Tuberculosis (TB) is an infection caused by the bacterium Mycobacterium tuberculosis, and screening is used to detect its presence in the body. TB testing is considered safe and is often a recommended part of prenatal care for individuals who have specific risk factors for infection. Timely testing is important, as an untreated TB infection poses a far greater risk to both the mother and the baby than the testing procedures themselves.
The Safety of the Standard Skin Test (TST)
The Tuberculin Skin Test (TST), also known as the Mantoux test, is a commonly used method that is safe to perform at any point during pregnancy. The procedure involves injecting a small amount of fluid called purified protein derivative (PPD) just under the top layer of the forearm skin. PPD is a derivative of proteins from the TB bacterium, not a live vaccine or active bacteria, meaning it cannot cause an infection. The injection is superficial and does not enter the bloodstream or cross the placenta, ensuring no direct exposure to the fetus.
A healthcare provider must examine the injection site 48 to 72 hours later to check for a raised, firm bump, known as induration. The size of this reaction indicates whether the individual has been exposed to the TB bacteria. Because the test requires a second visit for reading, it can sometimes be impractical for patients who may have difficulty returning to the clinic. No documented episodes of TST-related harm to the mother or the fetus have been reported despite decades of use in pregnant populations.
When Blood Tests (IGRA) Are Used
Another safe and widely accepted testing option during pregnancy is the Interferon-Gamma Release Assay (IGRA), often marketed under names such as QuantiFERON-TB Gold Plus. This test is a blood draw that measures the immune system’s reaction to specific TB-related proteins in a laboratory setting. IGRAs are preferred in certain situations because they offer a single-visit testing option, eliminating the need for a return appointment to read a skin reaction.
IGRAs are particularly advantageous for individuals who have received the Bacille Calmette-Guérin (BCG) vaccine. The BCG vaccine can cause a false-positive result on the TST, as the skin test cannot distinguish between a reaction caused by the vaccine and one caused by the actual TB bacteria. Since the IGRA uses more specific TB-related antigens, it is not affected by prior BCG vaccination, providing a more accurate result.
Managing Positive TB Results During Pregnancy
A positive result from either a TST or an IGRA indicates an infection with M. tuberculosis and necessitates a thorough medical evaluation to determine if the infection is latent or active. This evaluation must include a chest X-ray (CXR), performed with the abdomen shielded by a lead apron to minimize potential fetal radiation exposure. The radiation dose from a shielded CXR is extremely low and is not associated with harm to the developing baby. If possible, the CXR may be deferred until the second trimester, but it should be done immediately if the patient has symptoms or is at high risk, such as being HIV-positive.
If the CXR is clear and the patient has no symptoms, the diagnosis is Latent TB Infection (LTBI), where the bacteria are dormant. Treatment for LTBI is often delayed until after delivery to minimize the fetus’s exposure to medications, unless the mother is at high risk for the infection progressing to active disease. High-risk factors that warrant immediate treatment include recent infection or co-infection with HIV, as the danger of the infection advancing outweighs the medication risks.
In contrast, a diagnosis of active TB disease requires immediate and mandatory treatment, regardless of the stage of pregnancy. Untreated active TB carries a high risk of poor outcomes for both mother and infant. The standard treatment regimen typically involves a combination of medications, such as isoniazid, rifampin, and ethambutol, administered for a minimum of nine months. Pyrazinamide, another common TB drug, is generally reserved for severe or drug-resistant cases during pregnancy due to limited safety data.
All pregnant individuals taking isoniazid must also receive a daily supplement of pyridoxine (Vitamin B6) at a dose of 25 to 50 milligrams. This supplementation is necessary because isoniazid can interfere with the body’s use of Vitamin B6, potentially leading to peripheral neuropathy in the mother. The increased need for this vitamin during pregnancy makes this a standard protective part of the treatment plan.