint – WBC >4,200/µL or differential >80% neutrophils
    • Gram stain – low sensitivity; diagnostic if organisms are identified
    • Culture – moderately high sensitivity if positive
      • <50% positive in gonococcal arthritis
        • Recommend diagnosis of gonococcal arthritis be made based on clinical presentation and cultures of cervix, rectum, urethra or oropharynx
    • Crystal scan with polarized microscope – evaluate for crystalline arthritis
      • Monosodium urate demonstrates negative birefringence
      • Calcium pyrophosphate dehydrate (CPPD) crystals have weak birefringence
    • Not recommended
      • Glucose, lactate dehydrogenase (LD), and protein are neither sensitive nor specific
      • PCR for specific organisms is not currently recommended
  • C-reactive protein (CRP)
    • Usually elevated; absence of increased concentrations does not exclude septic arthritis
    • CRP >10 mg/L (>1.0 mg/dL) increases likelihood of septic arthritis (LR 1.6)
      • CRP ≥13.5 mg/L (≥1.35 mg/dL) in prosthetic joints – sensitivity 73-91%, specificity 81-86%
      • CRP remains elevated up to 2 months post-arthroplasty, then becomes normal
    • Inflammatory parameters may remain high for up to two weeks post-surgery
  • Cultures
    • Blood cultures
      • Positive in 50-70% of patients with non-gonococcal bacterial arthritis
        • Diagnostic if positive
        • Limited usefulness but may be helpful when ruling out other diseases, particularly in children
      • Lower rate of positivity in prosthetic joints
    • Tissue cultures
      • Prosthetic joints – multiple intraoperative tissue samples should be sent for culture (ideal is 5-6)
        • Antimicrobial susceptibility testing may help guide therapy
    • Other site cultures dependent on patient history – skin ulcer, urine, throat, genitourinary
  • Serologic testing for Lyme disease in patient with negative cultures and who resides in an endemic area


  • Prosthetic joints – intraoperative frozen sections often show >5-10 polymorphonuclear neutrophils per high-power field (PMNs/hpf) which indicates acute inflammation

Imaging Studies

  • X-ray/ultrasound – useful in detecting the presence of fluid; not useful in diagnosis of osteomyelitis unless late in course of disease
    • Prosthetic joints – periprosthetic lucency, osteolysis or prosthesis migration may be seen
  • Bone scan or MRI – may be necessary to rule out osteomyelitis
    • Sensitive for detecting failed implants but not specific for infection
    • Artifact from implants may obscure information

Differential Diagnosis

  • Adult noninfectious inflammatory arthritis
  • Pediatric inflammatory arthritis Intra-articular injury
    • Fracture
    • Meniscal tear
    • Osteonecrosis
    • Traumatic effusion
    • Hemarthrosis
  • Other
    • Malignancy (eg, synovial sarcoma)
    • Osteomyelitis
    • Cellulitis overlying joint


  • CRP levels – nonspecific, but often elevated during infection

Clinical Background

Septic arthritis may be caused by any number of different microorganisms and results in erythematous, painful, swollen joints.


  • Incidence – 2-10/100,000 in the U.S.
  • Transmission
    • Most cases are hematogenously acquired
    • Other mechanisms for infection
      • Surgery
      • Trauma
      • Percutaneous puncture
      • Spread from contiguous structure infection

Organisms most commonly involved

  • Bacteria
    • Children – Staphylococcus aureus, group A streptococcus, Kingella kingae
    • Adults
      • S. aureus – most common (50% of cases)
      • Streptococcus spp (groups A and B)
      • Neisseria gonorrhoeae – almost exclusively in sexually active patients
      • Gram-negative bacilli – elderly, IV drug abusers, immunocompromised persons
        • E. coli
        • Pseudomonas aeruginosa
        • Salmonella – sickle cell disease, immunocompromised
      • Coagulase negative staphylococci – prosthetic joint
      • Listeria (rare) – rheumatoid arthritis and immunosuppression
      • Anaerobes (rare) – prosthetic joints, bite victims
      • Polymicrobial – up to 20% of arthroplasty patients; most commonly methicillin-resistant Staphylococcus aureus (MRSA) or anaerobes plus other organisms
      • Borrelia burgdorferi – areas where tick is endemic
  • Virus – rare; most common is parvovirus B19
  • Fungi – uncommon
  • Parasites – rare
    • Helminths
    • Filaria

Risk Factors

  • Nonprosthetic joint
  • Prosthetic joint
    • Patient-related
      • Previous arthroplasty
      • Tobacco abuse
      • Obesity
      • Rheumatoid arthritis
      • Diabetes mellitus
      • Immunosuppression
      • Bacteremia, endocarditis
    • Surgery-related
      • Simultaneous bilateral arthroplasty
      • Operative time >2.5 hours
      • Allogenic blood transfusion
      • Post-operative complications
        • Delayed wound healing
        • Atrial fibrillation
        • Myocardial infarction
        • Urinary tract infection (UTI)
        • Prolonged hospital stay


  • Organism accesses joint space either directly or hematogenously
  • Organisms cause release of inflammatory cell cytokines, proteases
    • Leads to destruction of cartilage, inhibition of new cartilage synthesis, and bone loss

Clinical Presentation

  • Fever
  • Warm, swollen, erythematous, painful joint
  • Prosthetic joint
    • Draining sinus
    • Loosening of prosthesis
    • Pain in the area around the prosthesis
  • Infection may disseminate systemically

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential
Initial screening test in septic arthritis

Normal WBC count does not rule out septic arthritis

Cell Count, Body Fluid 0095019
Method: Cell Count/Differential

May assist in evaluating for joint disease, systemic disease, or inflammation

Cornerstone test for differential diagnosis on synovial fluid aspirate

Gram Stain 0060101
Method: Stain/Microscopy

Detect WBCs and presence and type of microorganisms in specimen

Low sensitivity (30-50%) negative gram stain does not rule out septic arthritis

C-Reactive Protein 0050180
Method: Quantitative Immunoturbidimetry

May be helpful in initial diagnosis of septic arthritis

Normal CRP does not rule out septic arthritis

Blood Culture 0060102
Method: Continuous Monitoring Blood Culture/Identification

Identifies presence of bacteremia 

Positive in 50-70% of patients with septic arthritis 

Time-sensitive test

Testing is limited to the University of Utah Health Sciences Center only

Body Fluid Culture and Gram Stain 0060108
Method: Stain/Culture/Identification

80-97% sensitivity for non-gonococcal septic arthritis

Anaerobe culture is NOT included with this order

Tissue Culture and Gram Stain 0060127
Method: Stain/Culture/Identification

Identifies bacteria in periprosthetic tissues

Anaerobe culture is NOT included with this order

Additional Tests Available
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Sedimentation Rate, Westergren (ESR) 0040325
Method: Visual Identification

May be helpful in initial diagnosis of septic arthritis

Normal ESR does not rule out septic arthritis

Borrelia burgdorferi Antibodies, Total by ELISA with Reflex to IgG and IgM by Western Blot (Early Disease) 0050267
Method: Semi-Quantitative Enzyme-Linked Immunosorbent Assay/Qualitative Western Blot

Preferred reflex test to detect Lyme disease in individuals with  ≤4 weeks of clinical symptoms or exposure to tick

Consider in patient with negative cultures and who resides in an endemic area

Reflex pattern – if ELISA result is 1.00 LIV or greater, then IgG and IgM Western blot will be added