Septic Arthritis

Septic arthritis, also known as infectious arthritis, may be caused by any number of different microorganisms and results in erythematous, painful, swollen joints. Synovial (joint) fluid analysis is the cornerstone of diagnosis and should be performed before beginning antibiotic therapy. Blood or joint fluid culture may also be used to identify the microorganism causing the infection and guide subsequent treatment.

  • Diagnosis
  • Monitoring
  • Background
  • Lab Tests
  • References
  • Related Topics
  • Videos

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
    • Kawasaki disease
    • Toxic (or transient) synovitis
    • Slipped capital femoral epiphysis
    • Acute rheumatic fever
    • Legg-Calvé-Perthes disease (LCPD)
    • Osteochondrosis
    • Sickle cell disease
    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


  • Incidence – 2-10/100,000 in the U.S.
    • 30-40/100,000 in patients with rheumatoid arthritis
    • 40-70/100,000 in patients with prosthetic joints
  • 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 aureusgroup A streptococcusKingella 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
    • Joint disease
    • Loss of skin integrity
      • Psoriasis
      • Eczema
      • Skin ulcers
    • Trauma
    • Surgery – arthroscopy
    • Procedure related – intra-articular steroids, Synvisc-One
    • Chronic disease
    • Immunosuppression
    • Intravenous drug use
    • Bacteremia, endocarditis
  • 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


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.

CBC with Platelet Count and Automated Differential 0040003
Method: Automated Cell Count/Differential


Normal WBC count does not rule out septic arthritis

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

Gram Stain 0060101
Method: Stain/Microscopy


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

C-Reactive Protein 0050180
Method: Quantitative Immunoturbidimetry


Normal CRP does not rule out septic arthritis

Blood Culture 0060102
Method: Continuous Monitoring Blood Culture/Identification


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


Anaerobe culture is NOT included with this order

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


Anaerobe culture is NOT included with this order


Choosing Wisely. An initiative of the ABIM Foundation. [Accessed: Nov 2017]

Coakley G, Mathews C, Field M, Jones A, Kingsley G, Walker D, Phillips M, Bradish C, McLachlan A, Mohammed R, Weston V, British Society for Rheumatology Standards, Guidelines and Audit Working Group. BSR & BHPR, BOA, RCGP and BSAC guidelines for management of the hot swollen joint in adults. Rheumatology (Oxford). 2006; 45(8): 1039-41. PubMed

Weston V, Coakley G, British Society for Rheumatology (BSR) Standards, Guidelines and Audit Working Group, British Society for Antimicrobial Chemotherapy, British Orthopaedic Association, Royal College of General Practitioners, British Health Professionals in Rheumatology. Guideline for the management of the hot swollen joint in adults with a particular focus on septic arthritis. J Antimicrob Chemother. 2006; 58(3): 492-3. PubMed

General References

Courtney P, Doherty M. Joint aspiration and injection and synovial fluid analysis. Best Pract Res Clin Rheumatol. 2009; 23(2): 161-92. PubMed

Del Pozo JL, Patel R. Clinical practice. Infection associated with prosthetic joints. N Engl J Med. 2009; 361(8): 787-94. PubMed

García-Arias M, Balsa A, Mola EM. Septic arthritis. Best Pract Res Clin Rheumatol. 2011; 25(3): 407-21. PubMed

Horowitz DL, Katzap E, Horowitz S, Barilla-LaBarca M. Approach to septic arthritis. Am Fam Physician. 2011; 84(6): 653-60. PubMed

Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis? JAMA. 2007; 297(13): 1478-88. PubMed

Mathews CJ, Kingsley G, Field M, Jones A, Weston VC, Phillips M, Walker D, Coakley G. Management of septic arthritis: a systematic review. Ann Rheum Dis. 2007; 66(4): 440-5. PubMed

Mathews CJ, Weston VC, Jones A, Field M, Coakley G. Bacterial septic arthritis in adults. Lancet. 2010; 375(9717): 846-55. PubMed

Punzi L, Oliviero F. Arthrocentesis and synovial fluid analysis in clinical practice: value of sonography in difficult cases. Ann N Y Acad Sci. 2009; 1154: 152-8. PubMed

Sharff KA, Richards EP, Townes JM. Clinical management of septic arthritis. Curr Rheumatol Rep. 2013; 15(6): 332. PubMed

Vassilopoulos D, Calabrese LH. Virally associated arthritis 2008: clinical, epidemiologic, and pathophysiologic considerations. Arthritis Res Ther. 2008; 10(5): 215. PubMed

Medical Reviewers

Last Update: November 2017