If a patient has symptoms of Tuberculosis (TB) and a respiratory culture positive for Methicillin-resistant Staphylococcus aureus (MRSA), should you still check for TB?

Testing for Tuberculosis in Patients with Respiratory Cultures Positive for MRSA

Yes, you should still test for tuberculosis (TB) in patients with symptoms of TB even if their respiratory culture shows MRSA. 1 The presence of one respiratory pathogen does not exclude the possibility of concurrent TB infection, which requires specific diagnostic testing and treatment.

Rationale for TB Testing Despite MRSA Positivity

• TB should be considered in patients with persistent respiratory symptoms (cough lasting ≥2-3 weeks), especially with associated symptoms like fever, night sweats, weight loss, or hemoptysis 1, 2
• The identification of MRSA in respiratory cultures does not rule out concurrent TB infection, as mixed infections can occur 3
• Failure to diagnose TB can lead to:
  • Continued disease transmission to others
  • Increased morbidity and mortality for the patient
  • Delayed appropriate treatment

Diagnostic Approach for Suspected TB

Initial Assessment

1. Evaluate TB risk factors 1:

   • Close contacts of persons with known/suspected TB
   • Foreign-born persons from high-incidence areas (Africa, Asia, Eastern Europe, Latin America, Russia)
   • Residents/employees of congregate settings (correctional facilities, long-term care facilities)
   • Immunocompromised status (HIV, diabetes, immunosuppressive medications)

2. Obtain appropriate specimens 1:

   • Collect at least 3 early morning sputum samples on separate days
   • For patients unable to produce sputum, consider induced sputum or bronchoscopy
   • Samples should be sent for:
     • AFB smear microscopy
     • Mycobacterial culture (gold standard)
     • Nucleic acid amplification testing (NAAT)

Diagnostic Testing

• AFB smear microscopy: Provides rapid results but has limited sensitivity (detects 104-105 AFB/ml) 1
• NAAT: Should be performed on initial respiratory specimen for rapid detection of M. tuberculosis 2
• Mycobacterial culture: Essential for definitive diagnosis and drug susceptibility testing 1
• Chest imaging: Obtain chest X-ray; consider CT in cases with high clinical suspicion but negative X-ray 2

Special Considerations

Infection Control

• Patients with suspected TB should be placed in respiratory isolation until determined to be non-infectious 1
• Isolation can be discontinued after:
  • Effective treatment for at least 2 weeks
  • Clinical improvement
  • Three consecutive negative sputum smears 4

Treatment Implications

• If TB is confirmed, standard treatment includes a combination of isoniazid, rifampin, ethambutol, and pyrazinamide for 2 months, followed by isoniazid and rifampin for 4 months 5, 6
• MRSA infection would require separate appropriate antibiotic therapy (e.g., vancomycin) 3
• Drug susceptibility testing should be performed on all TB isolates to guide appropriate therapy 1

Common Pitfalls to Avoid

• Diagnostic anchoring: Don't assume MRSA is the only pathogen present; TB can coexist with other infections
• Premature discontinuation of isolation: Maintain respiratory isolation until TB is ruled out, especially in high-risk patients
• Inadequate sampling: Collect multiple sputum specimens to increase diagnostic yield
• Relying solely on smear microscopy: Negative smears do not exclude TB; cultures and molecular tests are essential 1
• Failure to consider atypical presentations: TB may present atypically in immunocompromised patients 2

By following this systematic approach, clinicians can ensure that TB is appropriately diagnosed and treated, even when other respiratory pathogens like MRSA are present.