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Laboratory Diagnosis

The diagnosis of leprosy is histopathological; the laboratory tests may help but because of the possible stigma attached to leprosy, where there is doubt there can be delay before making a definitive diagnosis. Many of the laboratory tests are useful indications of the patient's reaction to infection, but not always a positive way to make the diagnosis.

There is no confirmed method of in vitro cultivation of M. leprae, although many claims have been made. On the contrary, M. leprae can be identified because it does not grow in media suitable for other mycobacteria. When inoculated into the foot pads of mice or the nine-banded armadillo, there is a characteristic growth pattern: this may take several weeks to develop, unless the mice have an altered immune system. The imaging diagnosis is seldom in doubt, except in the early stages, but because the clinical record may refer to several tests, that information is given below. Many classical tests have been replaced by specific antigen and DNA probes.

Lepromin Test

Some authorities (but not all) regard the lepromin test as useless. Lepromin is a crude, semistandardized extract of bacilli from a lepromatous nodule of an armadillo. One-tenth of a milliliter is injected intradermally and the site is examined after 72 hours (the Fernandez reaction) and 3-4 weeks (the Mitsuda reaction). The palpable nodule which develops is measured and graded. A positive test is a nodule more than 3-5 mm in size and indicates the presence of delayed hypersensitivity to M. leprae antigens. Unfortunately this is not a diagnostic test for leprosy, as many people who have never been exposed to this bacillus show a positive reaction, for example, due to tuberculosis, immunization with BCG, or even previous skin testing with lepromin. The Mitsuda reaction is positive in cases of tuberculoid leprosy and borderline leprosy of the tuberculoid type, and therefore is helpful in classification and prognosis. It is negative in lepromatous leprosy and the borderline lepromatous variety. At the early, indeterminate stage a negative test excludes the diagnosis of leprosy or, alternatively, indicates that the lesion is differentiating into lepromatous leprosy and may thus be helpful in assessing the direction in which the disease will progress. Reversal of a negative to a positive Mitsuda test in leprosy is considered an improvement and indicates a better prognosis with shift away from the lepromatous pole. BCG injection tends to change a negative reactor to a positive reactor by shifting the body's reaction towards the tuberculoid end of the spectrum; it is not clear whether this is a false-positive reaction.

Histamine Test

Tuberculoid leprosy causes hypopigmented macules: the response of the skin to histamine will be delayed, diminished, or even absent. In normal patients a drop of 0.001 histamine on the skin followed by pricking with a needle will cause a flare during the subsequent 10 minutes. In a hypopigmented patch due to leprosy this flare may be altered or absent. The normal and the hypopigmented area should be tested simultaneously in the same patient, but in dark skins the results may be difficult to assess.

Pilocarpine Test

Sweating is dependent upon the integrity of the parasympathetic nerve endings. If hypopigmentation is due to leprosy the response of the sweat glands will be diminished. Although described in detail in all standard textbooks on leprosy, it is also noted in each book that the histamine and pilocarpine tests are seldom required.

Skin or Nerve Biopsy

Histological diagnosis by skin biopsy is indicated whenever the diagnosis is in doubt, as it will be particularly at the earliest stage, indeterminate leprosy. Biopsy of the cutaneous nerve may also be necessary. Skin biopsy is helpful in the accurate classification of the pattern of the disease, judged particularly by the extent of the cellular infiltration below or up to the epidermis.

Skin Smears

Smears from suspect lesions, particularly from the forehead, ear lobes, chin, the extensor surfaces of the forearms, buttocks, and trunk, and the examination of nasal discharge obtained when the patient blows his or her nose provide an accurate method of identifying the leprosy bacillus with suitable staining. The bacillary index should be recorded and falls after about 1 year of successful treatment. The morphological index may also be obtained from the same smear because bacilli that stain irregularly or are fragmented are dead. It provides an accurate index of the success of therapy, or of developing drug resistance.

Metabolic factors in leprosy have been investigated by various authors, but whatever the extent of skeletal involvement, no significant changes have been found in blood calcium, phosphorus or phosphatase. Both hypercalcemia and hypocalcemia have been reported in different series, but this is probably the result of other factors and is not specific to leprosy.

Immunological studies show a cell-mediated immunity. In an endemic area for leprosy, the lymphocyte transformation test is positive in about a quarter of those in the population who are not suffering from lepromatous leprosy. A false-positive serology for syphilis may occur. The erythrocyte sedimentation rate, C-reactive protein, rheumatoid factor, lupus erythematosus (LE) cells, antithyroglobulin antibody, and serum protein may be elevated. There may be reversal of the albumin-globulin ratio.

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