What is the recommended initial workup for suspected joint infection?

Laboratory Workup for Suspected Joint Infection

Yes, you should order labs immediately for suspected joint infection, specifically ESR and CRP, followed by arthrocentesis with synovial fluid analysis including cell count with differential and cultures, plus blood cultures if systemic signs are present. 1

Essential Laboratory Tests

Inflammatory Markers

• Order both ESR and CRP in all patients with suspected joint infection when the diagnosis is not clinically obvious 1
• The combination of abnormal ESR and CRP provides the best sensitivity and specificity for detecting joint infection 1
• CRP is particularly valuable as it has been shown to have 73-91% sensitivity and 81-86% specificity for joint infection 2
• These markers also establish baseline values critical for monitoring treatment response 2

Synovial Fluid Analysis (Arthrocentesis)

Perform diagnostic arthrocentesis in all patients with suspected acute joint infection unless:

• The diagnosis is clinically evident AND
• Surgery is already planned AND
• Antimicrobials can be safely withheld prior to surgery 1

The synovial fluid analysis must include:

• Total cell count and differential leukocyte count (essential for distinguishing inflammatory from non-inflammatory conditions) 1, 2
• Aerobic and anaerobic cultures (definitive for septic arthritis diagnosis) 1, 2
• Crystal analysis if gout or pseudogout is clinically suspected 1, 2

Blood Cultures

Obtain blood cultures for aerobic and anaerobic organisms if: 1

• Fever is present
• Acute onset of symptoms
• Suspected bloodstream infection (e.g., Staphylococcus aureus)
• Any condition making bacteremia more likely

Plain Radiographs

• Order plain radiographs of the affected joint in all cases to establish baseline and evaluate for alternative diagnoses 1

Critical Timing Considerations

Antibiotic Withholding

If the patient is medically stable, withhold antimicrobial therapy for at least 2 weeks prior to collecting synovial fluid for culture 1, 2

• This significantly increases the likelihood of recovering an organism 1, 2
• Antibiotic administration prior to aspiration is the most common cause of false-negative cultures 3, 2
• Monitor closely for signs of sepsis during this period 3

Prosthetic Joint Infection Considerations

For suspected prosthetic joint infections specifically:

• The same laboratory workup applies (ESR, CRP, arthrocentesis, blood cultures) 1
• If arthrocentesis is negative but clinical suspicion remains high, obtain 3-6 intraoperative tissue samples for aerobic and anaerobic cultures during surgery 3, 4
• This represents the gold standard when preoperative aspiration fails 3

What NOT to Order Routinely

Do not routinely order advanced imaging studies (bone scans, leukocyte scans, MRI, CT, or PET scans) to diagnose joint infection 1, 3

• These tests do not change management when clinical suspicion is already high and surgery is planned 3

Common Pitfalls to Avoid

• Never start antibiotics before obtaining cultures unless the patient is septic or hemodynamically unstable 3, 2
• Do not rely on Gram stain alone—it has poor sensitivity and specificity for joint infection 2
• If initial aspiration is negative but clinical suspicion remains high, repeat aspiration is necessary 2
• Do not assume absence of synovial fluid excludes infection 3
• Serum urate levels alone are insufficient for diagnosing gout 2

Algorithmic Approach

1. Obtain ESR and CRP immediately 1
2. Order plain radiographs 1
3. Perform arthrocentesis with complete synovial fluid analysis (cell count, differential, cultures) 1, 2
4. Draw blood cultures if fever, acute onset, or suspected bacteremia 1
5. Withhold antibiotics for at least 2 weeks before aspiration if patient is stable 1, 2
6. If aspiration negative but suspicion high, proceed to surgery for tissue sampling 3