Surprisingly high specificity of the PPD skin test for M. tuberculosis infection from recent exposure in The Gambia.
BACKGROUND
Options for intervention against Mycobacterium tuberculosis infection are limited by the diagnostic tools available. The Purified Protein Derivative (PPD) skin test is thought to be non-specific, especially in tropical settings. We compared the PPD skin test with an ELISPOT test in The Gambia.


RESULTS
Household contacts over six months of age of sputum smear positive TB cases and community controls were recruited. They underwent a PPD skin test and an ELISPOT test for the T cell response to PPD and ESAT-6/CFP10 antigens. Responsiveness to M. tuberculosis exposure was analysed according to sleeping proximity to an index case using logistic regression. 615 household contacts and 105 community controls were recruited. All three tests assessed increased significantly in positivity with increasing M. tuberculosis exposure, the PPD skin test most dramatically (OR 15.7; 95% CI 6.6-35.3). While the PPD skin test positivity continued to trend downwards in the community with increasing distance from a known case (61.9% to 14.3%), the PPD and ESAT-6/CFP-10 ELISPOT positivity did not. The PPD skin test was more in agreement with ESAT-6/CFP-10 ELISPOT (75%, p = 0.01) than the PPD ELISPOT (53%, p<0.0001). With increasing M. tuberculosis exposure, the proportion of ESAT-6/CFP-10 positive contacts who were PPD skin test positive increased (p<0.0001), and the proportion of ESAT-6/CFP-10 negative contacts that were PPD skin test negative decreased (p<0.0001); the converse did not occur.


CONCLUSIONS
The PPD skin test has surprisingly high specificity for M. tuberculosis infection from recent exposure in The Gambia. In this setting, anti-tuberculous prophylaxis in PPD skin test positive individuals should be revisited.
Hill
Philip C
PC
Bacterial Diseases Programme, Medical Research Council Unit, Banjul, The Gambia. phill@mrc.gm
Brookes
Roger H
RH
Fox
Annette
A
Jackson-Sillah
Dolly
D
Lugos
Moses D
MD
Jeffries
David J
DJ
Donkor
Simon A
SA
Adegbola
Richard A
RA
McAdam
Keith P W J
KP
D000293:Adolescent; D000328:Adult; D000368:Aged; D000942:Antigens, Bacterial / Q000276:immunology; D001426:Bacterial Proteins / Q000276:immunology; D016022:Case-Control Studies; D002648:Child; D002675:Child, Preschool; D016358:Contact Tracing / Q000379:methods* / Q000706:statistics & numerical data; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods / Q000706:statistics & numerical data; D005260:Female; D005714:Gambia / Q000453:epidemiology; D006798:Housing; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D012680:Sensitivity and Specificity; D013601:T-Lymphocytes / Q000276:immunology; D014374:Tuberculin Test / Q000706:statistics & numerical data*; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis* / Q000453:epidemiology / Q000517:prevention & control / Q000635:transmission
D000942:Antigens, Bacterial; D001426:Bacterial Proteins; C117940:CFP-10 protein, Mycobacterium tuberculosis; C093276:ESAT-6 protein, Mycobacterium tuberculosis