Interpretation of a serial Mantoux test taking into account the annual risk of tuberculous infection.
BACKGROUND
Interpretation of tuberculin tests (TSTs) can be difficult. However, it is even more difficult to classify an individual as infected or non-infected if he or she has undergone a prior TST, as the difference between the booster effect and true conversion is not always clear.


OBJECTIVE
To determine the size of the induration that is indicative of true infection after a second Mantoux test, based on the fact that the annual risk of tuberculous infection (ARTI) is equivalent, regardless of whether it is calculated directly or indirectly.


METHODS
We performed two TSTs in the same population (aged 6 years at the first examination) with a 1-year interval. We calculated the prevalence of infection the first year (1.16%, 95%CI 0.67-1.65) and the ARTI using the indirect method (0.18%, 95%CI 0.00-0.37). For the second year, we considered different induration sizes and finally accepted a diameter of >10 mm as the criterion according to which the ARTI, calculated directly, corresponded to the value of the previous year (0.18%).


CONCLUSIONS
Although they served as an aid in the interpretation of the TST, calculations of this type will probably be unnecessary in the future, when in vitro diagnostic tests for tuberculosis are made available in every health centre, thus enabling the classification of those individuals with indurations of intermediate size in the TST.
del Río Camacho
G
G
Paediatrics Department, Costa del Sol Hospital, Marbella, Spain. vevirio@hotmail.com
Perea-Milla
E
E
Romero González
J
J
Pérez Frías
J
J
D002648:Child; D003430:Cross-Sectional Studies; D005500:Follow-Up Studies; D006801:Humans; D010507:Periodicity; D011237:Predictive Value of Tests; D015995:Prevalence; D012306:Risk; D014374:Tuberculin Test / Q000379:methods* / Q000706:statistics & numerical data*; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis* / Q000453:epidemiology*