LEPROMIN TEST The test is performed by injecting intradermally 0.1 ml of lepromin into the inner aspect of the forearm of the individual. As a routine, the reaction is read at 48 hours and 21 days. Two types of positive reactions have been described: (a) EARLY REACTION : The early reaction is also known as Fernandez reaction. An inflammatory response develops within 24 to 48 hours and this tends to disappear after 3 to 4 days. It is evidenced by redness and induration at the site of inoculation. If the diameter of the red area is more than 10 mm at the end of 48 hours, the test is considered positive. The early positive reaction indicates whether or not a person has been previously sensitized by exposure to and infection by the leprosy bacilli. In this sense, the early reaction is much superior to the late reactfon. The early reaction has been described as delayed hypersensitivity reaction to "soluble" constituents of the leprosy bacilli. The reaction corresponds to the Mantoux reaction in tuberculosis, caused by the soluble antigens (PPD) of the tubercle bacilli. (b) LATE REACTION : This is the classical Mitsuda reaction. The reaction develops late, becomes apparent in 7-10 days following the injection and reaching its maximum in 3 or 4 weeks. The test is read at 21 days. At the end of 21 days, if there is a nodule more than 5 mm in diameter at the site of inoculation, the reaction is said to be positive. The nodule may even ulcerate and heal with scarring if the antigen is crude. It may be noted that the diameter of the red area in the early reaction, and the diameter of the nodule in the late reaction are measured. The early reaction is induced by the soluble constituents of the leprosy bacilli; and the late reaction by the bacillary component of the antigen. It indicates cell-mediated immunity. In the first 6 months of life, most children are lepromin negative; some may become positive by the end of first year. Data obtained from different pas of the world indicate that in endemic areas, lepromin reaction is already positive in 20 per cent of children under 5 years of age, and this propoion increases to around 60 per cent or more in the 10-14 years age group, and to 80 per cent or more in persons over 19 years of age. BCG vaccination is capable of conveing the lepra reaction from negative to positive in a large propoion of individuals. Value of the lepromin test Lepromin test is not a diagnostic test. The two drawbacks that stand in the way of this test being used for diagnosis are : (i) positive results in non-cases, and (ii) negative results in lepromatous and near-lepromatous cases. The test has been generally accepted as a useful tool in evaluating the immune status (CMI) of leprosy patients. It is of considerable value in confirming the results ofclassification of cases of leprosy on clinical and bacteriological grounds. In other words, the test is widely used as an aid to classify the type of disease. The test is also of great value in estimating the prognosis in cases of leprosy of all types. The test is usually strongly positive in the typical tuberculoid cases, and the positivity getting weaker as one passes through the spectrum to the lepromatous end, the typical lepromatous cases being lepromin negative indicating a failure of CMI. It is known that lepromin negative individuals are at a higher risk of developing progressive multibacillary leprosy, but those who are lepromin positive either escape the clinical disease (the majority) or develop paucibacillary disease (the minority). Ref: Park 25th edition Pgno : 338