What is the next step for a patient with a skin tuberculosis (TB) test induration of 15 mm?

Management of 15 mm TB Skin Test Induration

A tuberculin skin test induration of 15 mm is positive regardless of risk category and requires chest radiography to exclude active tuberculosis, followed by treatment for latent tuberculosis infection (LTBI) if active disease is ruled out. 1, 2

Immediate Next Steps

1. Obtain Chest Radiograph

• All patients with positive tuberculin skin tests must undergo chest radiography (posterior-anterior view) to exclude active pulmonary tuberculosis before initiating LTBI treatment. 1, 3, 2
• Additional lateral views should be obtained at the physician's discretion, and are specifically indicated for children younger than 5 years. 1
• If the chest radiograph shows fibrotic changes consistent with prior TB or a Ghon complex, this indicates LTBI requiring treatment. 1
• If abnormal findings suggest active disease (such as cavitary lesions or infiltrates with mediastinal lymphadenopathy), sputum examination for acid-fast bacilli is mandatory. 1

2. Clinical Assessment for Active Disease

• Evaluate for symptoms of active tuberculosis including cough, fever, night sweats, weight loss, and hemoptysis. 1, 3
• If symptoms are present or chest radiograph is abnormal, collect sputum specimens (ideally three samples) for AFB smear and mycobacterial culture before starting treatment. 1
• Do not delay treatment for active TB if clinical suspicion is high—initiate multi-drug therapy empirically while awaiting culture results. 1

Treatment for Latent Tuberculosis Infection

Baseline Evaluation Before Treatment

• Obtain baseline liver function tests, particularly for patients over 35 years old, those with chronic liver disease, HIV infection, alcohol use, or pregnant/postpartum women. 3
• Assess for contraindications to isoniazid therapy including acute liver disease or previous isoniazid-associated hepatitis. 3, 4
• Review concomitant medications for potential drug interactions, especially with rifamycin-based regimens. 3

Preferred Treatment Regimens

The preferred regimen is isoniazid 300 mg daily for 9 months (strength of recommendation A-II), which provides maximal benefit in preventing progression to active TB. 1, 3

Alternative acceptable regimens include:

• Rifampin 600 mg daily for 4 months (strength B-II for HIV-negative, B-III for HIV-positive patients), particularly when isoniazid toxicity is a concern. 1, 3
• Isoniazid 300 mg daily for 6 months (strength B-I for HIV-negative, C-I for HIV-positive), which provides substantial protection though less than 9 months. 1
• Rifampin plus pyrazinamide for 2 months (strength B-II for HIV-negative, A-I for HIV-positive), though this regimen should be reserved for patients unlikely to complete longer courses due to increased hepatotoxicity risk. 1

Monitoring During Treatment

Monthly clinical assessments are mandatory to evaluate for hepatotoxicity symptoms including nausea, vomiting, jaundice, abdominal pain, and dark urine. 3

• Educate patients to stop medication immediately and seek care if symptoms of hepatotoxicity develop. 3
• Routine laboratory monitoring (monthly liver function tests) is recommended for patients with abnormal baseline tests, age over 35, chronic liver disease, HIV infection, alcohol use, or pregnancy/postpartum status. 3
• Withhold isoniazid if transaminases exceed 3 times the upper limit of normal with symptoms, or 5 times the upper limit without symptoms. 3

Special Considerations

Risk Stratification Context

While 15 mm induration is positive in all risk categories, understanding the patient's risk profile helps guide urgency and monitoring:

• ≥5 mm is positive for: HIV-positive persons, recent TB contacts, immunosuppressed patients, or those with fibrotic chest radiograph changes. 1, 2
• ≥10 mm is positive for: Recent immigrants from high-prevalence countries, injection drug users, healthcare workers, residents of congregate settings, or persons with diabetes, chronic renal failure, silicosis, or malignancies. 1, 2
• ≥15 mm is positive for: Low-risk persons with no identified risk factors. 1, 2

BCG Vaccination History

• Prior BCG vaccination does not contraindicate tuberculin skin testing. 1, 2
• A positive reaction in BCG-vaccinated persons should be interpreted as indicating M. tuberculosis infection when the person has increased risk for recent infection or medical conditions increasing disease risk. 1, 2

Common Pitfalls to Avoid

• Do not dismiss a positive test as "just BCG" in patients from high-prevalence countries or with other risk factors—treat as true LTBI. 1, 2
• Do not start LTBI treatment without first obtaining chest radiography to exclude active disease. 1, 3
• Do not use single-drug therapy if active TB is suspected or confirmed—this requires multi-drug regimens. 1, 4, 5
• Do not continue isoniazid if significant hepatotoxicity develops—the risk of severe liver injury outweighs benefits. 3