A skin test using the subcellular fraction T12E antigen of Trypanosoma cruzi can be used to evaluate patients for cellular immunity to T. cruzi.
Method:
(1) 100 µL of the antigen preparation containing 20 µg of protein is injected intradermally.
(2) The size of induration at 48 hours is recorded, with measurements along the long and short axes taken in centimeters.
(3) The area of induration is calculated using the equation for an ellipse:
area of induration in square cm = area of an ellipse =
= ((diameter along long axis in cm) / 2) * ((diameter along short axis in cm) / 2) * π
Interpretation:
• A person who does not have cellular immunity to T. cruzi will have an erythematous area at the site of injection that measures < 1 square cm at 48 hours.
• A person with cellular immunity will show an area of induration measuring > 1.5 square cm.
Limitations:
• A person may be anergic to the antigen with hereditary or acquired immunodeficiency.
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