TB remains a serious public health concern worldwide, making testing important for controlling its spread. The frequency of a tuberculosis test is not standardized across the general population. Testing schedules are determined by an individual’s specific risk factors, potential exposure, and medical history. Understanding the disease and diagnostic tools is necessary to determine an appropriate testing timeline.
Understanding Latent and Active TB Infection
Tuberculosis infection exists in two distinct states that influence testing strategy. Latent TB Infection (LTBI) occurs when a person is infected with Mycobacterium tuberculosis but shows no symptoms. The immune system contains the bacteria, meaning the person with LTBI is not contagious and cannot spread the infection.
Active TB Disease means the bacteria are multiplying and causing symptoms, typically affecting the lungs. Individuals with active disease are usually symptomatic, experiencing symptoms like a prolonged cough, weight loss, and night sweats. This state is contagious, especially if the infection is pulmonary, and requires immediate treatment with antibiotics. Screening tests primarily detect LTBI, which has an estimated 5% to 15% chance of progressing to active disease if left untreated.
Different Methods for Screening
Two primary tests screen for TB infection by detecting the body’s immune response to the bacteria. The Tuberculin Skin Test (TST), or Mantoux test, involves injecting a small amount of tuberculin purified protein derivative (PPD) beneath the skin. A healthcare worker must examine the injection site 48 to 72 hours later to measure the diameter of any raised area.
Interferon Gamma Release Assays (IGRAs) are blood tests requiring only a single patient visit. These assays, such as QuantiFERON and T-Spot, measure the amount of interferon-gamma released by white blood cells exposed to specific TB antigens. A primary advantage of IGRAs is that their results are not affected by prior vaccination with the Bacille Calmette-Guérin (BCG) vaccine, which can cause a false-positive TST result. Both the TST and IGRA identify infection but cannot distinguish between latent and active disease.
Recommended Testing Schedules for Specific Groups
For the general population in countries with low TB incidence, routine testing is not recommended. Testing frequency is targeted toward specific high-risk groups based on ongoing exposure risk. This targeted approach focuses resources on individuals most likely to benefit from diagnosis and preventive treatment.
Healthcare workers (HCWs) require baseline testing upon hire, including a risk assessment, symptom evaluation, and a TB test. Following this initial screening, routine annual testing is generally not standard practice unless there is a known recent exposure or ongoing TB transmission within the facility. If HCWs have untreated LTBI, they should receive annual screening for symptoms of active TB disease.
Individuals in close, prolonged contact with someone diagnosed with infectious Active TB Disease require immediate attention and a specific retesting schedule. The protocol is to test immediately after the exposure is identified. If the initial test is negative, retesting should occur eight to ten weeks after the last known exposure. This delayed retest allows the body sufficient time to develop a detectable immune response.
Individuals with compromised immune systems (e.g., those with HIV or receiving immunosuppressive therapies) are at a significantly higher risk for progression from latent infection to active disease. These individuals often require more frequent monitoring or testing if their risk factors persist or change. Testing is also recommended upon entry for residents and employees of communal settings, such as homeless shelters and correctional facilities, and for individuals who have recently immigrated from high TB prevalence countries.
Interpreting Results and Next Steps
A positive result from a TST or an IGRA indicates infection with Mycobacterium tuberculosis. It signifies the presence of infection but does not confirm if the infection is the non-contagious latent state or the active, transmissible disease. Further diagnostic steps are necessary immediately following a positive screening result.
The next steps involve a medical evaluation, typically including a symptom evaluation and a chest X-ray to look for signs of active disease. If the chest X-ray is abnormal or symptoms are reported, a sputum sample may be collected to confirm the presence of multiplying bacteria. If the evaluation rules out Active TB Disease, the result is interpreted as Latent TB Infection.
Preventive treatment, or prophylaxis, is strongly encouraged once LTBI is confirmed, especially for high-risk individuals. Taking medication significantly reduces the risk of the latent infection progressing to active disease. The decision to treat LTBI is based on a risk-benefit analysis, considering the individual’s risk factors and potential side effects.