The laboratory diagnosis of Leprosy is made by:
1. Histology
2. Examination of appropriate Ziehl-Neelsen-stained skin smears
for AFB
Bacilli are found in polar Lepromatous leprosy (LL), Borderline
(BB) and its variants Borderline Lepromatous (BL). They are scanty
in Borderline tuberculoid (BT) and are not found in polar tuberculoid
(TT).
Instructions for preparation of Skin Smears
These are best performed by experienced staff. The nodule of plaque
is cleaned with ether or any suitable disinfectant. The skin is
gripped tightly between thumb and forefinger of the left hand
to exsanguinate the lesion. An incision, 5 mm long and 3 mm deep,
is made in the skin between the fingers of the left hand with
a small bladed scalpel, the pressure on the fingers being maintained.
The base of the wound is scraped several times in the same direction,
so that tissue fluid and pulp collects on the blade and this is
smeared on a labelled glass slide. There should be minimal blood
on the specimen. Smears from ear lobes are made in the same manner.
The incision is made along the lateral edge of the pinna, the
latter being compressed between thumb and forefinger. 6-8 smears
are made from each patient. The sites are recorded and it is preferable
to repeat smears from the same site when repeat specimens are
taken later. Slides should be heat fixed if there is a delay in
reading in the laboratory.
Smears from Nasal Secretions
These smears are essential in deciding whether a patient is infectious.
They are always positive in untreated (LL), some (BL) and negative
in all cases of (BB), (BT), and (TT). The patient is requested
to blow into a tissue and smears are made with a scalpel onto
glass slides (2-3).
Reporting conventions
Bacterial Index (BI) and Morphological Index (MI).
The BI is an index of the bacillary load in the patient. This
is expressed on a semi-logarithmic scale as given below.
BI Interpretation
1+ 1 to 10 bacilli per 100 high power (oil immersion) fields
2+ 1 to 10 bacilli per 10 high power fields
3+ 1 to 10 bacilli per high power field
4+ 10 to 100 bacilli per high power field
5+ 100 to 1000 bacilli per high power field
6+ >1000 bacilli per high power field
Reference may be made to the term 'globi' in some reports. These
are clumps of bacilli and are generally found in LL. The clumps
are derived from micro-colonies in macrophages.
MI interpretation
The MI is an index of viability of the bacilli. Solid bacilli
are deemed to be viable while fragmented or granular bacilli are
considered to be non-viable. Two hundred discrete bacilli are
evaluated if possible. The MI is equal to the percentage of viable
bacilli. Note that pauci-bacillary lesions may not be assessable
for the MI. The MI in untreated multi-bacillary leprosy usually
ranges between 25 and 75 and should decline to 0 after 4 to 6
months of effective modern chemotherapy.
