Step-by-Step Guide to Performing Slit Skin Smear for Leprosy Diagnosis
Slit skin smear (SSS) is a bacteriological technique used to detect acid-fast bacilli (Mycobacterium leprae) in skin lesions, serving as one of the cardinal signs for definitive leprosy diagnosis, though its routine use for classification is increasingly questioned. 1, 2, 3
Pre-Procedure Preparation
Site Selection
- Select 4-6 sites total: both earlobes (mandatory) plus 1-2 active skin lesions 1, 2
- Choose lesions with raised, erythematous borders or nodules rather than flat hypopigmented patches 2
- Avoid recently traumatized or ulcerated areas 1
- For suspected paucibacillary cases with minimal lesions, earlobe sampling alone may suffice 2
Materials Needed
- Sterile surgical blade (No. 15 or similar)
- Glass microscope slides (labeled with patient ID and site)
- 70% alcohol for skin preparation
- Gauze or cotton swabs
- Gloves
- Ziehl-Neelsen or modified acid-fast staining reagents 2
Procedure Steps
1. Skin Preparation
- Cleanse the selected site thoroughly with 70% alcohol and allow to dry completely 1
- Avoid leaving residual alcohol, which can interfere with specimen quality 1
2. Creating the Incision
- Pinch the skin firmly between thumb and forefinger to create ischemia and minimize bleeding 1, 2
- Make a small incision (3-5 mm long, 2-3 mm deep) through the dermis using the sterile blade 2
- The incision should be deep enough to reach dermal tissue but shallow enough to avoid excessive bleeding 1, 2
3. Specimen Collection
- Scrape the sides and base of the incision vigorously with the blade edge, rotating it 90 degrees 1, 2
- Collect tissue fluid and dermal material—NOT blood or surface exudate 1, 2
- If bleeding occurs, wait briefly and re-scrape after blotting 2
4. Slide Preparation
- Spread the collected material thinly and evenly on a labeled glass slide 2
- Create separate smears for each site sampled 2
- Allow slides to air-dry completely before staining 2
5. Staining and Microscopy
- Perform Ziehl-Neelsen or modified acid-fast staining 2
- Examine under oil immersion (100x objective) for red-stained acid-fast bacilli within macrophages or extracellularly 2, 4
- Report bacterial index (BI) on a logarithmic scale of 0-6+ 2
Critical Clinical Considerations
Diagnostic Limitations
- SSS positivity automatically classifies a patient as multibacillary (MB), but a negative result does NOT rule out leprosy or exclude MB disease 2, 3
- In a recent Ethiopian study, 54.7% of clinically diagnosed MB patients had negative SSS, yet remained classified as MB based on clinical criteria (≥6 lesions) 3
- SSS should complement—not replace—clinical diagnosis based on cardinal signs: definite sensory loss in skin lesions and/or thickened peripheral nerves 2, 4
When SSS Changes Management
- SSS is most valuable when clinical classification is uncertain (1-5 lesions with equivocal features) 3
- For clear-cut paucibacillary cases (<5 lesions), SSS rarely changes classification and may represent unnecessary resource expenditure 3
- For clear-cut multibacillary cases (≥6 lesions), clinical classification alone suffices for treatment initiation 3
Common Pitfalls to Avoid
- Inadequate depth of incision: Superficial scraping yields only epidermal debris with low bacillary yield 1, 2
- Excessive bleeding: Dilutes the specimen and obscures bacilli on microscopy 2
- Sampling healed or inactive lesions: Active borders have higher bacterial loads 1, 2
- Relying solely on SSS for diagnosis: Clinical cardinal signs (sensory loss, nerve thickening) remain the primary diagnostic criteria 2, 4
Practical Algorithm for SSS Use
Perform SSS when:
- Clinical diagnosis is uncertain (atypical presentation, 1-5 lesions without clear sensory loss) 3
- Baseline documentation is needed for monitoring treatment response in confirmed MB cases 2
- Relapse is suspected in previously treated patients 2
SSS is NOT routinely indicated when: