Septic arthritis, also known as infectious arthritis, is a painful joint infection that can result in significant acute or chronic disability. The causative agent is usually bacteria, but in some cases, the infection may be viral or fungal. Most commonly, hematogenous seeding causes septic arthritis, although other causes include direct inoculation of bacteria into a joint (eg, as a result of an animal bite) or complications of surgery or trauma. Synovial (joint) fluid analysis that includes culture is the cornerstone of diagnosis and should be performed before antibiotic therapy is administered. Blood testing is often recommended to detect biomarkers like C-reactive protein (CRP), identify causative microorganisms, and guide subsequent treatment.
Quick Answers for Clinicians
Prosthetic joint infections (PJIs) are a challenging potential complication of total joint arthroplasty. Although many of the same laboratory tests are used for diagnosis, the diagnostic strategies for PJI and septic arthritis are different. More information about PJI is presented in the Infectious Diseases Society of America (IDSA) Diagnosis and Management of Prosthetic Joint Infection clinical practice guidelines.
Septic arthritis is most likely to form in the joints of patients with preexisting conditions such as rheumatoid arthritis, lupus, or gout. Additional risk factors include advanced age, recent joint surgery or injection, diabetes, intravenous drug use, or immunosuppression. Diagnosis of septic arthritis in patients with these risk factors is often difficult or delayed because symptoms may be confused with or suppressed by an underlying condition. As such, high clinical suspicion for septic arthritis is appropriate in high-risk populations to ensure timely diagnosis.
The most common cause of septic arthritis is Staphylococcus aureus. The number of septic arthritis cases involving methicillin-resistant S aureus (MRSA) is increasing, especially in the United States. Streptococci and other gram-positive organisms are also relatively common causes of septic arthritis. Gram-negative bacilli are uncommon except in cases of trauma, immunosuppression, or intravenous drug use. Viral septic arthritis is rare, but the most common cause of this condition is parvovirus B19. In patients who are immunocompromised, Candida spp are a possible cause of fungal infection.
Radiography, ultrasound, and magnetic resonance imaging (MRI) may be useful in the diagnosis of septic arthritis. However, the utility of these techniques depends on the afflicted joint. Generally, standard radiography is performed first to identify possible tissue swelling or fluid accumulation. Ultrasound is a rapid and noninvasive test that detects the presence of a joint effusion and is particularly useful in the shoulder and hip, where detection of effusion by palpation is difficult. MRI is an excellent imaging modality to differentiate between bone and soft tissue infection. Imaging should be used in addition to comprehensive clinical examination and laboratory testing for the diagnosis of septic arthritis.
Indications for Testing
Laboratory testing is appropriate for patients who present with fever, rigors, and a warm, swollen, painful joint. However, not all patients exhibit these classic symptoms. For example, adults often do not present with fever. Patients who are immunosuppressed, such as those on corticosteroids, may also experience blunted pain. Clinical judgment should be exercised to determine an appropriate diagnostic strategy.
In most cases, there is no single laboratory test with sufficient sensitivity, specificity, and predictive value to diagnose septic arthritis definitively. The standard testing strategy involves laboratory analysis of synovial fluid and blood, and should include cell count, gram stain, and culture. This testing should be performed before the initiation of antimicrobial treatment.
Removal of synovial fluid from the affected joint for testing serves not only as a diagnostic procedure, but also as a treatment approach, because it reduces pressure in the joint. Synovial fluid should be analyzed using cell count, gram stain, and culture.
Elevated synovial white blood cell (WBC) count is highly suggestive of septic arthritis. Confounding factors include immunosuppression and the presence of other diseases such as reactive arthritis. Results should be considered in conjunction with other laboratory tests and clinical evaluation.
Gram stains of synovial fluid should be performed in any case of undiagnosed arthritis to determine the presence of gram-positive bacteria. A positive gram stain is extremely helpful in diagnosis, but a negative result does not preclude a diagnosis of septic arthritis.
Positive synovial fluid culture is considered diagnostic for septic arthritis. Fastidious organisms and previous antibiotic treatment may cause false-negative results. Inoculation of joint fluid in blood culture bottles is recommended for pediatric samples, but in all cases can increase the yield of fastidious organisms and reduce the rate of false-negative results.
Blood testing for septic arthritis may include peripheral WBC count and CRP. In cases of septic arthritis, results for all of these assays are generally elevated. Gout or other inflammatory processes may also cause these results, so further testing is required for definitive diagnosis. Blood cultures obtained during febrile episodes and before antimicrobial treatment may also help to establish the severity of disease, identify causative agents, and guide medical management.
ARUP Laboratory Tests
Automated Cell Count/Differential
Montgomery NI, Epps HR. Pediatric septic arthritis. Orthop Clin North Am. 2017;48(2):209-216.
Nair R, Schweizer ML, Singh N. Septic arthritis and prosthetic joint infections in older adults. Infect Dis Clin North Am. 2017;31(4):715-729.
Osmon DR, Berbari EF, Berendt AR, et al. Diagnosis and management of prosthetic joint infection: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2013;56(1):e1-e25.