TB Blood Test

TB Blood Tests

What is an Interferon Gamma Release

Assay (IGRA)?

An IGRA is a blood test that can determine if a person has been infected with TB bacteria. An IGRA measures how strong a person’s immune system reacts to TB bacteria by testing the person’s blood in a laboratory.Two IGRAs are approved by the U.S. Food and Drug  Administration (FDA) and are available in the United States:

1) QuantiFERON®-TB Gold In-Tube test


2) T-SPOT®.TB test (T-Spot)

How does the IGRA work?

Blood is collected into special tubes using a needle. The blood is delivered to a laboratory as directed by the  IGRA test instructions. The laboratory runs the test and reports the results to the health care provider.

What does a positive IGRA result mean?

Positive IGRA: This means that the person has been infected with TB bacteria. Additional tests are needed to determine if the person has latent TB infection or TB disease. A health care worker will then provide treatment as needed.

Negative IGRA: This means that the person’s blood did not react to the test and that latent TB infection or TB disease is not likely.

Who can receive an IGRA?

Anyone can have an IGRA in place of a TST. This canbe for any situation where a TST is recommended. In general, a person should have either a TST or an IGRA, but not both. There are rare exceptions when results from both tests may be useful in deciding whether a

person has been infected with TB.

IGRAs are the preferred method of TB infection testing

for the following:

  • People who have received the BCG shot
  • People who have a difficult time returning for a

second appointment to look at the TST after the test  was given

How often can an IGRA be given?

There is no problem with repeated IGRAs.  Who Should Get Tested for TB?


Certain people should be tested for TB bacteria because  they are more likely to get TB disease, including:

  • People who have spent time with someone who has TB disease
  • People with HIV infection or another medical problemthat weakens the immune system
  • People who have symptoms of TB disease (fever, night sweats, cough, and weight loss)
  • People from a country where TB disease is common (most countries in Latin America, the Caribbean, Africa, Asia, Eastern Europe, and Russia)
  • People who live or work somewhere in the United States where TB disease is more common (homeless shelters, prison or jails, or some nursing homes)
  • People who use illegal drugs


When Should You Suspect Tuberculosis (TB)?

TB is a disease caused by Mycobacterium

tuberculosis. TB disease should be suspected in persons who have the following symptoms:

  • Unexplained weight loss
  • Loss of appetite
  • Night sweats
  • Fever
  • Fatigue

If TB disease is in the lungs (pulmonary), symptoms may include:

  • Coughing for ≥3 weeks
  • Hemoptysis (coughing up blood)
  • Chest pain

If TB disease is in other parts of the body (extrapulmonary), symptoms will depend on the area affected.


How Do You Evaluate Persons Suspected of Having TB Disease?

A complete medical evaluation for TB includes the following:

  • Medical History

Clinicians should ask about the patient’s history of TB exposure, infection, or disease.

It is also important to consider demographic factors (e.g., country of origin, age, ethnic or racial group, occupation) that may increase the patient’s risk for exposure to TB or to drug resistantAlso, clinicians should determine whether the patient has medical conditions, especially HIV infection, that increase the risk of latent TB infection progressing to TB disease.

  • . Physical Examination

A physical exam can provide valuable information about the patient’s overall

condition and other factors that may affect how TB is treated, such as HIV infection or other illnesses.

  1. Test for TB Infection
    The Mantoux tuberculin skin test (TST) or the TB blood test can be used to test for M. tuberculosis infection. Additional tests are required to confirm TB disease. The Mantoux tuberculin skin test is performed by injecting a small amount of fluid called tuberculin into the skin in the lower part of the arm. The test is read within 48 to 72 hours by a trained
    health care worker, who looks for a reaction  (induration) on the arm.
  2. Chest Radiograph
    A posterior-anterior chest radiograph is used to detect chest abnormalities. Lesions may appear anywhere in the lungs and may differ
    in size, shape, density, and cavitation. These abnormalities may suggest TB, but cannot be used to definitively diagnose TB. However, a
    chest radiograph may be used to rule out the possibility of pulmonary TB in a person who has had a positive reaction to a TST or TB blood
    test and no symptoms of disease.
  3. Diagnostic Microbiology
    The presence of acid-fast-bacilli (AFB) on a sputum smear or other specimen often indicates TB disease. Acid-fast microscopy is easy and quick, but it does not confirm a diagnosis of TB because some acid-fast-bacilli are not M. tuberculosis. Therefore, a culture is done on all initial samples to confirm the diagnosis. (However, a positive culture
    is not always necessary to begin or culture for M. tuberculosis confirms
    the diagnosis of TB disease. Culture examinations should be completed on all specimens, regardless of AFB smear results. Laboratories should report positive results on smears and cultures within 24 hours by telephone or fax to the primary health care provider and to
    the state or local TB control program, as  required by law.
  4. Drug Resistance

    For all patients, the initial M. tuberculosis isolate should be tested for drug resistance. It is crucial to identify drug  resistance as early as possible to ensure  effective treatment. Drug susceptibility  patterns should be repeated for patients.

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