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If the redness
of Mantoux is more than 10 mm it is considered positive but please try to understand 2 things:-
1-When the a person has symptoms like loss of weight;evening fever
or continuous mild fever; cough
;loss of appetite then a doctor can suspect TB
2-It can be said that a person has TB when all the test like ESR;X-ray chest sputum for AFB;IgM; are positive and the latest is TB Gold which is confirmatory these test are diagnostic of TB.Mantoux is also a supportive test but alone Mantoux cannot be considered as diagnostic of TB just for a simple reason in most of the kids who have been reactive to BCG vaccine
it comes positive.I would suggest you to stay calm and then consult a Paediatrician who will guide you better.There are minimal chances that your child will have TB and if at all it is present it is easily treatable.Please do not worry and do any thing in a haste.