A 48-hour-old term neonate born to a mother with prolonged rupture of membranes (PROM >18 hours) presents with fever (38.5°C), poor feeding, and lethargy. Clinical examination reveals no focal signs of infection. Which investigation is most appropriate to establish the diagnosis of neonatal sepsis?

## Diagnostic Approach to Neonatal Sepsis **Key Point:** Blood culture remains the gold standard and most specific investigation for confirming bacterial sepsis in neonates, as it identifies the causative organism and allows antimicrobial susceptibility testing. ### Why Blood Culture + CBC? **High-Yield:** Blood culture is the ONLY investigation that definitively confirms bacteremia and identifies the pathogen. In neonatal sepsis: - Sensitivity: ~80–90% if drawn before antibiotics - Specificity: ~99% (positive culture = true infection, not contamination, if proper technique used) - Allows targeted therapy based on organism and sensitivities **Clinical Pearl:** CBC with differential aids risk stratification: - Immature-to-total neutrophil ratio (I:T ratio) >0.2 suggests infection - Thrombocytopenia (<100,000/μL) is a marker of severe sepsis - However, CBC alone is NOT diagnostic — it has poor sensitivity and specificity ### Investigation Hierarchy in Neonatal Sepsis | Investigation | Role | Diagnostic Value | |---|---|---| | **Blood culture** | Gold standard; identifies organism | Definitive (if positive) | | CBC with differential | Adjunctive; risk assessment | Supportive only | | Blood glucose, lactate | Metabolic markers | Prognostic, not diagnostic | | CRP, procalcitonin | Inflammatory markers | Adjunctive; poor specificity | | CSF culture | If meningitis suspected | Gold standard for meningitis | | Urine culture | If UTI suspected (>72 hrs) | Not first-line in early sepsis | **Mnemonic: BCCR** — **B**lood culture first, **C**omplete blood count adjunctive, **C**erebrospinal fluid if meningitis suspected, **R**epeat cultures if clinical deterioration. ### Timing and Technique 1. Draw blood culture BEFORE starting antibiotics (if clinically safe) 2. Use aseptic technique; clean skin with chlorhexidine or povidone-iodine 3. Minimum 1 mL blood in culture bottle (neonatal bottles preferred) 4. Simultaneously send CBC, blood glucose, lactate **Warning:** Do NOT delay antibiotics waiting for culture results in a septic neonate — empiric broad-spectrum therapy (ampicillin + gentamicin ± cefotaxime) is started immediately after cultures are drawn.