Slit Skin Smear and Bacteriological Index in Leprosy
On this page
Direct answer
The slit skin smear is a test in which dermal tissue fluid is taken from standard sites, stained for acid-fast bacilli and examined to measure the bacterial load in leprosy. The bacteriological index (BI) is a logarithmic score from 0 to 6+ that reflects how many bacilli are present. The morphological index (MI) is the percentage of solid-staining, viable bacilli, and together the two indices guide classification, treatment and follow-up.
What you must remember
- Sites: usually both earlobes plus other sites such as the elbows or an active lesion.
- Technique: a small, shallow incision is made, the dermal fluid is smeared on a slide, and the smear is stained by a modified Ziehl-Neelsen method.
- Bacteriological index (BI): a logarithmic score from 0 to 6+ averaged across sites, with each grade representing roughly a ten-fold rise in bacilli.
- Morphological index (MI): the percentage of solid-staining, viable bacilli among all bacilli seen.
- Solid staining: a solid-staining bacillus is viable, whereas a fragmented or granular bacillus is dead.
- Paucibacillary (PB) leprosy: up to five skin lesions with no more than one nerve involved, usually with a negative smear.
- Multibacillary (MB) leprosy: more than five skin lesions, involvement of more than one nerve, or a positive smear.
- Interpretation: a persistent or rising index after treatment may suggest relapse or drug resistance.
A stepwise smear technique
A patient with suspected leprosy and no clear lesion still needs a smear, because bacilli can be found in apparently normal skin, especially in lepromatous patients. The steps below follow the usual technique from the site to the reported index.
- Clean the site: Wash the earlobe or lesion with alcohol and let it dry, so the smear is not contaminated.
- Make a small incision: Use a sterile blade to make a shallow incision in the skin at each chosen site.
- Collect the dermal fluid: Squeeze the incision and scrape the cut surface, collecting the dermal tissue fluid on the blade.
- Smear and fix: Spread the material thinly on a clean slide, allow it to air dry and heat fix it.
- Stain and grade: Stain by Ziehl-Neelsen, count the bacilli per field and assign the Ridley logarithmic grade for each site.
- Calculate the indices: Average the BI across the sites and record the MI from the proportion of solid-staining bacilli.
- Repeat during treatment: Repeat smears at intervals to monitor the response, since the index falls as bacilli die.
Where the exam tries to catch you
Most traps involve mixing up the two indices or reading a negative smear as proof of absence.
- Trap: equating the bacteriological index with the number of lesions Correct: the index measures bacilli, so a smear result must always be read alongside the clinical picture.
- Trap: confusing the morphological index with the bacteriological index Correct: the BI is the overall logarithmic bacillary load, whereas the MI is the percentage of solid-staining, viable bacilli.
- Trap: taking a negative smear as exclusion of leprosy Correct: a negative smear is typical of paucibacillary disease, so it does not rule leprosy out.
- Trap: sampling only the active lesion Correct: the earlobes and other standard sites are included, since bacilli can be found there even without active lesions.
Frequently asked questions
Which sites are used for a slit skin smear in leprosy?
Slit skin smears are usually taken from both earlobes plus other sites such as the elbows or an active lesion. These standard sites give the best yield, and bacilli can be found in apparently normal skin.
What does the bacteriological index in leprosy measure?
The bacteriological index measures the bacillary load on a logarithmic scale from 0 to 6+, with each grade representing roughly a ten-fold rise in bacilli. A higher index means more bacilli and usually more extensive, multibacillary disease.
What is the morphological index in leprosy?
The morphological index is the percentage of solid-staining, viable bacilli among all bacilli seen on the smear. It falls during effective treatment, because dead bacilli fragment and stain granularly rather than solidly.
Does a negative slit skin smear exclude leprosy?
A negative slit skin smear does not exclude leprosy, because paucibacillary disease often gives negative smears. Clinical findings, sensory testing and skin biopsy are needed alongside the smear before leprosy is ruled out.