How long should a baby take isoniazid (INH) prophylaxis, 3 months or 6 months?

Duration of INH Prophylaxis for Infants

For infants exposed to infectious tuberculosis, isoniazid prophylaxis should be given for 3 months, followed by tuberculin skin testing to determine if continuation is needed. 1, 2

Clinical Algorithm for Prophylaxis Duration

If Mother is Infectious at Birth (Sputum Smear-Positive or <2 Weeks of Treatment)

• Start isoniazid immediately at standard pediatric dosing (10-15 mg/kg daily, maximum 300 mg) for 3 months 1, 2
• Perform tuberculin skin test (TST) at 3 months after starting prophylaxis 1
• If TST remains negative at 3 months AND chest radiograph is normal, stop prophylaxis and give BCG vaccination 3, 1
• If TST converts to positive (Heaf grade 2-4 or Mantoux ≥5mm induration), complete full 6 months of prophylaxis 3

If Mother Has Completed TB Treatment and is Non-Infectious

• No prophylaxis is required 1
• Give BCG vaccination at birth or shortly after 1

Key Decision Points

The 3-month checkpoint is critical because it allows time for tuberculin conversion to occur if infection was transmitted. 3 This approach prevents unnecessary prolonged treatment in uninfected infants while ensuring adequate prophylaxis for those who are infected. 1

The British Thoracic Society guidelines explicitly recommend either isoniazid alone for 6 months OR rifampicin and isoniazid for 3 months as acceptable regimens for children with latent TB infection. 3 However, for initial prophylaxis in exposed newborns where infection status is unknown, the 3-month checkpoint strategy is preferred. 1

Alternative Regimen

• Rifampicin plus isoniazid for 3 months is an acceptable alternative to 6 months of isoniazid monotherapy 3, 2
• This combination regimen has comparable efficacy with potentially better adherence due to shorter duration 3

Important Caveats

Do not reflexively start 6 months of prophylaxis without assessing the mother's current infectiousness status. 1 Many infants are unnecessarily treated when the mother has already completed treatment and is non-infectious. 1

For children under 5 years with documented household exposure to infectious TB, the American Academy of Pediatrics recommends 9 months of isoniazid as the standard regimen for confirmed latent infection. 2, 4 This differs from the initial 3-month prophylaxis strategy for newborns of uncertain infection status. 1, 2

If the source case has isoniazid-resistant TB, use rifampicin alone for 4-6 months instead. 2 Drug susceptibility testing of the source case is essential. 3, 2