When, and when not, to use the interferon-gamma TB blood test

The Journal of Family Practice. 2005 October;54(10):873-875

October 1, 2005|Family Medicine

Gray areas. While there is some indication that QFTG is more sensitive for detecting TB infection in those exposed to an infectious patient, it is unknown whether it will predict as well as a skin test which patients are at risk of developing active disease.² Therefore, it is not clear at this time if all those who have a positive QFTG should be considered candidates for treatment of latent TB infection, or if this should be offered only to those who have both a positive QFTG and TB skin test.

Level of risk influences interpretation. The current CDC recommendations, which have not been updated since QFTG was licensed, state that low-risk patients with a positive QFT and negative TB skin test should not receive treatment for latent infection.³ However, clinical judgment and perceived risk should be the basis for deciding on treatment in those at increased risk who have a positive QFT and negative TB skin test.

It is also not clear what effect a recent TB skin test has on QFTG results, and performing a QFTG soon after a TB skin test is not recommended by the US Food and Drug Administration or the manufacturer.

FAST TRACK

Cost of QFTG ($80–$100 per test) should be weighed with the cost of staff time to read and follow-up a skin test

How to interpret QFTG results. If the QFTG result is positive, the patient needs clinical evaluation and a chest x-ray to rule out active disease. The diagnosis and treatment of active and latent TB has been covered in a previous Practice Alert.⁴ If the QFTG result is negative, no further evaluation is indicated unless symptoms of TB exist. The QFTG can have an indeterminate result, in which case a skin test can be useful.

Weighing the cost. The cost of a QFTG (about $80–$100 per test) needs to be compared with cost of staff time to read and interpret a skin test and to follow up with patients who fail to return for a skin test measurement.

TABLE 2
Recommendations for use of the QFT

SITUATIONS WHERE QFT IS RECOMMENDED AS A POSSIBLE DIAGNOSTIC TOOL
Persons at increased risk for latent TB infection
Recent immigrants from countries with high rates of endemic TB Injection drug users Residents and employees in correctional institutions Homeless persons Some health care workers
Persons at increased risk of TB infection if exposed
HIV-positive persons Those receiving other immunosuppressive medications
Persons with conditions that cause increased risk of TB disease if infected, including those with:
Diabetes Chronic renal failure Silicosis Malnutrition Renal dialysis Chest x-ray consistent with old TB Immunosuppressive conditions and treatments
Persons at low risk of TB infection who require initial or periodic testing
Most health care workers When required for school or work Members of the military Some emergency first-responders
SITUATIONS WHERE QFT IS NOT CURRENTLY RECOMMENDED
Patients receiving anti-TB medications for active TB disease Children aged <17 years Pregnant women
SITUATIONS WHERE QFT IS PROMISING BUT FUTURE VALUE IS UNCERTAIN
Testing contacts of persons with infectious TB Confirmation testing after a positive skin test

Conclusion

The QFTG test is relatively new; as more evidence becomes available, its place among the tools available for the diagnosis of latent and active TB will clarify. Check with your state and local public health departments to find out the situations for which they are recommending this new diagnostic tool, as practice varies across the country.

CORRESPONDING AUTHOR
Doug Campos-Outcalt, MD, MPA, 4001 North Third Street #415, Phoenix, AZ 85012. E-mail: dougco@u.arizona.edu

References

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