## Investigation of Choice in Impetigo **Key Point:** Culture on blood agar with antibiotic sensitivity testing is the gold standard for confirming bacterial impetigo and determining antibiotic susceptibility, particularly to detect methicillin-resistant *Staphylococcus aureus* (MRSA). ### Why Culture is Essential 1. **Organism Identification**: Gram stain suggests gram-positive cocci but cannot definitively differentiate *S. aureus* from *Streptococcus pyogenes* (Group A Streptococcus). 2. **Antibiotic Susceptibility**: Determines resistance patterns (MRSA vs. MSSA), which guides therapy selection. 3. **Clinical Relevance**: In India, MRSA prevalence in community-acquired impetigo is rising; culture prevents empiric failures. ### Specimen Collection - **Swab the lesion** under the crust (not the crust itself). - Use sterile cotton or calcium alginate swab. - Send in sterile tube without transport medium (culture is time-sensitive). ### Culture Media & Interpretation | Medium | Growth Pattern | Identification | |--------|---|---| | Blood agar | β-hemolytic (S. aureus) or non-hemolytic (S. pyogenes) | Catalase test, coagulase test, CAMP test | | Mannitol salt agar | Yellow colonies (S. aureus) | Selective for Staph | | Gram stain | Gram-positive cocci in clusters | Presumptive Staph | **High-Yield:** Coagulase-positive, catalase-positive gram-positive cocci = *Staphylococcus aureus*. Coagulase-negative = *Streptococcus pyogenes* or other Streptococci. **Clinical Pearl:** In non-bullous impetigo (most common form), *S. aureus* is now the predominant pathogen in many regions, including India, replacing *S. pyogenes*. Culture guides whether to add anti-staphylococcal coverage. ### Timing - Results available in **24–48 hours**. - Sensitivities available in **48–72 hours**. - Empiric therapy should be started immediately while awaiting culture. [cite:Irvine's Dermatology Ch 14] 
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