Septic Arthritis — Adult

Key Points

• Acute painful loss of both active and passive joint motion is a red flag for septic arthritis.
• Arthrocentesis is the central diagnostic test and should precede antibiotics when the patient is not septic or unstable.
• No single synovial WBC threshold can rule septic arthritis in or out.
• Finding urate or calcium pyrophosphate crystals does not exclude simultaneous infection.
• Gram stain is insensitive; synovial culture and blood cultures are essential microbiologic tests.
• Empiric therapy should always cover S. aureus, with additional gram-negative coverage based on host and exposure risk.
• Large-joint infection usually requires urgent drainage or operative washout in addition to antibiotics.
• Failure to improve after source control requires renewed evaluation for inadequate drainage, resistant pathogen, osteomyelitis, or an alternative diagnosis.

Algorithm

1. Recognize an acutely painful swollen joint and assess for sepsis, bacteremia, prosthesis, trauma, or overlying cellulitis.
2. Obtain blood cultures when indicated and perform urgent arthrocentesis of the native joint.
3. Send synovial WBC/differential, Gram stain, aerobic and anaerobic cultures, and crystals.
4. If septic or unstable, start antibiotics immediately after rapidly obtained cultures; otherwise begin after joint aspiration.
5. Consult orthopedics for large-joint lavage, repeated drainage, or open source control according to anatomy.
6. Narrow antibiotics when organism and susceptibility data return.
7. Assess for endocarditis, osteomyelitis, gonococcal infection, or another source when clinical clues are present.
8. Begin rehabilitation after control of acute infection and follow inflammatory and functional recovery.

Clinical Synopsis & Reasoning

Acute monoarthritis with severe pain, reduced active and passive range of motion, swelling, warmth, fever, or inability to bear weight should raise concern for septic arthritis. Fever is absent in many cases. Risk factors include older age, diabetes, immunosuppression, injection drug use, recent bacteremia, skin infection, joint surgery or injection, and pre-existing joint disease. Prosthetic-joint infection follows a different diagnostic and treatment pathway. The knee is most commonly affected, but hip, shoulder, wrist, ankle, and sternoclavicular infection can be serious. Obtain blood cultures when systemic infection is suspected. Arthrocentesis is the key diagnostic procedure and should not be delayed for advanced imaging in an accessible joint. Synovial WBC values above 50,000 cells/mm3 with neutrophil predominance increase probability, but lower counts occur early, after antibiotics, with immunosuppression, and in gonococcal infection. Crystals do not exclude concurrent bacterial infection. Gram stain has limited sensitivity, so culture remains essential. Ultrasound helps confirm effusion and guide aspiration in deeper joints; MRI is useful for adjacent osteomyelitis, abscess, or sacroiliac and spinal-region disease. Consider gout, pseudogout, inflammatory arthritis, hemarthrosis, reactive arthritis, Lyme arthritis in endemic settings, and gonococcal disease.

Treatment Strategy & Disposition

After synovial and blood cultures are obtained, begin empiric antibiotics promptly. Regimen selection should cover Staphylococcus aureus and streptococci, adding gram-negative coverage for older, immunocompromised, injection-drug, health care-associated, or otherwise high-risk patients. Vancomycin plus ceftriaxone or cefepime is a common broad starting framework in seriously ill adults, modified for local susceptibility and host factors. Disseminated gonococcal infection requires ceftriaxone-based treatment and testing for other sexually transmitted infections. Native large-joint infection usually needs prompt arthroscopic lavage or repeated drainage, with open surgery for selected anatomy or failed less invasive source control. The 2023 SANJO guideline emphasizes obtaining joint samples before antibiotics unless sepsis requires immediate therapy. Tailor antibiotic route and total duration to organism, joint, surgical findings, bacteremia, osteomyelitis, and response. Early mobilization after infection control helps prevent stiffness, but the joint should not be exercised through uncontrolled acute pain before source control. Persistent fever, positive cultures, rising inflammatory markers, or failure of function to improve should trigger repeat aspiration, imaging, and surgical reassessment.

Epidemiology / Risk Factors

• S. aureus is the dominant pathogen in adult native-joint septic arthritis.
• Streptococci are also common, while gram-negative bacilli occur more often in older, immunocompromised, and health care-exposed patients.
• Disseminated gonococcal infection remains an important cause in sexually active adults.
• Pre-existing inflammatory or degenerative joint disease increases susceptibility and can make diagnosis more difficult.
• Delay in source control increases the risk of permanent joint damage.

Investigations

Synovial Fluid (Typical Adult)

Pharmacology

Prognosis / Complications

• Early drainage and active antimicrobial therapy improve survival and preserve joint function.
• Mortality is highest in older patients, polyarticular infection, bacteremia, and severe comorbidity.
• Residual pain, stiffness, and cartilage loss can persist despite microbiologic cure.
• Hip and shoulder infections often have greater functional consequences because diagnosis and drainage can be delayed.
• Recurrent or persistent infection should prompt evaluation for osteomyelitis, endocarditis, or an incompletely controlled source.

Patient Education / Counseling

• Explain that joint infection can damage cartilage quickly and generally needs both antibiotics and drainage.
• Tell patients that crystals on the aspiration do not automatically mean gout alone if infection is still plausible.
• Complete the prescribed antimicrobial course and attend surgical and infectious-disease follow-up.
• Seek urgent reassessment for recurrent fever, increasing swelling, wound drainage, or worsening joint function.
• After recovery, follow the rehabilitation plan gradually to restore range of motion without compromising healing.

Notes

Septic arthritis should be approached as a source-control infection rather than a laboratory-number diagnosis. The decisive step is obtaining synovial fluid and then ensuring that an infected joint is actually drained. A high synovial cell count strengthens the diagnosis, but lower counts, negative Gram stain, or crystals cannot safely rule it out when the clinical syndrome remains convincing. Rapid orthopedic involvement, organism-directed antibiotics, and reassessment of any poor response protect both the patient and the joint.

References

1. 2023 SANJO Guideline for Management of Septic Arthritis in Native Joints — Link