Mycobacterium tuberculosis infection in health care workers in rural India: comparison of a whole-blood interferon gamma assay with tuberculin skin testing.
BACKGROUND
Mycobacterium tuberculosis infection in health care workers has not been adequately studied in developing countries using newer diagnostic tests.


OBJECTIVE
To estimate latent tuberculosis infection prevalence in health care workers using the tuberculin skin test (TST) and a whole-blood interferon gamma (IFN-gamma) assay; to determine agreement between the tests; and to compare their correlation with risk factors.


METHODS
A cross-sectional comparison study of 726 health care workers aged 18 to 61 years (median age, 22 years) with no history of active tuberculosis conducted from January to May 2004, at a rural medical school in India. A total of 493 (68%) of the health care workers had direct contact with patients with tuberculosis and 514 (71%) had BCG vaccine scars.


METHODS
Tuberculin skin testing was performed using 1-TU dose of purified protein derivative RT23, and the IFN-gamma assay was performed by measuring IFN-gamma response to early secreted antigenic target 6, culture filtrate protein 10, and a portion of tuberculosis antigen TB7.7.


METHODS
Agreement between TST and the IFN-gamma assay, and comparison of the tests with respect to their association with risk factors.


RESULTS
A large proportion of the health care workers were latently infected; 360 (50%) were positive by either TST or IFN-gamma assay, and 226 (31%) were positive by both tests. The prevalence estimates of TST and IFN-gamma assay positivity were comparable (41%; 95% confidence interval [CI], 38%-45% and 40%; 95% CI, 37%-43%, respectively). Agreement between the tests was high (81.4%; kappa = 0.61; 95% CI, 0.56-0.67). Increasing age and years in the health profession were significant risk factors for both IFN-gamma assay and TST positivity. BCG vaccination had little impact on TST and IFN-gamma assay results.


CONCLUSIONS
Our study showed high latent tuberculosis infection prevalence in Indian health care workers, high agreement between TST and IFN-gamma assay, and similar association between positive test results and risk factors. Although TST and IFN-gamma assay appear comparable in this population, they have different performance and operational characteristics; therefore, the decision to select one test over the other will depend on the population, purpose of testing, and resource availability.
Pai
Madhukar
M
Department of Medicine, Mahatma Gandhi Institute of Medical Sciences, Sevagram, India. madhupai@berkeley.edu
Gokhale
Kaustubh
K
Joshi
Rajnish
R
Dogra
Sandeep
S
Kalantri
Shriprakash
S
Mendiratta
Deepak K
DK
Narang
Pratibha
P
Daley
Charles L
CL
Granich
Reuben M
RM
Mazurek
Gerald H
GH
Reingold
Arthur L
AL
Riley
Lee W
LW
Colford
John M
JM
D000328:Adult; D003430:Cross-Sectional Studies; D003906:Developing Countries; D005260:Female; D006282:Health Personnel; D006801:Humans; D007194:India / Q000453:epidemiology; D007371:Interferon-gamma / Q000276:immunology; D008297:Male; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000302:isolation & purification / Q000502:physiology; D015995:Prevalence; D012307:Risk Factors; D012424:Rural Population; D014374:Tuberculin Test; D014376:Tuberculosis / Q000175:diagnosis* / Q000453:epidemiology*; D017735:Virus Latency
D007371:Interferon-gamma