What tests should be performed before initiating isoniazid therapy for latent tuberculosis infection?

Testing Prior to Isoniazid for Latent Tuberculosis Infection

Before starting isoniazid, you must obtain a chest radiograph to exclude active tuberculosis, and baseline liver function tests are only required for specific high-risk patients—not routinely for everyone. 1

Mandatory Pre-Treatment Evaluation

Clinical Assessment

• Active tuberculosis must be definitively excluded through detailed history, physical examination, chest radiography, and when indicated, bacteriologic studies before initiating any single-drug therapy for latent TB. 1
• Assess specifically for TB symptoms including fever, night sweats, weight loss, cough, hemoptysis, and any respiratory complaints. 1
• Document HIV status, pregnancy status, alcohol use, history of liver disease, and current medications. 1

Chest Radiography (Required for All Patients)

• A posterior-anterior chest X-ray is mandatory for all persons being considered for latent TB treatment to exclude active pulmonary disease. 1
• Children younger than 5 years require both posterior-anterior and lateral radiographs. 1
• Pregnant women with positive tuberculin tests or recent TB exposure should have chest radiographs with appropriate shielding as soon as feasible, even during the first trimester. 1
• If the chest radiograph is normal and no TB symptoms are present, the patient may be a candidate for latent TB treatment. 1

Sputum Examination (Selective, Not Routine)

• Sputum examination is not indicated for most persons being considered for latent TB treatment. 1
• Three consecutive sputum samples (obtained on different days) for acid-fast bacilli smear and culture are required if chest radiograph shows findings suggestive of prior, healed TB infections. 1
• HIV-infected persons with respiratory symptoms should have sputum specimens submitted for mycobacterial examination even if the chest radiograph is normal. 1
• Patients with only calcified pulmonary nodules on radiograph do not require bacteriologic examination. 1

Baseline Laboratory Testing

Patients Who Do NOT Need Routine Baseline Labs

• Baseline laboratory testing is not routinely indicated for all patients at the start of latent TB treatment. 1
• Baseline testing is not routinely indicated based solely on older age, though it may be considered individually for patients taking other medications for chronic conditions. 1

Patients Who DO Need Baseline Liver Function Tests

Baseline hepatic measurements (AST/ALT and bilirubin) are indicated for: 1

• HIV-infected patients
• Pregnant women and women in the immediate postpartum period (within 3 months of delivery)
• Persons with history of chronic liver disease (hepatitis B or C, alcoholic hepatitis, or cirrhosis)
• Persons who use alcohol regularly
• Patients whose initial evaluation suggests a liver disorder
• Persons at risk for chronic liver disease

Additional Baseline Considerations

• Active hepatitis and end-stage liver disease are relative contraindications to isoniazid for latent TB treatment. 1
• Patients with baseline liver function abnormalities require routine laboratory monitoring during treatment. 1

Critical Pitfalls to Avoid

• Never start isoniazid without first obtaining a chest X-ray—a normal chest X-ray does not completely exclude active TB in immunocompromised patients, so maintain high clinical suspicion. 2
• Never assume all patients need baseline liver function tests—this wastes resources and is not supported by guidelines for low-risk patients. 1
• Never initiate single-drug treatment until active TB is definitively excluded—treating unrecognized active disease with isoniazid alone promotes drug resistance. 1, 2
• Never delay chest radiography in pregnant women with positive tuberculin tests or recent TB exposure due to pregnancy concerns—appropriate shielding makes this safe even in the first trimester. 1

Monitoring During Treatment

• Monthly clinical evaluations are required for patients receiving isoniazid alone, including questioning about side effects and brief physical assessment for signs of hepatitis. 1
• Educate all patients to stop treatment immediately and seek medical evaluation if they develop fever, malaise, vomiting, jaundice, or unexplained deterioration. 1
• Routine laboratory monitoring during treatment is indicated only for persons whose baseline liver function tests were abnormal or who are at risk for hepatic disease. 1
• Some experts recommend withholding isoniazid if transaminase levels exceed 3 times the upper limit of normal with symptoms or 5 times the upper limit of normal if asymptomatic. 1

Pyridoxine Supplementation

• Concomitant administration of pyridoxine (vitamin B6, 10-25 mg daily) is recommended in malnourished patients and those predisposed to neuropathy, including alcoholics and diabetics. 1, 3