A 3-week-old exclusively breastfed infant from rural India presents with failure to thrive, poor latch, and the mother reports severe breast pain during feeding. Examination shows a tender, erythematous, localized swelling on the left breast with purulent discharge from the nipple. What is the most appropriate investigation to identify the causative organism?

## Diagnosis: Puerperal Mastitis with Possible Abscess ### Clinical Presentation The combination of localized erythema, tenderness, purulent discharge, and systemic signs in a postpartum mother suggests bacterial mastitis, likely progressing to abscess formation. The causative organisms are typically skin commensals (Staphylococcus aureus, Streptococcus agalactis) or, in resource-limited settings, gram-negative organisms. ### Investigation of Choice **Key Point:** Culture and sensitivity of expressed breast milk is the gold standard for identifying the causative organism and guiding antibiotic therapy in mastitis. **High-Yield:** Breast milk culture provides: 1. **Organism identification** — essential for targeted antibiotic therapy 2. **Antibiotic susceptibility** — critical in the era of MRSA and other resistant pathogens 3. **Confirmation of infection** — differentiates true infection from contamination (>10^5 CFU/mL is diagnostic) ### Diagnostic Approach for Mastitis | Investigation | Indication | Sensitivity | Specificity | Utility | |---|---|---|---|---| | **Breast milk culture & sensitivity** | Confirmed/suspected mastitis | High | High | **Gold standard** — identifies organism & resistance | | **Ultrasound** | Assess for abscess formation | High | High | **Adjunct** — guides drainage if needed | | **Mammography** | Chronic/recurrent mastitis, rule out malignancy | Moderate | Moderate | Not for acute infection; contraindicated in lactation | | **Serology for TB** | Granulomatous mastitis (rare) | Variable | Variable | Only if clinical suspicion for TB mastitis (chronic, non-responsive) | **Clinical Pearl:** In acute bacterial mastitis, culture is obtained by expressing milk into a sterile container after cleaning the nipple with 70% alcohol and discarding the first few drops. A count >10^5 CFU/mL confirms infection. **Mnemonic:** **MASTITIS** = **M**ilk culture **A**nd sensitivity, **S**taphylococcus most common, **T**argeted therapy, **I**dentify organism, **T**reat empirically pending results, **I**nfection confirmed, **S**upport breastfeeding. ### Management Algorithm ```mermaid flowchart TD A[Puerperal Mastitis]:::outcome --> B{Purulent discharge or signs of abscess?}:::decision B -->|No| C[Empiric antibiotics + supportive care]:::action B -->|Yes| D[Obtain breast milk culture & sensitivity]:::action D --> E[Ultrasound to assess for abscess]:::action E --> F{Abscess present?}:::decision F -->|No| G[Antibiotics based on culture results]:::action F -->|Yes| H[Percutaneous drainage + antibiotics]:::urgent C --> I[Continue breastfeeding]:::action G --> I H --> I ``` **Tip:** Do NOT discontinue breastfeeding; breast milk culture itself is safe for the infant, and continuing feeds helps drain the breast and prevents abscess formation.