Summary
The University of Iowa Hospitals and Clinics Department of Pathology has become aware that the referral laboratory for QuantiFERON-TB Gold In-Tube testing (Mayo Medical Laboratories) has revised months of positive tests for Mycobacterium tuberculosis to indicate the possibility of false positive testing generated by a possible contamination event at the manufacturer.

The Assay
QuantiFERON-TB Gold In-Tube (QFT-GIT) is a 3-tube test that measures T-cell production of the cytokine interferon gamma in response to antigens from Mycobacterium tuberculosis (MTB) in comparison to positive and negative control tubes.  As there are only recombinant MTB antigens in the first tube, a positive result is interpreted as infection by MTB.  QFT-GIT therefore may be useful in the diagnosis of latent MTB infection, even in those who have been vaccinated with BCG and have a positive tuberculin skin test (TST).

Recall Scope

• All positive QFT-GIT tests nationally from April through September 2012 are suspect.
• Negative tests during this period are reportedly reliable, although true false negative results can always occur in children <5 years of age or the immunosuppressed.
• 25 patients at the University of Iowa Hospitals and Clinics tested positive during the recall period; all ordering physicians and patients have been contacted.
• Patients from Student and Employee Health were tested at the State Hygienic Laboratory (Coralville), which did not receive defective tubes.
• Testing at the State Hygienic Laboratory (Ankeny) and other outside sites was affected by the recall.

Recommendations

• TST and QFT-GIT are indicated for diagnosing latent tuberculosis infection but avoid using both in the same patient.
• Recent TST, within 2-3 months, can induce sensitization and cause false-positive QFT-GIT.
• Neither TST nor QFT-GIT can distinguish latent from active tuberculosis.
• Negative QFT-GIT cannot exclude active infection in patients with symptoms, signs, or radiographic evidence of active tuberculosis, even when the test is working properly.
• Order TST or QST-GIT testing mainly on individuals at risk for latent TB infection, and when positive, further investigation such as a chest x-ray and/or consultation are required.

For further questions about laboratory testing, please contact Bradley Ford (Associate Director, Clinical Microbiology) 6-2990, J. Stacey Klutts (Director, Clinical Microbiology, University of Iowa Hospitals and Clinics & VA Health Care System) or Matthew Krasowski (Clinical Chemistry and Serology) 4-9380.  For clinical questions, please contact Drs. Daniel Diekema (Infectious Disease) 4-5626 or Douglas Hornick (Pulmonary, Critical Care and Occupational Medicine) 6-8266.