Tuberculosis (TB) is a disease caused by the bacterium Mycobacterium tuberculosis, which primarily affects the lungs but can impact other organs. Screening for TB infection is a routine part of prenatal care for individuals who have specific risk factors. Screening aims to identify latent infection (non-contagious) or active disease (transmissible), which poses a risk to both the pregnant individual and the fetus. Common risk factors include a history of travel to or birth in a country with a high TB prevalence, close contact with someone who has active TB disease, or having a compromised immune system.
Safety and Procedure of the Tuberculin Skin Test
The Tuberculin Skin Test (TST), also known as the Mantoux or PPD test, is a well-established method for screening for TB infection. This test is considered safe to perform during any stage of pregnancy because it does not contain any live bacteria and cannot cause infection in the mother or the fetus. The procedure involves injecting a small amount of tuberculin, a sterile protein extract derived from the TB bacteria, just beneath the skin on the forearm. The material is not circulated systemically throughout the body. The individual must return to the clinic for a follow-up reading, which is a necessary part of the procedure.
The TST must be read between 48 and 72 hours after the injection to ensure an accurate result. The provider assesses the injection site by feeling for a raised, hardened area, known as induration, rather than just looking at the redness. The diameter of this induration is measured in millimeters, and the interpretation of whether the result is positive depends on the size of the induration and the patient’s specific risk factors. A positive TST indicates that the body has developed an immune response to the TB bacteria, suggesting a prior exposure or infection.
Blood Test Alternatives for TB Screening
Interferon-Gamma Release Assays (IGRAs) offer an alternative to the traditional TST for detecting M. tuberculosis infection. These blood tests, such as the QuantiFERON-TB Gold Plus, measure the immune system’s response by assessing the amount of interferon-gamma released by white blood cells after exposure to specific TB-related antigens. Like the TST, IGRAs are considered safe for use throughout pregnancy, as they only require a single blood draw.
A significant advantage of IGRAs is that the results are not affected by prior vaccination with the Bacille Calmette-Guérin (BCG) vaccine, which is commonly administered in countries with high TB rates. The BCG vaccine can sometimes cause a false-positive result on the TST, leading to unnecessary follow-up evaluations. IGRAs are also preferred when it is difficult for a patient to return for the mandatory TST reading. Although the blood test may be more costly, it provides a definitive result from a single visit, simplifying the screening process for some individuals. Regardless of which test is used, a positive result warrants further medical investigation to rule out active disease.
Managing a Positive TB Screening Result During Pregnancy
A positive screening result, whether from a TST or an IGRA, indicates the presence of TB bacteria but does not distinguish between Latent TB Infection (LTBI) and Active TB Disease. LTBI means the bacteria are dormant in the body and the individual is not contagious, while Active TB Disease means the bacteria are multiplying and causing illness, making the person infectious. The immediate next step is to rule out active disease, which is a greater health threat to both mother and fetus than latent infection.
This exclusion process typically requires a medical evaluation and a chest X-ray (CXR). Performing a CXR during pregnancy is safe and standard practice when indicated, as the radiation dose is minimal and focused. Safety protocols mandate the use of a lead apron to shield the abdomen and uterus, minimizing any radiation exposure to the developing fetus.
If the CXR is normal and the individual has no symptoms of illness, LTBI is diagnosed. Treatment for LTBI often involves delaying medication until after the baby is delivered to reduce potential fetal exposure to anti-TB drugs. However, treatment may be initiated during the pregnancy, often starting after the first trimester, if the risk of the latent infection progressing to active disease is considered high. High-risk factors include recent close contact with an infectious person or a co-existing condition such as HIV infection.