A 32-year-old woman presents with a painful, fluctuant swelling on her left forearm that appeared 5 days ago. She has fever (38.5°C) and mild leukocytosis. On examination, the lesion is warm, erythematous, and tender. Gram stain of aspirated pus shows gram-positive cocci in clusters. Which investigation is most appropriate to confirm the diagnosis and guide antibiotic therapy?

Explanation

## Investigation of Choice for Suspected S. aureus Skin Infection ### Why Culture + Biochemical Tests is the Gold Standard **Key Point:** Culture on blood agar followed by catalase and coagulase testing remains the gold standard for presumptive identification of *Staphylococcus aureus* in routine clinical microbiology laboratories. **High-Yield:** The diagnostic algorithm for gram-positive cocci in clusters is: 1. Culture on blood agar → beta-hemolytic colonies 2. Catalase test → positive (differentiates from *Streptococcus*) 3. Coagulase test → positive (confirms *S. aureus*) ### Advantages of Culture + Biochemical Testing | Feature | Benefit | |---------|----------| | **Cost-effective** | Affordable for resource-limited settings; suitable for India | | **Antibiotic susceptibility** | Allows simultaneous AST (VITEK, disc diffusion) to guide therapy | | **MRSA detection** | Oxacillin/cefoxitin disc or E-test identifies methicillin resistance | | **Routine availability** | Available in all hospital microbiology labs | | **Sensitivity & Specificity** | >95% for *S. aureus* identification | **Clinical Pearl:** In a community-acquired skin infection (furuncle, abscess) with gram-positive cocci in clusters, culture is mandatory because it simultaneously: - Confirms the organism - Identifies MRSA vs. MSSA (critical for empiric therapy in India, where MRSA prevalence is 30–50%) - Provides AST for beta-lactams, fluoroquinolones, and glycopeptides ### Why Other Options Are Suboptimal **PCR for mecA gene:** While specific for methicillin resistance, it does NOT confirm *S. aureus* presence and does NOT provide AST for other antibiotics. It is a supplementary test, not a primary diagnostic tool. **MALDI-TOF:** Rapid and accurate for species identification but requires prior culture isolation. It does NOT provide AST and is not a first-line investigation in routine practice. **16S rRNA sequencing:** Overkill for a presumptive diagnosis; reserved for atypical or fastidious organisms. It is expensive and not available in most Indian labs. [cite:Collee et al. Mackie & McCartney Practical Medical Microbiology 14e Ch 17]