Slit skin smear in Leprosy

Slit skin smear (SSS) -  3 types of staining of lepra bacilli 

 (Source: IAL textbook)

Note: Even though SSS is traditionally recommended to classify cases of leprosy as Paucibacillary (PB) and multibacillary (MB), lately histopathology and demonstration of bacilli in biopsies (bacterial index of granuloma or BIG) is considered better, especially in diagnosing cases with less lepra bacilli load

Introduction

• High specificity (100%)

• Low sensitivity (10-50%) (As SSS sample is a superficial nick with a blade, not a deep sample, hence, many patients with significant number of bacilli in deep dermis will be declared SSS negative). This falsely negative sample will then be classified as PB case and treated wrongly. Thus, histological biopsy examination is the gold standard and depending on the presence/abence of bacilli in dermis (bacterial index of granuloma or BIG index), classification between PB and MB is done. This BIG positivity in SSS negative patients is explained by the deeper AFB presence in deep  dermis where they remain inaccessible to superficial blade nicks for SSS samples)

•  WHO no longer considers SSS essential to make diagnosis of leprosy in field programs as reading requires expertise and skill

Procedure

• From ear lobes, eyebrows and active skin lesions (the number of sites vary- WHO says ideally 4 sites, minimum 3 sites-one ear lobe and two active lesions). The importance of smears from dorsa of fingers the diagnosis of long treated lepromatous cases and impending relapse too has been emphasized in many studies (MCQ) 

• Small superficial nicks with a sterile blade

• Stained and approximately 100 fields per smear examined

• Presence of acid-fast bacilli observed. They can be uniformly distributed (solid bacilli) or unevenly distributed (fragmented and granulated bacilli). Clumps of bacilli are called globi. 

• Solid bacilli = viable organisms 

• Fragmented/ Granular= Dead bacilli

Interpretation

1) Bacterial Index (BI) 

Note: 
1) Bacilli are abundant in lepromatous leprosy (BI 5+ or 6+), can not be demonstrated in tuberculoid leprosy, and may have intermediate counts in borderline leprosy patients

2) Over 99.9% live bacilli get killed from action of rifampicin (MCQ). Thereafter BI reflects only the presence of dead bacilli with occasional live ones. As the dead bacilli can be cleared from the body by natural mechanisms of the host, BI in skin smears start falling (See below) 


Use of BI: 

1) Useful in classifying leprosy within the Ridley and Jopling spectrum and between the two treatment groups, namely paucibacillary (PB) or skin smear negative leprosy and, multibacillary (MB) or skin smear positive leprosy (MCQ) . 

2) The monitoring of the response to treatment in MB patients is assisted by periodical, normally annual SSS (BI reduces by 1+ every year on treatment eg: If BI is 4+ to begin with , it becomes 3+ after 1 year of treatment. After this stage, BI reduces automatically by 1+ every year even without treatment.Thus, BI may not become at completion of treatment (MCQ))

3) Diagnosing relapse after completion of treatment - BI increases by 2+ from previous baseline value (MCQ))

4) If Initial BI before treatment ≥ 4+ , then instead of standard 1 year therapy, 2 year therapy is preferable (Extra edge point)


2) Morphological Index (MI)
If only the solid living bacilli are counted. It will become negative rapidly on treatment (within 5 weeks), as all living bacilli are killed (contrast this with BI)

Use of MI
More sensitive parameter of therapeutic failure, non compliance, drug resistance, or relapse (MCQ)