How should septic arthritis in an adult be urgently diagnosed and treated, including joint aspiration, empiric intravenous antibiotics covering methicillin‑susceptible and methicillin‑resistant Staphylococcus aureus and gram‑negative organisms, surgical drainage, and duration of therapy?

Septic Arthritis: Urgent Diagnosis and Treatment

Septic arthritis is an orthopedic emergency requiring immediate joint aspiration before antibiotics, followed by empiric intravenous vancomycin 15 mg/kg every 6 hours to cover both methicillin-susceptible and methicillin-resistant Staphylococcus aureus, with surgical drainage for most cases and a total treatment duration of 3-4 weeks. 1, 2

Immediate Diagnostic Approach

Determine Surgical Emergency Status

• Proceed directly to operating room if the patient presents with sepsis/septic shock, immunocompromised status, or evidence of extensive joint destruction—these cases require immediate surgical debridement with intraoperative cultures rather than aspiration 3, 1, 4
• For non-emergency presentations, perform joint aspiration immediately before any antibiotics are administered to maximize organism identification 1, 5

Joint Aspiration Technique

• Use image-guided aspiration (ultrasound or fluoroscopy) to ensure proper needle placement and reduce complications 3, 1
• Send synovial fluid for: white blood cell count with differential, Gram stain, aerobic and anaerobic cultures, and crystal analysis 3, 1
• Synovial fluid WBC ≥50,000 cells/mm³ is highly suggestive of septic arthritis, though this threshold is not absolute and can be influenced by various factors 3, 1
• Obtain blood cultures before antibiotics if fever is present or bacteremia is suspected 1, 2

Critical Pitfall: Antibiotic Timing

• Never administer antibiotics before joint aspiration unless the patient is in septic shock 5
• Antibiotics given before aspiration dramatically reduce diagnostic yield: culture sensitivity drops from 79% to 28%, and microscopy sensitivity plummets from 58% to 12% 5
• If antibiotics were already given, patients should ideally be off antibiotics for at least 2 weeks before aspiration (with careful clinical monitoring for sepsis progression) 3
• A "dry tap" does not exclude infection and warrants further workup 3, 1

Empiric Antibiotic Therapy

First-Line Regimen for Adults

• Vancomycin 15 mg/kg IV every 6 hours (or 30-60 mg/kg/day divided into 2-4 doses) immediately after obtaining cultures 1, 2
• This covers both MSSA (36% of cases) and MRSA (17.6% of cases), with Staphylococcus aureus accounting for >66% of all septic arthritis 2, 6

Add Gram-Negative Coverage When Indicated

• Add cefepime 2 g IV every 12 hours OR ciprofloxacin 400 mg IV every 12 hours for patients with recent hospitalization, healthcare exposure, or immunocompromised status 2
• Never use ciprofloxacin or rifampin as monotherapy for staphylococcal infections due to rapid resistance emergence 2
• For suspected Pseudomonas or multidrug-resistant gram-negatives, dual antipseudomonal coverage is mandatory 2

Pediatric Empiric Therapy

• Vancomycin 15 mg/kg/dose IV every 6 hours (40 mg/kg/day divided into 4 doses) 3, 2
• Clindamycin 10-13 mg/kg/dose IV every 6-8 hours is an alternative if local clindamycin resistance is low (<10%) and the patient is stable without bacteremia 3

Culture-Directed Definitive Therapy

Once Organism Identified

• For MSSA: Switch to nafcillin or oxacillin 1-2 g IV every 4 hours, OR cefazolin 1 g IV every 8 hours 3, 2
• For MRSA: Continue vancomycin; consider adding rifampin 600 mg PO daily or 300-450 mg twice daily for enhanced bone and biofilm penetration 3, 2
• For Streptococcal infections: Penicillin G 20-24 million units IV daily or ceftriaxone 1-2 g IV every 24 hours 2
• For gonococcal arthritis: Ceftriaxone 1-2 g IV daily for 7-14 days, and evaluate for other sexually transmitted infections 4

Surgical Management

Indications for Surgical Drainage

• Surgical debridement is indicated for most cases of septic arthritis to achieve adequate source control 1, 2
• Absolute indications include: surgical emergencies (sepsis, immunocompromised, extensive destruction), persistent infection despite aspiration and antibiotics, loculated effusions, or compartmentalization 1, 4
• Hip joints in children require surgical debridement, whereas arthrocentesis may be adequate for other joints 3
• Synovial fluid culture positivity is approximately 80% in non-gonococcal septic arthritis; if aspirate is positive, proceed with treatment—if negative with high clinical suspicion, consider percutaneous image-guided bone biopsy 3, 1

Duration of Therapy

Native Joint Infections

• Traditional duration: 3-4 weeks total for uncomplicated bacterial arthritis 3, 2
• Recent high-quality evidence supports 2 weeks after surgical drainage for select cases, though this was predominantly studied in smaller joints—the 3-4 week duration remains more prudent for large joints (knee, hip, shoulder) 2
• Extend to 4-6 weeks if concomitant osteomyelitis is present (occurs in up to 30% of pediatric cases) 3, 2

Prosthetic Joint Infections

• Debridement with implant retention: 12 weeks (superior to 6 weeks per 2021 DATIPO trial) 2
• Hip prosthesis with one- or two-stage exchange: 3 months total 2
• Knee prosthesis with one- or two-stage exchange: 6 months total 2

Route Transition

• Switch to oral antibiotics after 2-4 days if patient is clinically improving, afebrile, and tolerating oral intake 2
• Oral options for MRSA include: linezolid 600 mg PO every 12 hours, TMP-SMX (trimethoprim 4 mg/kg/dose) every 8-12 hours plus rifampin 600 mg daily 2

Special Populations and Pathogens

High-Risk Patient Characteristics

• Age >80 years, diabetes mellitus, rheumatoid arthritis, immunosuppression, prosthetic joint, recent intra-articular injection, IV drug use, or recent joint surgery all increase septic arthritis risk 2
• Shoulder septic arthritis is most commonly caused by MRSA, while MSSA predominates in other joints 6
• Group B Streptococcus causes shoulder SA significantly more often than knee or hip infections 6

Age-Specific Pathogens

• Gonococcal arthritis: Consider in sexually active young adults 1
• Kingella kingae: Consider in children <4 years 1, 2
• Salmonella species: Consider in patients with sickle cell disease 1, 2

Immunocompromised Patients

• Require vancomycin plus antipseudomonal β-lactam (cefepime or carbapenem) to cover resistant gram-negatives 2
• Consider adding antifungal coverage (echinocandin) if severely immunosuppressed 2
• Longer treatment courses are generally needed beyond the standard 3-4 weeks 2

Monitoring and Follow-Up

Treatment Response

• Monitor CRP and ESR to assess treatment response 1
• Weekly repeat aspirations may be necessary if initial aspiration is negative and clinical suspicion remains high 3, 1
• Repeat imaging with MRI if clinical response is suboptimal to detect extracapsular spread, abscess formation, or concurrent osteomyelitis 1
• For persistent bacteremia, repeat imaging to identify undrained foci of infection 3

Vancomycin-Specific Monitoring

• Monitor vancomycin trough levels and adjust dosing to avoid toxicity 2
• Be vigilant for drug interactions, especially in elderly patients 3

Linezolid Safety Considerations

• Avoid concurrent use with SSRIs or other serotonergic agents due to serotonin syndrome risk 2
• Monitor for bone marrow suppression and peripheral neuropathy during prolonged therapy 2

Key Clinical Pearls

• MRSA patients are typically older, have more comorbidities, higher inflammatory markers, more complications, longer antibiotic courses, and increased mortality compared to MSSA 7
• Negative Gram stain does not exclude bacterial infection—it has poor sensitivity and specificity 3, 1
• Negative joint aspirate culture does not rule out infection, especially if antibiotics were given beforehand 1, 4, 5
• Treatment initiated within 1 week of symptom onset yields the best outcomes and minimizes irreversible joint damage 8, 9
• Delayed or inadequate treatment can result in joint destruction, osteomyelitis, sepsis, and permanent disability 8, 9, 6