Latent Tuberculosis Infection and Preventive Therapy
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Direct answer
Latent tuberculosis infection is the presence of Mycobacterium tuberculosis in the body without clinical or radiological evidence of active disease, detected by a positive tuberculin test or IGRA. Household contacts under five (under six in the national programme) have the highest risk of progression, so they are the priority group. Preventive therapy starts only after active tuberculosis is excluded, following national programme guidance.
What you must remember
- Definition: infection with M. tuberculosis without symptoms, abnormal examination or a suggestive chest X-ray, and no bacteriological evidence of disease.
- Highest-risk group: children under five, especially infants, and children with HIV, malnutrition or recent close contact with an infectious case.
- Contact screening: every child in household contact with a sputum-positive adult is screened for both active disease and infection.
- Exclude active disease first: symptoms, examination and a chest X-ray must rule out active tuberculosis before preventive therapy starts.
- Isoniazid: six months of daily isoniazid is the classic preventive regimen, and shorter rifamycin-based combinations are used where the programme permits.
- Pyridoxine: vitamin B6 is given with isoniazid to children at risk of peripheral neuropathy, such as those with HIV or malnutrition.
- Drug-resistant source: if the source case has drug-resistant tuberculosis, the preventive regimen changes and is set by specialist guidance.
Choosing a preventive regimen for a child contact
A 3-year-old lives with a father who has just been diagnosed with sputum-positive pulmonary tuberculosis. The child has no symptoms, a normal chest X-ray and a positive tuberculin test, so latent infection is the working diagnosis. Each regimen needs counselling and regular review, because adherence decides whether the protection is complete.
| Regimen | Drugs | Duration | Points to note |
|---|---|---|---|
| 6H | Isoniazid daily | Six months | Classic regimen; watch for hepatitis and neuropathy |
| 3HR | Isoniazid and rifampicin daily | Three months | Shorter course; rifampicin interacts with many drugs |
| 3HP | Isoniazid and rifapentine weekly | Three months | Weekly directly observed dosing; availability varies |
| 4R | Rifampicin daily | Four months | Used when isoniazid cannot be given |
Shorter regimens are often preferred because they improve completion, but the national programme decides which regimen applies to a given child. Adherence matters more than the choice of regimen in practice. Hepatitis and peripheral neuropathy are checked at each visit, and the family is taught to report jaundice or persistent vomiting promptly.
Where the exam tries to catch you
Most lost marks come from confusing infection with disease, and from starting therapy before excluding disease.
- Trap: treating a positive test as active disease Correct: a positive test in an asymptomatic child with a normal chest X-ray indicates latent infection, so preventive therapy is given instead of full treatment.
- Trap: starting isoniazid before excluding active tuberculosis Correct: active disease is excluded clinically and radiologically first, because giving preventive drugs alone to an active case can select for resistance.
- Trap: deciding on preventive therapy from the BCG scar Correct: a BCG scar says nothing about infection, and a tuberculin or IGRA test is needed.
Frequently asked questions
Which children should receive preventive therapy for latent tuberculosis?
Children under five (under six in the national programme) who are household contacts of a sputum-positive case are the priority group for preventive therapy, and children with HIV are also a high-priority group. Eligibility after active disease is excluded follows the national programme guidance.
Why must active tuberculosis be excluded before preventive therapy in children?
Active tuberculosis must be excluded before preventive therapy because giving preventive drugs alone to an active case can select for drug resistance. Symptoms, examination and a chest X-ray are used to exclude active disease before the regimen begins.
Which preventive regimen is used for tuberculosis contacts in children?
The classic regimen is six months of daily isoniazid, and rifampicin-based combinations are used for shorter courses where the national programme permits. The choice depends on drug availability, adherence and drug interactions, so the national guidance is followed.
Why is vitamin B6 given with isoniazid in children?
Vitamin B6 is given with isoniazid to prevent peripheral neuropathy, a known side effect that is more likely in children with HIV or malnutrition. Pyridoxine is given alongside the isoniazid for the full duration of the course.