“Anybody can get TB. It’s an infectious disease,” says Lee Reichman, MD, MPH, a professor of medicine and epidemiology and the executive director emeritus at Rutgers Global Tuberculosis Institute in Newark, New Jersey. But the disease is pretty hard to spread. It takes six months of eight-hours-per-day contact to get infected with TB or two months of 24-hour contact, he explains. About 30 percent of people who are exposed to Mycobacterium tuberculosis will develop latent TB, and if that’s left untreated, around 5 to 10 percent of those people could end up getting active tuberculosis disease at some point in their lifetime. (2) Since tuberculosis bacteria grow slowly and symptoms can be confused with those of other conditions, careful testing is required. Several laboratory and imaging tests may need to be done to definitively diagnose active TB. Slowing the spread of TB requires catching it early and treating it early. If caught in the latent stage, TB can be treated before the disease becomes active and infectious. In the United States, screening for latent TB is done in the following populations: (1,3,4)
People who have recently come to the United States from a country with a high rate of tuberculosis (including Russia and countries in Latin America, the Caribbean, Africa, and Eastern Europe)People whose work or living arrangements put them in contact with people who have active tuberculosisPeople who are starting to take a drug that suppresses the immune system, which may reactivate latent tuberculosisPeople with other diseases that increase the risk of developing active TB once infected, such as insulin-requiring diabetes, end-stage renal disease, prior gastrectomy, or HIV infectionPeople who are taking drugs that block tumor necrosis factor alpha, such as Remicade (infliximab), Humira (adalimumab), or Enbrel (etanercept)
If you’re in a high-risk group, it’s worth having a conversation with your doctor to decide if the testing is right for you, says Hayan Yacoub, MD, an internal medicine practitioner at Austin Regional Clinic in Texas. The Centers for Disease Control and Prevention (CDC) recommends screening anyone with the following symptoms for active TB: (4)
Coughing that lasts for three weeks or longerWeight loss that can’t be explainedCoughing up bloodChest painLoss of appetiteNight sweatsFeverFatigue
Before ordering tests, your doctor will check your lymph nodes for swelling, listen to your lungs, discuss your symptoms, and ask a few questions to get a sense of your possible TB exposure. (5)
Screening Tests for Tuberculosis and Who Should Get Them
Screening tests are more often used to find latent TB in people with a weakened immune system or those who may have recently been exposed to someone with active TB. (3,4) Three tests are used to screen for tuberculosis:
A chest X-ray (best used to screen for active TB)A tuberculin skin testBlood tests (interferon gamma release assay, QuantiFERON-TB Gold, and T-Spot)
A chest X-ray is taken to look for changes in the lungs that could show signs of active TB or scars from a previous TB infection. Doctors will look for lesions or anything else that doesn’t appear normal, which could mean a person has pulmonary TB. Spots on your lungs could also mean the immune system is trying to contain the spread of TB bacteria. A tuberculin skin test, also known as a Mantoux test or PPD (purified protein derivative), is done by injecting a solution containing a protein made from tuberculosis bacteria just under the top layer of skin on the forearm. It’s done in two parts; first, an injection is given, and then the person will return to the doctor’s office in 48 or 72 hours to have the injection site examined. If the skin at the injection site develops a raised red bump, it indicates that the person may be infected with TB. (4,5,6) If a skin test is positive, doctors will consider a person’s risk factors and then order additional testing to figure out the best course of treatment. The skin test can give a false-positive result if a person has had a prior vaccination with the bacille Calmette-Guérin (BCG) vaccine or is infected with other bacteria that are close relatives of tuberculosis. For this reason, doctors don’t rely on the skin test alone to confirm a diagnosis. (5) It’s also possible for the skin test to give a false negative. This can happen in people who have been infected very recently or who have HIV/AIDS, or in children and elderly adults. The brand-name blood tests are approved in the United States for the diagnosis of latent TB infection. A blood test may be ordered on its own or after a positive skin test. Like the tuberculin skin test, the different blood tests offer ways to measure the body’s immune response to the presence of Mycobacterium tuberculosis. The tests are done in a lab after a blood sample is drawn. The blood tests do not react to the BCG vaccine, so they can help rule out a false positive from a skin test in those who may have had the vaccine.
Diagnostic Tests to Determine Active TB Disease vs. Latent TB Infection
Neither the tuberculin skin test nor any of the blood tests can tell the difference between active and latent disease. Chest X-rays also have their limitations because the effects of TB on the lungs are similar to those of many other conditions. For a more conclusive diagnosis, other tests are necessary: (6,7,8,9)
Sputum is the mucus that comes up when you cough. If a skin or blood test is positive, doctors will take samples of sputum and look for the presence of Mycobacterium tuberculosis. Sputum can also be tested for drug-resistant strains; the results help doctors choose the right medications for treatment. Lab results take about four to eight weeks.Molecular tests can be used to detect the bacteria’s genetic material, which helps identify drug-resistant strains.Samples of sputum or other bodily fluids, as well as tissue samples obtained by a biopsy of the lungs, lymph nodes, or other tissues, may be cultured to grow the bacteria and make it easier to see under a microscope. Culturing TB bacteria can take four to eight weeks before any growth appears, making this a slow process for detecting TB.
A variety of lab tests can also be done to see which drugs will best treat the strain of tuberculosis a person has. Imaging tests may be used to assist in providing a diagnosis of active tuberculosis: (4,5,6,7,10)
X-rays may be done to look for pulmonary TB in the lungs as well as tuberculosis affecting the bones or spine.Computed tomography (CT scans) may be used to look for spinal TB or to get a better view of the lungs if X-ray findings are nonspecific. CT scans offer more detail than X-rays.MRIs of the spine or brain may be done if tuberculosis infection of either is suspected.Bone scans can be used to tell the difference between metastatic (cancerous) lesions and those caused by TB.
Why Testing for HIV Is Recommended When TB Is Diagnosed
Because tuberculosis is closely associated with HIV, it’s become common practice to test for HIV in someone with suspected tuberculosis whose HIV status is not known. This allows treatment for HIV to be started as well, if appropriate, and coordinated with TB treatment. (11) The CDC recommends HIV testing for anyone who may have TB or who is diagnosed with latent TB. That’s because HIV is a known risk factor for getting infected with latent TB and having it become active. TB is also more deadly for people with HIV. When HIV is identified and treated, outcomes for TB treatment improve. (11) “Once the HIV is under control, risk of TB recurrence is decreased,” says Alexea Gaffney-Adams, MD, an internist and pediatrician with subspecialty training in infectious disease at Stony Brook Medicine in Smithtown, New York.
Starting Treatment for Tuberculosis After Diagnosis
It can take some time to positively diagnose active tuberculosis. In cases where tuberculosis is strongly suspected, doctors often start treatment before the diagnosis is confirmed by laboratory isolation of the TB bacteria. Tuberculosis is treated by taking a series of drugs over several months to kill the bacteria. Active TB takes longer to treat than latent TB because the bacteria have multiplied. (6) Additional reporting by Ingrid Strauch.
