The lepromin test is useful in determining the extent of host immune reactivity to Mycobacterium leprae.
0.1 ml of lepromin, prepared from a crude extract of organisms, is injected intradermally. The reaction is read at 48 hours (Fernandez) or at 3 to 4 weeks (Mitsuda).
A positive Fernandez reaction, induration and erythema, indicates that the host is capable of an immune response to M. leprae of the delayed type.
It is equivalent to the Mantoux (PPD) test for M. tuberculosis.
The Mitsuda reaction is graded +1 - a papule 4-6 mm; +2, a papule 7-10 mm; +3, a papule greater than 10 mm, or a papule that ulcerates. The degree of Mitsuda reaction correlates well with the host immune response, and therefore with prognosis. Histologic classification of tissue biopsied from the Mitsuda reaction probably has better clinical correlation than histology from a leprosy skin lesion.