Use of Interferon Gamma Release Assay (IGRA) for Contact Investigation in Coworkers of a Fast Food Worker with Pulmonary Tuberculosis

Background:   An employee (Worker X) of a fast food restaurant located in a hospital was admitted with pneumonia.  He complained of cough for one year and a ten-pound involuntary weight loss. Chest CT showed bronchiectatic changes in the right lower lobe, hilar adenopathy, and a cavity. Expectorated sputum was positive for acid fast bacilli (4+) that subsequently grew sensitive Mycobacterium tuberculosis.  The patient was discharged home on directly observed therapy.

Objective:   To describe the use of IGRA in contact investigation among coworkers exposed to Worker X.

Methods:   An epidemiologic investigation was performed by the hospital's infection control department and the Cuyahoga County TB program.  Family members were assessed using tuberculin skin testing (TST) by the Cuyahoga County TB program.  Coworkers were assessed by infection control personnel.  Proof of TST is required prior to employment, and none were known to be positive.  All coworkers were screened with IGRA testing (QuantiFERON-TB Gold In-Tube, Cellestis) at baseline and at 10 weeks.  An IGRA converter is defined as an individual who had a negative test at baseline and a positive test 10 weeks later. 

Results:  Four of 11 (36%) household and family contacts had reactive TST with normal chest radiographs, and offered isoniazid (INH) therapy for latent tuberculosis infection (LTBI).  None of the 42 coworkers were symptomatic (mean age of 29.8 years, range 17-59 yrs). There were three (7%) who had positive IGRA at baseline with negative chest radiographs and one (2%) coworker who had an indeterminate IGRA result. Of the 38 who had negative IGRA tests at baseline, 33 had follow-up testing done.  Only one of these 33 (3%) coworkers converted at 10 weeks, with a negative chest radiograph. All workers with positive IGRA received INH therapy for LTBI. One of the four (25%) coworkers on the same shift as Worker X had a positive IGRA test versus three of thirty eight (8%) who did not work on the same shift (p=0.3, two-tailed Fisher's exact test).   There were no identified cases of active tuberculosis (TB) linked to Worker X, who is doing well with clinical and radiographic improvement, and completing 9 months of therapy.

Conclusions: A case of pulmonary TB in a fast food worker may have accounted for 10% transmission of infection among coworkers. IGRA testing was an efficient method for obtaining baseline and follow-up assessments of TB infection in this relatively young, mobile population.