Acute Septic Arthritis

Acute septic arthritis is an infection of the joint that requires prompt diagnosis and treatment to prevent joint destruction. The infection is most commonly caused by Staphylococcus or Streptococcus species, followed by gram-negative bacilli. Gram-negative bacilli (such as Escherichia coli) are more commonly implicated in patients with advanced age, after joint trauma, or in immunosuppressed patients. Neisseria gonorrhoeae may be suspected in sexually active patients. Tuberculosis may be suspected in patients with an appropriate history (e.g., prior exposure to tuberculosis or with compromised immune function), and rarely, infectious arthritis is due to other mycobacterial species or fungi.

Acute septic arthritis usually presents with a single hot, erythematous, painful joint, sometimes with systemic symptoms. The presentation of crystal arthritis can be quite similar and needs to be ruled out. Of note, acute crystal arthritis and septic arthritis can coexist.

Aspiration and culture of the affected joint is essential when septic arthritis is suspected. In a patient with suspected septic arthritis, the presence of crystals on synovial fluid examination should not exclude the diagnosis. In addition, it should be noted that many patients with septic arthritis do not have a fever on presentation, so the absence of fever similarly should not exclude the diagnosis.

Risk Factors

• existing joint disease (e.g., rheumatoid arthritis or osteoarthritis)

• concomitant immunosuppressive medication

• prosthetic joints

• low socioeconomic status

• intravenous drug use

• alcohol use disorder

• diabetes

• recent intra-articular glucocorticoid injection or other intra-articular procedure

• cutaneous ulcers

• direct joint trauma (including recent joint surgery, aspiration, or injection)

Diagnosis

Laboratory tests to support the diagnosis of a generalized inflammatory response that might be driven by infection include complete blood count (CBC), C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR).

• Synovial fluid analysis:

  • A white blood cell (WBC) count >50,000 cells/mm³ is suggestive of septic arthritis and a WBC count >100,000 is even more specific (predominantly with neutrophils). However, a WBC count <50,000 does not exclude septic arthritis, and WBC counts >50,000 can be seen in other conditions, as well (e.g., crystal-induced arthritis).

  • Synovial fluid WBC counts closer to 20,000 are common with gonococcal septic arthritis.

  • There is no single cutoff for synovial fluid WBC count that can rule in or rule out septic arthritis. In general, the higher the WBC count and percentage of neutrophils, the higher the suspicion for septic arthritis.

  • Synovial fluid Gram stain and culture are required to evaluate for infection.

  • The presence of urate or calcium pyrophosphate crystals is diagnostic for gout or pseudogout, respectively. However, their presence does not exclude concomitant septic arthritis.

  • Watch a video on performing arthrocentesis here.

• Blood cultures: Most cases of septic arthritis unrelated to trauma develop via bacteremia. Peripheral blood cultures are a necessary part of the overall workup but are often negative. Additional sources of infection should be considered (e.g., urinary tract infection, pneumonia, abscess, intravenous drug use, and osteomyelitis).

Plain radiographs can help exclude fracture as a cause of acute joint pain and swelling and serve as a baseline assessment of damage for future comparison. They are not typically diagnostic in the setting of acute septic arthritis.

Management

The management of septic arthritis has not been studied in placebo-controlled trials. Treatment options differ for septic arthritis of a native joint versus a prosthetic joint.

Native joint: In general, the management of acute septic arthritis of a native joint includes the prompt initiation of empiric antibiotic therapy (find suggested empiric regimens here). Patients treated with antibiotics prior to arthrocentesis may have lower synovial-fluid WBC counts than is typical; such patients may also have negative microbiologic studies despite having septic arthritis.

Drainage and decompression of the infected joint either via surgical lavage or bedside arthrocentesis is appropriate, particularly when the effusion is large, when septic arthritis is highly suspected (i.e., elevated synovial-fluid WBC count), or there is inadequate improvement with antibiotics; whether arthroscopic washout is routinely necessary is controversial.

Prompt surgical intervention is necessary for patients with hemodynamic instability and for most cases of septic arthritis involving deep joints, such as the shoulder or hip (these should be surgically drained due to difficulty assessing source control).

Prosthetic joint: In cases of suspected septic arthritis of a prosthetic joint, discussion with an experienced surgeon prior to joint aspiration and administration of antibiotics is desirable. Infected prosthetic joints require surgical management if cure is to be achieved; otherwise, long-term suppressive therapy with antimicrobial agents is necessary. The introduction of a percutaneous needle into a prosthetic joint without aseptic conditions should be avoided.

The following algorithm describes a general management approach:

(Source: Infection Associated with Prosthetic Joints. N Engl J Med 2009.)