Procedure

The suitable dilution of standard dose of tuberculin (standard dose 5TU) is injected in the inner aspect of forearm. A weal of 5mm should be raised. The     reaction is read after 48 to 72 hours. The induration is measure not the redness (erythema) surrounding it.

Dosage of tuberculin

 The dose is 0.1ml.

 In 0.1 ml of diluent contains 0.0001mg of Purified protein derivative (PPD) which is equivalent to 5Tuberculin units (TU).

 PPD is obtained by culturing Mycobacterium tuberculosis H37RA strain with TWEEN 80 on a protein free medium Quinsol.

Interpretation of the reaction

The reaction is read by measuring the induration at the site of injection and not the redness surrounding it. The induration is measured in millimeters. If there is no induration than it is measured as 0mm and considered it as negative reaction. Redness with no induration is again negative reaction. Another tuberculin test is required if the reaction is negative. 

Negative reaction

If there is no induration than it is measured as 0mm and considered it as negative reaction. Redness with no induration is again negative reaction. Another tuberculin test is required if the reaction is negative.

Positive reaction.

Natural infection- in a child less than two years of age if the induration is more than 10mm than it is considered as recent infection and treatment is required with anti tubercular drugs.

False negative reaction

This is a situation where in the proven tuberculosis patients the tuberculin reaction reads negative. False negative reaction can happen in following conditions.

• During the incubation period of the disease
• Steroid and other immunosuppressive therapy
• In overwhelming infection of tuberculosis
• Wrong technique
• Inactive tuberculin units
• Following measles