What are the basic tests for diagnosing cutaneous tuberculosis in a patient?

Diagnostic Testing for Cutaneous Tuberculosis

For cutaneous tuberculosis diagnosis, obtain a skin biopsy for both histopathology and mycobacterial culture, perform tuberculin skin testing (TST) with proper Mantoux technique, and obtain a chest radiograph to evaluate for concurrent pulmonary involvement. 1, 2

Initial Clinical Evaluation

• Assess for constitutional symptoms including persistent cough (≥3 weeks), night sweats, weight loss, anorexia, and fever, particularly in patients with TB exposure history, travel to endemic areas, or residence in high-prevalence regions 1, 3
• Determine HIV status immediately, as this critically affects both diagnostic interpretation and treatment approach 1
• Evaluate risk factors including immunosuppression, diabetes, chronic renal failure, malignancy, recent immigration from high-prevalence countries, and injection drug use 4

Core Diagnostic Tests

Tuberculin Skin Testing (TST)

• Administer PPD using the Mantoux technique: inject 0.1 mL of PPD (5 TU) intracutaneously on the volar or dorsal forearm surface, producing a discrete 6-10 mm wheal 4, 1
• Reading timing is critical: trained personnel (not patient self-reading) must measure induration only (not erythema) at 48-72 hours post-injection 4, 1
• Interpretation cutoffs based on risk stratification:
  • ≥5 mm positive for: HIV-infected patients, recent TB contacts, immunosuppressed patients, or those with fibrotic chest radiograph changes 4, 1
  • ≥10 mm positive for: recent immigrants from high-prevalence countries, injection drug users, healthcare workers with TB exposure, patients with diabetes/chronic renal failure/malignancy, children <4 years, residents of high-risk congregate settings 4
  • ≥15 mm positive for: low-risk individuals with no known risk factors 4

Skin Biopsy (Essential for Cutaneous TB)

• Submit tissue for both mycobacterial culture and histopathology as the primary diagnostic approach for cutaneous lesions 2
• Culture remains the gold standard for species identification and drug susceptibility testing 4, 2
• Histopathology may show classic granuloma formation or nonspecific inflammation; when granulomas are absent, proceed with additional testing 2
• PCR testing demonstrates superior diagnostic performance: 88% sensitivity and 83% specificity, outperforming culture, immunohistochemistry, and acid-fast bacilli staining 5
• PCR amplification is invaluable for identifying M. tuberculosis from skin biopsies when cultures are negative 2, 5

Chest Radiography

• Obtain chest radiograph in all suspected cutaneous TB cases to evaluate for pulmonary involvement, as approximately one-third have concurrent systemic disease 1
• Look for Ghon complex (indicative of latent TB), or Ghon complex with enlarged mediastinal lymph nodes (suggesting active primary TB) 4
• Apical cavitary lesions, infiltrates, and scarring may indicate active or prior pulmonary disease 4

Additional Diagnostic Considerations

Sputum Examination (When Indicated)

• Collect three sputum specimens on different days for AFB smear microscopy and mycobacterial culture if respiratory symptoms present or chest radiograph abnormal 4, 3
• Perform nucleic acid amplification testing (NAAT) on initial respiratory specimen for rapid diagnosis 4

Interferon-Gamma Release Assays (IGRAs)

• Consider IGRA as alternative or adjunct to TST in adults, particularly in BCG-vaccinated populations where specificity is higher 4, 3
• For children <5 years, TST is preferred over IGRA 4

Critical Diagnostic Pitfalls to Avoid

• Never dismiss positive TST results in BCG-vaccinated patients—prior BCG vaccination should not alter interpretation of positive results in high-risk individuals 1
• Do not rely on anergy testing in HIV-infected or immunocompromised patients due to lack of standardization, poor reproducibility, and uncertain clinical benefit 4, 1
• Only trained personnel should read TST results—patient or healthcare worker self-reading is unacceptable 4, 1
• Measure only induration, never erythema when reading TST 4, 1
• Do not exclude cutaneous TB based on negative cultures alone—clinical correlation is paramount, and PCR may be positive when cultures are negative 5
• If uncertain about TST validity, repeat on the opposite arm within five weeks rather than accepting a potentially false-negative result 4

Diagnostic Algorithm Summary

1. Perform TST with proper Mantoux technique and trained reading at 48-72 hours 1
2. Obtain skin biopsy for culture, histopathology, and PCR testing 2, 5
3. Order chest radiograph to assess for pulmonary involvement 1
4. If respiratory symptoms or abnormal chest radiograph: collect sputum for AFB smear, culture, and NAAT 4, 3
5. Interpret all results in clinical context—bacteriological/histological confirmation achieved in 88% of cases, but clinical findings remain critical when laboratory tests are inconclusive 5, 6