The Mantoux skin test must be read 2 to 3 days (48 to 72 hours) after it is done. The widest transverse diameter (across the arm) of induration (raised, swollen, thickened area of skin) is measured. It is important that the induration and not the area of redness is measured. The diameter of the induration is best measured with a ruler. The result should be reported in millimeters and not simply as positive or negative. The interpretation is as follows:
- If the diameter of induration is 0 to 4 mm the test is negative.
- A diameter of induration of 5 to 9 mm is intermediate and may be due to BCG or TB infection. In a healthy HIV negative child this usually indicates that BCG has been given. However, in HIV infected or severely malnourished children, it may indicate tuberculosis.
- A diameter of induration of 10 mm or more is positive and indicates infection with TB bacilli.
A Mantoux skin test of 10 mm or more indicates tuberculous infection. However, an induration between 5 and 9 mm cannot differentiate between children who have a tuberculous infection and those who have had BCG in the past 2 years. Furthermore, a result of 10 mm or more cannot differentiate between a recent healed TB infection and active tuberculosis. It is unfortunate that BCG may confuse the interpretation of the Mantoux skin test in the first 2 years. It is important to understand that a positive Mantoux test suggest tuberculous infection but does not necessarily mean that the child has tuberculosis.
A positive Mantoux skin test indicates tuberculous infection but not necessarily tuberculosis.
An area of induration of 5 to 9 mm may also indicate infection with a non-tuberculous Mycobacterium.