CS227840_G

National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention 

Division of Tuberculosis Elimination

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TB Elimination

Interferon-Gamma Release Assays 

(IGRAs) – Blood Tests for TB Infection

What are they?

Interferon-Gamma Release Assays (IGRAs) are 

whole-blood tests that can aid in diagnosing 

Mycobacterium tuberculosis infection. They do not 

help differentiate latent tuberculosis infection (LT BI) 

from tuberculosis disease. Two IGRAs that have 

been approved by the U.S. Food and Drug 

Administration (FDA) are commercially available 

in the U.S. They are:

• QuantiFERON® – TB Gold In-Tube test (QFT–GIT);

• SPOT® TB test (T–Spot)

How do they work?

IGRAs measure a person’s immune reactivity 

to M. tuberculosis. White blood cells from most 

persons that have been infected with M. tuberculosis

will release interferon-gamma (IFN-g) when mixed 

with antigens (substances that can produce an 

immune response) derived from M. tuberculosis. 

To conduct the tests, fresh blood samples are mixed 

with antigens and controls. The antigens, testing 

methods, and interpretation criteria for IGRAs differ 

(see Table 1). 

What are the advantages of IGRAs?

• Requires a single patient visit to conduct

the test.

• Results can be available within 24 hours.

• Does not boost responses measured by

subsequent tests.

• Prior BCG (bacille Calmette-Guérin)

vaccination does not cause a false-positive

IGRA test result.

What are the disadvantages and 

limitations of IGRAs?

• Blood samples must be processed within

8-30 hours after collection while white blood

cells are still viable.

• Errors in collecting or transporting blood

specimens or in running and interpreting the

assay can decrease the accuracy of IGRAs.

• Limited data on the use of IGRAs to predict

who will progress to TB disease in the future.

Table1: Differences in Currently Available IGRAs

QFT–GIT T–Spot

Initial Process Process whole blood within 16 hours Process peripheral blood mononuclear 

cells (PBMCs) within 8 hours, or if T-Cell 

Xtend® is used, within 30 hours.

M. tuberculosis Antigen Single mixture of synthetic peptides 

representing ESAT-6, CFP-10 and TB7.7

Separate mixtures of synthetic peptides 

representing ESAT–6 and CFP-10

Measurement IFN-g concentration Number of IFN-g producing cells (spots)

Possible Results Positive, negative, indeterminate Positive, negative, borderline, 

invalid
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Limited data on the use of IGRAs for:

» Children younger than 5 years of age; 

» Persons recently exposed to M. tuberculosis;

» Immunocompromised persons; and 

» Serial testing. 

• Tests may be expensive.

What are the steps in administering 

an IGRA test?

Confirm arrangements for testing in a qualified 

laboratory, and arrange for delivery of the 

blood sample to the laboratory in the time the 

laboratory specifies to ensure testing of samples 

with viable blood cells. 

• Draw a blood sample from the patient 

according to the test manufacturer’s 

instructions. 

• Schedule a follow-up appointment for the 

patient to receive test results.

• Based on test results, provide follow-up 

evaluation and treatment as needed.

How do you interpret IGRA test 

results?

IGRA interpretations are based on the amount 

of IFN-g that is released or on the number 

of cells that release IFN-g. Both the standard 

qualitative test interpretation (positive, 

negative, or indeterminate) and the quantitative 

assay measurements (Nil, TB, and Mitogen 

concentrations or spot counts) should be 

reported. 

As with the tuberculin skin tests (TSTs), IGRAs 

should be used as an aid in diagnosing infection 

with M. tuberculosis. A positive test result suggests 

that M. tuberculosis infection is likely; a negative 

result suggests that infection is unlikely. An 

indeterminate result indicates an uncertain 

likelihood of M. tuberculosis infection. A borderline 

test result (T-Spot only) also indicates an uncertain 

likelihood of M. tuberculosis infection.

A diagnosis of LTBI requires that TB disease be 

excluded by medical evaluation. This should 

include checking for signs and symptoms 

suggestive of TB disease, a chest radiograph, and, 

when indicated, examination of sputum or other 

clinical samples for the presence of M. tuberculosis. 

Decisions about a diagnosis of M. tuberculosis

infection should also include epidemiological and 

historical information. 

Recommendations on when to use 

IGRA tests

• IGRAs can be used in place of (but not in 

addition to) TST in all situations in which CDC 

recommends TST as an aid in diagnosing 

M. tuberculosis infection, with preferences 

and special considerations noted below. 

This includes contact investigations, testing 

during pregnancy, and screening of health 

care workers and others undergoing serial 

evaluation for M. tuberculosis infection. 

Despite the indication of a preference, use of 

the alternative test (FDA-approved IGRA or 

TST) is acceptable medical and public health 

practice. Caution in interpretation should 

be used when testing certain populations 

because of limited data on the use of 

IGRAs (see Updated Guidelines for Using 

Interferon Gamma Release Assays to Detect 

Mycobacterium tuberculosis Infection, United 

States). 

• Populations in which IGRAs are preferred for 

testing:

» Persons who have received BCG (either as a 

vaccine or for cancer therapy); and 

» Persons from groups that historically have 

poor rates of return for TST reading.

• TST is preferred over IGRAs for testing children 

less than 5 years of age.

• As with TST, IGRAs generally should not be 

used for testing persons who have a low risk 

of infection and a low risk of disease due to 

M. tuberculosis.

• Each institution and TB control program 

should evaluate the availability and benefits 

of IGRAs in prioritizing their use.
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• Routine testing with both TST and IGRA is 

not recommended. However, results from 

both tests might be useful in the following 

situations:

» When the initial test is negative and:

– The risk for infection, the risk for 

progression to disease, and the risk for a 

poor outcome are high (e.g., HIV infected 

persons or children under 5 years of 

age who are exposed to a person with 

infectious TB). 

– There is clinical suspicion for TB 

disease (e.g., signs, symptoms, and/or 

radiographic evidence suggestive of 

TB disease) and confirmation of 

M. tuberculosis infection is desired. 

– Taking a positive result from a second 

test as evidence of infection increases 

detection sensitivity. 

» When the initial test is positive and:

– Additional evidence of infection is 

required to encourage acceptance and 

adherence (e.g., foreign-born healthcare 

workers who believe their positive TST 

is due to BCG). A positive IGRA might 

prompt greater acceptance of treatment 

for LTBI as compared with a positive TST 

alone.

– The person has a low risk of both 

infection and progression from infection 

to TB disease. Requiring a positive result 

from the second test as evidence of 

infection increases the likelihood that 

the test reflects infection. An alternative 

is to assume, without additional testing, 

that the initial result is a false positive or 

that the risk for disease does not warrant 

additional evaluation or treatment, 

regardless of test results.

» In addition, repeating an IGRA or 

performing a TST might be useful when 

the initial IGRA result is indeterminate, 

borderline, or invalid and a reason for 

testing persists.

Multiple negative results from any combination 

of these tests cannot exclude M. tuberculosis

infection. Steps should be taken to minimize 

unnecessary and misleading testing of persons 

at low risk. 

Selection of the most suitable test or 

combination of tests for detection of 

M. tuberculosis infection should be based on 

the reasons and the context for testing, test 

availability, and overall cost of testing. 

Can IGRAs Be Given To Persons 

Receiving Vaccinations?

As with TST, live virus vaccines might affect 

IGRA test results. However, the effect of live 

virus vaccination on IGRAs has not been 

studied. Until additional information is 

available, IGRA testing in the context of live 

virus vaccine administration should be done as 

follows:

• Either on the same day as vaccination with 

live-virus vaccine or 4-6 weeks after the 

administration of the live-virus vaccine

• At least one month after smallpox 

vaccination

Additional Information

Centers for Disease Control and Prevention. 

Updated Guidelines for Using Interferon 

Gamma Release Assays to Detect 

Mycobacterium tuberculosis Infection, United 

States. (PDF) MMWR 2010; 59 (No.RR-5). http://

www.cdc.gov/mmwr/preview/mmwrhtml/

rr5905a1.htm?s_cid=rr5905a1_e

http://www.cdc.gov/tb

November 2011