What does a positive Tuberculin Skin Test (TST) or Interferon-Gamma Release Assay (IGRA) mean for a patient with recent close contact with a known case of Tuberculosis (TB)?

From the Guidelines

A positive TB skin test in a patient with recent close contact with known tuberculosis indicates latent tuberculosis infection (LTBI), and treatment is strongly recommended to prevent progression to active TB disease. The standard treatment regimen is isoniazid (INH) 300mg daily for 6-9 months, or a shorter course of rifampin 600mg daily for 4 months, as recommended by the American Thoracic Society 1. Another option is a 3-month regimen of once-weekly isoniazid plus rifapentine. The patient should be monitored for medication side effects, particularly liver toxicity with isoniazid. It's essential to understand that a positive test after known exposure doesn't necessarily mean the person has active TB disease; additional evaluation including chest X-ray, symptom screening (cough, fever, night sweats, weight loss), and possibly sputum tests are needed to rule out active disease before starting LTBI treatment, as suggested by the Centers for Disease Control and Prevention 1. The reason for treating LTBI is that without treatment, approximately 5-10% of infected persons will develop active TB disease at some point in their lives, with the highest risk occurring within the first two years after infection, as reported in the European Respiratory Journal 1. Key considerations for treatment include:

• The risk of progression from latent infection to active TB disease, which is higher in individuals with certain medical conditions, such as HIV infection or immunosuppression 1
• The importance of completing a full course of preventive therapy to ensure maximum efficacy, as emphasized by the American Thoracic Society 1
• The need for careful monitoring of medication side effects and potential interactions with other medications, as noted in the MMWR Recommendations and Reports 1. Overall, the goal of treatment is to prevent the development of active TB disease and reduce the risk of transmission to others, as highlighted in the American Journal of Respiratory and Critical Care Medicine 1.

From the FDA Drug Label

1. Close contacts of persons with newly diagnosed infectious tuberculosis (≥ 5 mm) In addition, tuberculin-negative (< 5mm) children and adolescents who have been close contacts of infectious persons within the past 3 months are candidates for preventive therapy until a repeat tuberculin skin test is done 12 weeks after contact with the infectious source. If the repeat skin test is positive (> 5 mm), therapy should be continued.

A positive TB spot (tuberculin skin test) in a patient who has been in recent close contact with a known tuberculosis case means that the patient has been infected with tuberculosis and is a candidate for preventive therapy 2. The patient should receive therapy to prevent the development of active tuberculosis.

• The patient should be treated with isoniazid for at least 12 months if they have HIV infection or other risk factors.
• If the patient is a child or adolescent, they should receive preventive therapy until a repeat tuberculin skin test is done 12 weeks after contact with the infectious source.
• If the repeat skin test is positive, therapy should be continued.

From the Research

Positive TB Spot Test Results

A positive TB spot test, also known as a tuberculin skin test (TST) or gamma interferon release assay, indicates that a person has been infected with tuberculosis (TB) bacteria 3. This is particularly relevant for individuals who have been in recent close contact with someone known to have tuberculosis.

Risk of Developing Active Tuberculosis

According to a study published in Prescrire international, persons exposed to TB through close, prolonged, or frequent contact may develop tuberculosis, usually within two years following exposure 3. The risk of developing active TB is higher for those with a positive TST result.

Treatment Options

Treatment for latent tuberculosis infection (LTBI) typically consists of isoniazid monotherapy for 6 to 12 months 3. However, this regimen can cause severe hepatic disorders and numerous drug interactions. Alternative treatment options, such as rifampicin monotherapy or a combination of isoniazid and rifampicin, have been shown to be effective but also carry risks of hepatotoxicity 3, 4, 5.

Monitoring and Management

Regular monitoring of liver function tests (LFTs) is essential for patients undergoing treatment for LTBI, especially those with pre-existing liver disease or abnormal baseline LFTs 6, 7. This allows for early detection of hepatotoxicity and prompt intervention to prevent severe liver damage.

Key Considerations

• A positive TB spot test result indicates infection with TB bacteria, but not necessarily active disease.
• Close contacts of individuals with active TB are at higher risk of developing the disease.
• Treatment for LTBI is available, but it carries risks of hepatotoxicity and other adverse effects.
• Regular monitoring of LFTs is crucial for patients undergoing treatment for LTBI.