At what age is Isoniazid (INH) prophylaxis recommended in infants?

INH Prophylaxis in Infants: Age-Based Recommendations

Isoniazid prophylaxis should be initiated immediately from birth (age 0) in infants born to mothers with infectious pulmonary tuberculosis who have not completed at least 2 weeks of effective treatment. 1

Clinical Algorithm Based on Maternal TB Status

For Newborns of Mothers with Active/Infectious TB

• Start INH prophylaxis immediately at birth if the mother is sputum smear-positive or has not completed at least 2 weeks of effective anti-tuberculous treatment 1
• The standard pediatric dose is 10-15 mg/kg/day as a single daily dose (maximum 300 mg) 1
• For term infants ages 0-8 months, use 3 mg/kg per dose twice daily 2
• Continue prophylaxis for 3 months, then perform tuberculin skin testing (TST) to determine next steps 1

For Newborns of Mothers Who Completed TB Treatment

• No prophylaxis is required if the mother has completed her full course of anti-tuberculous therapy and is confirmed non-infectious 1
• Proceed directly with BCG vaccination at birth or shortly after 1

Critical Decision Points at 3 Months

• If TST remains negative at 3 months and chest radiograph is normal: Stop prophylaxis and administer BCG vaccination 1
• If TST converts to positive: Complete a full 6 months of INH prophylaxis, as this indicates latent TB infection requiring full treatment 1

Special Considerations for High-Risk Infants

• Infants and children younger than 4 years are at highest risk for disseminated tuberculosis, making early treatment initiation critical 2
• Treatment should be started as soon as the diagnosis is suspected in this age group, even before confirmatory testing 2
• For tuberculin-negative children who have been close contacts within the past 3 months, start prophylaxis immediately and repeat TST 12 weeks after contact with the infectious source 3

Alternative Regimen Option

• Rifampicin plus isoniazid for 3 months is an acceptable alternative to 6 months of isoniazid monotherapy, offering comparable efficacy with potentially better adherence 1
• If the source case has isoniazid-resistant TB, use rifampicin alone for 4-6 months instead 1

Key Pitfalls to Avoid

• Do not delay prophylaxis while waiting for test results if the mother is known to be infectious 1
• Do not give BCG vaccination before completing the 3-month assessment in exposed infants, as this complicates interpretation of subsequent TST 1
• Do not reflexively start prophylaxis based solely on maternal TB history without assessing current infectiousness status 1
• Breastfeeding should continue during prophylaxis and is not contraindicated 1

Safety in Young Infants

• The 10 mg/kg daily dose achieves target INH blood plasma concentrations comparable to adult values in low-birth-weight and premature infants 4, 5
• Reduced elimination occurs in smaller, younger infants and slow acetylators, cautioning against higher doses 4
• Treatment is generally well tolerated with normal liver function tests in most cases 4, 5