Serial testing with the interferon-γ release assay in Portuguese healthcare workers.
OBJECTIVE
Evidence for the utility of the new Mycobacterium tuberculosis (MTB) specific IFN-γ release assays in diagnosing latent tuberculosis infection (LTBI) is growing. However, data concerning conversion and reversion rates in serial testing of healthcare workers (HCWs) with an interferon-γ release assay are sparse.


METHODS
Between February 2007 and September 2009, 670 HCWs in the University Hospital of Porto, Portugal were tested at least twice with QuantiFERON-TB® Gold In-Tube (QFT) for LTBI. The tuberculin skin test (TST) was performed simultaneously. QFT was considered positive if INF-γ ≥0.35 IU/mL. TST conversion was defined as an increase ≥10 or ≥6 mm compared to a baseline TST <10 mm.


RESULTS
The second QFT was positive in 4.8% of the 376 HCWs with an INF-γ concentration at baseline below 0.1 IU/mL but in 48.8% of the 41 HCWs with an INF-γ concentration of 0.2 to <0.35 IU/mL. Out of 74 HCWs with a baseline INF-γ concentration ≥3.0 IU/mL, 4 (5.4%) reversed while 27 out of 55 HCWs (49%) with a baseline INF-γ concentration ≥0.35 to <0.7 IU/mL reversed to a negative QFT. Those 61 HCWs with TST conversion (increase ≥10 mm) were most often (78.7%) negative in both consecutive QFTs.


CONCLUSIONS
Our data suggests the use of an uncertainty zone between 0.2 and 0.7 IU/mL in serial testing with QFT. As long as the knowledge regarding disease progression in QFT-positive persons is limited, persons pertaining to this zone should be retested before being offered preventive chemotherapy.
Torres Costa
José
J
Occupational Health Division, Hospital S. João, Alameda Professor Hernâni Monteiro, Porto, Portugal.
Silva
Rui
R
Sá
Raul
R
Cardoso
Maria João
MJ
Nienhaus
Albert
A
D000293:Adolescent; D000328:Adult; D005260:Female; D006282:Health Personnel; D006801:Humans; D007371:Interferon-gamma / Q000097:blood*; D057230:Limit of Detection; D008297:Male; D008403:Mass Screening; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000276:immunology / Q000302:isolation & purification; D011174:Portugal / Q000453:epidemiology; D011933:Reagent Kits, Diagnostic; D014374:Tuberculin Test / Q000379:methods*; D014376:Tuberculosis / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology; D055815:Young Adult
D011933:Reagent Kits, Diagnostic; D007371:Interferon-gamma