Identifying ELISPOT and skin test cut-offs for diagnosis of Mycobacterium tuberculosis infection in The Gambia.
METHODS
An urban area, The Gambia.


OBJECTIVE
To identify ELISPOT and PPD skin test cut-offs, targeting sensitivity and specificity equivalence.


METHODS
Tuberculosis cases >5 years of age and their household contacts underwent ELISPOT, HIV and PPD skin tests. Cases and contacts sleeping in a different house were used to estimate sensitivity and specificity, providing two planes for estimating cut-offs. Specificity was adjusted for infection from previous exposure using a multivariate discrimination algorithm.


RESULTS
The point on the line of intersection of the planes that maximised sensitivity and specificity equivalence occurred at 4 spots (95% confidence interval [CI] 3.5-5, multiplier=0 ) for CFP-10 and 5.5 spots (4.5-8, multiplier=0 for ESAT-6), yielding a sensitivity and specificity of 76% for both antigens. Combining ESAT-6 and CFP-10 using an 'or' statement yielded a maximum equivalence sensitivity and specificity of 76.5% at 6 spots for ESAT-6 and 11.5 spots for CFP-10. For the PPD skin test sensitivity and specificity, an equivalence of 78% occurred at 11 mm induration (9-13 mm).


CONCLUSIONS
An ELISPOT cut-off for ESAT-6 or CFP-10 could be set at 4-8 spot forming units (20-40 spots per million), with little benefit from combining the results. A cut-off of 9-13 mm for the PPD skin test is reasonable when comparing with the ELISPOT.
Jeffries
D J
DJ
Medical Research Council Labs, Bacterial Diseases Programme, Banjul, The Gambia.
Hill
P C
PC
Fox
A
A
Lugos
M
M
Jackson-Sillah
D J
DJ
Adegbola
R A
RA
Brookes
R H
RH
D000293:Adolescent; D000907:Antibodies, Bacterial / Q000032:analysis*; D003937:Diagnosis, Differential; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D005714:Gambia / Q000453:epidemiology; D006801:Humans; D009169:Mycobacterium tuberculosis / Q000276:immunology*; D015995:Prevalence; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D014374:Tuberculin Test / Q000379:methods; D014376:Tuberculosis / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology; D014505:Urban Population
D000907:Antibodies, Bacterial