Lepromin Test
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Direct answer
The lepromin test is an intradermal skin test that uses heat-killed Mycobacterium leprae to measure cell-mediated immunity against the bacillus. Its early reading is the Fernandez reaction at 48 hours, and its late reading is the Mitsuda reaction at 3 to 4 weeks. A positive result, seen in tuberculoid and borderline tuberculoid leprosy, indicates good immunity and a favourable prognosis, whereas a negative result in lepromatous disease indicates poor immunity.
What you must remember
- Antigen: lepromin, a heat-killed suspension of Mycobacterium leprae prepared from infected tissue.
- Route: injected intradermally, usually on the forearm.
- Fernandez reaction: an early erythematous, indurated response read at 48 hours, reflecting a rapid immune response.
- Mitsuda reaction: a late, firm granulomatous nodule read at about 3 to 4 weeks, reflecting delayed immunity.
- Pattern by type: positive in TT and BT, variable in BB, and negative in BL and LL.
- Prognosis: a positive test indicates a better prognosis, and a negative test indicates poor immunity and a higher risk of progression.
- Not diagnostic: a positive result does not confirm leprosy, since many healthy people in endemic areas also react.
- Current use: mainly a research and classification tool rather than a routine diagnostic test.
A stepwise reading of the test
A clinician wants to know how strong the patient's cell-mediated immunity is, not whether the bacillus is present. The lepromin test answers that question by measuring the immune response to a fixed bacillary antigen, read at two time points.
- Prepare the antigen: Lepromin is a standardised suspension of heat-killed Mycobacterium leprae, injected intradermally in a fixed volume, usually on the forearm.
- Read at 48 hours: The Fernandez reaction is an early erythematous induration, and a clear reaction indicates a rapid cell-mediated response.
- Read at 3 to 4 weeks: The Mitsuda reaction is a firm, raised granulomatous nodule that appears late, reflecting delayed immunity.
- Interpret by type: Clearly positive results in TT and BT indicate a robust response, responses in BB are variable, and negative results in BL and LL indicate a poor response.
- Relate to prognosis: A positive response tracks with lower bacillary load and better outcome, while a negative response suggests a higher risk of progression.
- Avoid diagnostic misuse: A positive result does not confirm leprosy, because many people in endemic areas also react to the antigen.
- Record the reading: Note the size of the induration and the time of reading, since both affect comparison between tests and between centres.
Where the exam tries to catch you
Most traps involve using lepromin for diagnosis or mixing up the two reading times.
- Trap: using lepromin to diagnose leprosy Correct: the test measures immunity, and healthy people in endemic areas can be positive, so diagnosis rests on clinical and smear findings.
- Trap: believing the Fernandez reaction is the late reading Correct: Fernandez is the early reading at 48 hours, while Mitsuda is the late reading at about 3 to 4 weeks.
- Trap: expecting LL patients to be lepromin positive Correct: LL and BL patients are lepromin negative because their cell-mediated immunity is weak.
- Trap: reading a negative test as a good prognostic sign Correct: a negative lepromin test indicates poor cell-mediated immunity and a worse prognosis.
Frequently asked questions
What is the difference between the Fernandez and Mitsuda reactions?
The Fernandez reaction is an early erythematous response read at 48 hours after intradermal lepromin. The Mitsuda reaction is a late granulomatous nodule read at about 3 to 4 weeks, and both reflect cell-mediated immunity to Mycobacterium leprae.
Which leprosy types are lepromin positive?
Lepromin is positive in tuberculoid and borderline tuberculoid leprosy, reflecting strong cell-mediated immunity. The response is variable in mid-borderline leprosy, and borderline lepromatous and lepromatous leprosy are lepromin negative.
Does a positive lepromin test diagnose leprosy?
A positive lepromin test does not diagnose leprosy, because the test only measures immunity to the bacillus. Many healthy people in endemic areas are also lepromin positive, so leprosy is diagnosed through clinical examination and smears.
Why is lepromin negative in lepromatous leprosy?
Lepromatous leprosy is lepromin negative because the patient has weak cell-mediated immunity against Mycobacterium leprae. The bacilli multiply freely in the skin, and the immune system does not form the granulomas that define a positive reaction.