What are the diagnostic steps for cutaneous tuberculosis in a patient with skin lesions?

Diagnostic Testing for Cutaneous Tuberculosis

For suspected cutaneous tuberculosis, obtain a skin biopsy for both histopathology and mycobacterial culture, perform tuberculin skin testing (TST) or interferon-gamma release assay (IGRA), and obtain a chest radiograph to evaluate for pulmonary involvement. 1, 2

Initial Clinical Assessment

When evaluating skin lesions suspicious for cutaneous TB, look for these specific features:

• Lesion morphology patterns: Verrucous plaques, ulcerations, nodules, abscesses, or lupus vulgaris-type lesions 1, 3
• Constitutional symptoms: Persistent cough >3 weeks, night sweats, weight loss, anorexia, or fever 4
• Exposure history: Contact with active TB cases, travel to endemic areas, or residence in high-prevalence regions 4
• HIV status: Must be determined as it affects diagnostic interpretation and treatment approach 4

Diagnostic Algorithm

Step 1: Skin Biopsy (Essential)

Obtain a skin biopsy and submit for BOTH culture and histopathology simultaneously 1, 2

• Culture: Request mycobacterial culture specifically, as M. tuberculosis has a low bacillary load in cutaneous lesions 2, 5
• Histopathology: Look for granulomatous inflammation, though some cases show only nonspecific inflammation without classic granuloma formation 1
• Acid-fast bacilli (AFB) staining: Perform on tissue, though sensitivity is limited in paucibacillary forms 2, 5

Critical caveat: Multibacillary forms (tuberculous chancre, scrofuloderma, orificial TB) are easier to diagnose bacteriologically, while paucibacillary forms (lupus vulgaris, tuberculosis verrucosa cutis) require correlation of clinical, epidemiological, and histological data 5

Step 2: Tuberculin Skin Testing

Administer PPD using the Mantoux technique: 0.1 mL of PPD (5 tuberculin units) intracutaneously on the volar or dorsal forearm 4, 6

• Proper injection: Should produce a 6-10 mm wheal 4, 6
• Reading timing: Must be read by trained personnel at 48-72 hours, measuring only induration (not erythema) in millimeters 4, 6

Interpretation cutoffs:

• ≥5 mm positive: HIV-infected patients, recent TB contacts, immunosuppressed patients (≥15 mg prednisone daily for ≥1 month), organ transplant recipients, or those with fibrotic chest radiograph changes 6, 7
• ≥10 mm positive: Moderate-risk individuals 6
• ≥15 mm positive: Low-risk individuals with no known risk factors 6

Important limitation: A negative TST does NOT rule out cutaneous TB, as false-negative rates reach 25% in active TB and are higher in HIV-infected patients 4, 7

Step 3: Chest Radiography

Obtain a chest radiograph in ALL suspected cutaneous TB cases 4, 7

• Approximately one-third of cutaneous TB cases have concurrent systemic involvement 3
• Look for apical cavitary disease, infiltrates in any lung zone, or mediastinal/hilar adenopathy 4
• HIV-infected patients may have atypical radiographic presentations or even normal chest radiographs despite active disease 4

Step 4: Molecular Testing (When Initial Tests Are Inconclusive)

If histopathology is nonspecific and cultures are negative or pending, utilize PCR amplification 1, 2

• PCR has proven invaluable for identifying M. tuberculosis from skin biopsies when cultures are negative 1
• Offers rapid results with high sensitivity and specificity 2
• Particularly useful in paucibacillary forms where AFB stains and cultures are typically negative 5

Alternative: Consider IGRA testing (QuantiFERON-TB Gold or T-SPOT.TB) as an adjunct or alternative to TST, especially if patient cannot return for TST reading 6

Step 5: Additional Bacteriologic Studies

If pulmonary symptoms are present or chest radiograph is abnormal, collect sputum specimens 4, 7

• Obtain 3-5 sputum specimens on different days for AFB smear and mycobacterial culture 4
• Negative sputum smears do not exclude pulmonary TB 4

Common Diagnostic Pitfalls

• Do not dismiss positive TST in BCG-vaccinated patients: Interpret positive reactions as M. tuberculosis infection when the patient is at increased risk 7
• Do not rely on anergy testing: The CDC explicitly recommends against anergy testing in HIV-infected or immunocompromised patients due to lack of standardization and poor reproducibility 4, 7
• Do not accept patient self-reading of TST: Only trained personnel should read and interpret results 4, 6
• Do not measure erythema: Only induration should be measured and recorded 4, 6, 7
• Do not delay biopsy: Cutaneous TB diagnosis is challenging and requires tissue confirmation, as clinical mimicry of other conditions is common 3
• Do not assume isolated cutaneous disease: Always evaluate for systemic involvement with chest radiograph and symptom assessment 3

Classification Context

Once diagnosis is established, patients should be classified according to the American Thoracic Society/CDC framework as Class 3 (clinically active tuberculosis) if they have clinical, bacteriological, and/or radiographic evidence of current disease 4