
Am Fam Physician. 2021;104(6):589-597
Author disclosure: No relevant financial affiliations.
Septic arthritis must be considered and promptly diagnosed in any patient presenting with acute atraumatic joint pain, swelling, and fever. Risk factors for septic arthritis include age older than 80 years, diabetes mellitus, rheumatoid arthritis, recent joint surgery, hip or knee prosthesis, skin infection, and immunosuppressive medication use. A delay in diagnosis and treatment can result in permanent morbidity and mortality. Physical examination findings and serum markers, including erythrocyte sedimentation rate and C-reactive protein, are helpful in the diagnosis but are nonspecific. Synovial fluid studies are required to confirm the diagnosis. History and Gram stain aid in determining initial antibiotic selection. Staphylococcus aureus is the most common pathogen isolated in septic arthritis; however, other bacteria, viruses, fungi, and mycobacterium can cause the disease. After synovial fluid has been obtained, empiric antibiotic therapy should be initiated if there is clinical concern for septic arthritis. Oral antibiotics can be given in most cases because they are not inferior to intravenous therapy. Total duration of therapy ranges from two to six weeks; however, certain infections require longer courses. Consideration for microorganisms such as Neisseria gonorrhoeae, Borrelia burgdorferi, and fungal infections should be based on history findings and laboratory results.
Septic arthritis should be considered in adults presenting with acute monoarticular arthritis. A delay in diagnosis and treatment of septic arthritis can lead to permanent morbidity and mortality. Subcartilaginous bone loss, cartilage destruction, and permanent joint dysfunction can occur if appropriate antibiotic therapy is not initiated within 24 to 48 hours of onset.1 The reported incidence of septic arthritis is four to 29 cases per 100,000 person-years, and risk increases with age, use of immunosuppressive medications, and lower socioeconomic status.2
Intra-articular infection is typically monoarticular, with up to 20% of cases occurring in multiple joints (oligoarticular [also called polyarticular]).1,3 A joint is most commonly infected hematogenously from bacteremia. Staphylococcus aureus and Streptococcus species are the most common causes. Septic arthritis is diagnosed through laboratory testing, particularly synovial fluid studies.
Diagnosis
HISTORY AND CLINICAL PRESENTATION
The presentation of septic arthritis may vary based on pathogen, underlying medical conditions, or exposures (Table 1).1,2,4–7 Septic arthritis may present similarly to other types of arthritis. More than 50% of patients with septic arthritis have a history of joint swelling, joint pain, and fever. Sweats or rigors are less common. Native joint infections most commonly occur in the knee, followed by the hip, shoulder, ankle, elbow, and wrist.1 Patients with septic arthritis may present with an acutely painful atraumatic joint, which should be differentiated from other causes of monoarticular joint pain (Figure 18–10 and Table 21 ).

Clinical history or exposure | Joint involvement | Pathogen |
---|---|---|
Cleaning fish tank | Small joints (fingers, wrists) | Mycobacterium marinum |
Dog or cat bite | Small joints (fingers, toes) | Capnocytophaga species, Pasteurella multocida |
Exposure to soil or dust containing decomposed wood (North Central and Southern United States) | Monoarticular; knee, ankle, or elbow | Blastomyces dermatitidis |
Ingestion of unpasteurized dairy products | Monoarticular, sacroiliac joint | Brucella species |
Intravenous drug abuse | Axial joints, such as sternoclavicular or sacroiliac joint | Pseudomonas aeruginosa, Staphylococcus aureus |
Nail through shoe | Foot | P. aeruginosa |
Older age | — | Increased risk of gram-negative infections |
Prosthetic joint | Any prosthetic joint | Coagulase-negative staphylococci, Pseudomonas species, Pneumococcus species |
Sexually active | Tenosynovial component in hands, wrists, or ankles | Neisseria gonorrhoeae |
Soil exposure/gardening | Monoarticular; knee, hand, or wrist | Nocardia species, Pantoea agglomerans, Sporothrix schenckii |
Southwestern United States, Central and South America (primary respiratory illness) | Knee | Coccidioides immitis |
Underlying medical conditions | ||
Diabetes mellitus or immunocompromise | — | Increased risk of fungal infection (most commonly Candida), Pseudomonas, and Escherichia coli |
Gout | — | Increased risk of Pseudomonas and E. coli |
Rheumatoid arthritis | Oligoarticular (also called polyarticular) | Increased risk of fungal (most commonly Candida) and pneumococcus infections |
Systemic lupus erythematosus (particularly with functional hyposplenism) | — | N. gonorrhoeae, Proteus species, Salmonella species |
Terminal complement deficiency | Tenosynovial component in hands, wrists, or ankles | N. gonorrhoeae |


Diagnosis | Etiology |
---|---|
Crystal-induced arthritis | Calcium oxalate, cholesterol, gout, hydroxyapatite crystals, pseudogout |
Infectious arthritis | Bacteria, fungi, mycobacteria, spirochetes, viruses |
Inflammatory arthritis | Behçet syndrome,* rheumatoid arthritis,* sarcoidosis, seronegative spondyloarthropathy (e.g., ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease), Still disease,* systemic lupus erythematosus,* systemic vasculitis* |
Osteoarthritis | Erosive/inflammatory variants* |
Systemic infection | Bacterial endocarditis, HIV infection |
Tumor | Metastasis, pigmented villonodular synovitis |
Other | Amyloidosis, avascular necrosis, clotting disorders/anti-coagulant therapy, familial Mediterranean fever,* foreign body, fracture, hemarthrosis, hyperlipoproteinemia,* meniscal tear |
Oligoarticular septic arthritis is more likely to present with symptoms of systemic infection and more commonly affects the shoulder, wrist, and elbow.11 Bacteremia is especially common with septic arthritis of the shoulder.
RISK FACTORS
Risk factors for septic arthritis are listed in Table 3.1,11 Patients with rheumatoid arthritis and a flare-up in one or multiple joints are at particularly high risk. In one study, the incidence of septic arthritis was 1.8 per 1,000 patient-years in those treated with nonbiologic disease-modifying antirheumatic drugs vs. 4.2 per 1,000 patient-years in those treated with anti–tumor necrosis factor therapy.12 People who smoke tobacco also have an increased risk of septic arthritis.2

Contiguous spread |
Skin infection, cutaneous ulceration |
Direct inoculation |
Previous intra-articular injection |
Prosthetic joint (within two years) |
Recent joint surgery |
Hematogenous spread |
Diabetes mellitus |
HIV infection |
Immunosuppressive medication use |
Intravenous drug abuse |
Osteoarthritis |
Other causes of sepsis |
Prosthetic joint (more than two years) |
Rheumatoid arthritis |
Sexual activity (gonococcal arthritis) |
Other |
Age older than 80 years |
Smoking |
PHYSICAL EXAMINATION
The physical examination of patients with septic arthritis almost always reveals a severely painful joint with motion, often including an obvious effusion. The presentation is typically more subtle in those with periprosthetic joint infections, small joint infections, atypical infections (e.g., fungal, Lyme disease, tuberculosis), or immunosuppression. An overlying skin infection can be the source of pain or the entry point of the intra-articular infection.
LABORATORY EVALUATION
Serum markers may be helpful in evaluating for septic arthritis but are not diagnostic. A 2011 study showed that serum erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level are each more than 90% sensitive for septic arthritis when low cutoffs are used (98% for ESR of 10 mm per hour or greater, 94% for ESR of 15 mm per hour or greater, and 92% for CRP of 2.0 mg per dL [20 mg per L] or greater), which is helpful in ruling out septic arthritis.8 A 2017 meta-analysis showed that with a cutoff of 0.5 ng per mL or greater, procalcitonin has a higher specificity than CRP (95% CI, 0.87 to 0.98; positive likelihood ratio = 10.97).13 Blood cultures should be considered when bacteremia or fungemia is suspected.
Analysis of synovial fluid obtained via arthrocentesis is necessary to differentiate septic arthritis from other forms of arthritis and to determine the causative pathogen.1,4 Synovial fluid analysis should include Gram stain, aerobic and anaerobic cultures, and white blood cell count with differential (Table 41,14–16).
Subscribe
From $145- Immediate, unlimited access to all AFP content
- More than 130 CME credits/year
- AAFP app access
- Print delivery available
Issue Access
$59.95- Immediate, unlimited access to this issue's content
- CME credits
- AAFP app access
- Print delivery available
Article Only
$25.95- Immediate, unlimited access to just this article
- CME credits
- AAFP app access
- Print delivery available