What test should be done after a positive Tuberculin (TB) skin test?

Management After a Positive TB Skin Test

A chest radiograph should be obtained immediately for all persons with a positive tuberculin skin test to exclude active pulmonary tuberculosis. 1

Algorithmic Approach to Positive TST

Step 1: Obtain Chest X-Ray

• All individuals with a positive TST require a chest radiograph as the mandatory next step, regardless of symptoms 1
• The chest X-ray serves to differentiate between latent TB infection (LTBI) and active pulmonary disease 1
• This applies even to pregnant women, who should have chest radiographs with appropriate shielding as soon as feasible, even during the first trimester 1
• Children younger than 5 years require both posterior-anterior and lateral views 1

Step 2: Clinical Evaluation

• Perform a symptom assessment focusing on: persistent cough (>2-3 weeks), fever, night sweats, weight loss, hemoptysis, and chest pain 2
• Healthcare personnel and exposed individuals with positive TST should undergo clinical evaluation for active tuberculosis 3

Step 3: Interpretation Based on Chest X-Ray Results

If Chest X-Ray is NORMAL and Patient is ASYMPTOMATIC:

• Consider the patient for treatment of latent tuberculosis infection (LTBI) 1
• Standard LTBI treatment is isoniazid for 6-9 months 4
• No repeat chest radiographs are needed unless symptoms develop in the future 3
• Counsel the patient to report any pulmonary symptoms promptly, as they remain at risk for future TB reactivation 3

If Chest X-Ray Shows ABNORMALITIES or Patient is SYMPTOMATIC:

• Proceed immediately to sputum examination for acid-fast bacilli (AFB) smear and mycobacterial culture 1
• Obtain at least 3 serial sputum samples for AFB smear and culture 2
• For radiographic findings suggestive of prior healed TB, three consecutive sputum samples should be collected 1
• Consider nucleic acid amplification testing as an adjunct in cases of moderate to high TB suspicion 2
• If active TB is confirmed, initiate standard four-drug therapy (isoniazid, rifampin, ethambutol, pyrazinamide) for 2 months, followed by isoniazid and rifampin for 4 additional months 2

Radiographic Findings to Recognize

Typical Active TB Patterns:

• Upper lobe infiltration, particularly with cavitation 1
• Patchy or nodular infiltrates in apical or subapical posterior upper lobes 1
• Cavitary disease is more common in patients with larger TST reactions (≥15 mm) 5

Atypical Presentations:

• HIV-infected patients may show infiltrates in any lung zone or mediastinal/hilar adenopathy rather than classic upper lobe disease 1
• Miliary or disseminated disease is more common in patients with negative or low TST reactions 5

Special Populations

Healthcare Workers:

• Those with TST ≥5 mm or symptoms require chest radiographs 3
• If chest X-ray is normal and asymptomatic, evaluate for preventive therapy 3
• Work restrictions apply only if active pulmonary or laryngeal TB is diagnosed 3

HIV-Infected Individuals:

• Offer HIV counseling and testing to all personnel diagnosed with active TB 3
• Evaluate HIV risk in those with positive TST; if HIV infection is possible, strongly encourage testing 3
• HIV-positive patients with positive TST should receive preventive therapy for 9-12 months 4

Critical Pitfalls to Avoid

• Never skip the chest X-ray: Even asymptomatic patients with positive TST require radiographic evaluation to exclude active disease 1
• Do not rely on TST alone to diagnose active TB: A negative TST does not rule out active tuberculosis, particularly in immunocompromised patients or those with disseminated disease 6, 5
• Do not treat suspected active TB as LTBI: If clinical or radiographic findings suggest active disease, obtain sputum studies before initiating therapy—active TB requires combination therapy, not isoniazid monotherapy 6
• Do not order routine follow-up chest X-rays: After the initial evaluation, repeat radiographs are only indicated if new symptoms develop 3

Documentation Requirements

• Record the exact induration measurement in millimeters, not just "positive" or "negative" 4
• Document risk factors that influenced the interpretation threshold (≥5 mm for high-risk, ≥10 mm for moderate-risk, ≥15 mm for low-risk) 1, 4
• Maintain records of chest X-ray results and clinical evaluation findings 3