A 6-month-old infant from rural India presents with a 3-week history of paroxysmal cough with inspiratory stridor (whooping sound), post-tussive vomiting, and apnoeic spells. Vaccination status is unknown. Which investigation is most appropriate to confirm the diagnosis?

## Diagnostic Approach to Pertussis **Key Point:** Culture of Bordetella pertussis on selective media (Bordet–Gengou or Regan–Lowe medium) remains the gold standard for diagnosis, especially in the catarrhal and early paroxysmal stages (first 2–3 weeks of illness). ### Specimen Collection & Culture - **Specimen:** Nasopharyngeal swab (not throat swab) — the organism colonizes the nasopharynx - **Media:** Bordet–Gengou (potato-glycerol-blood agar) or Regan–Lowe (charcoal-blood agar) - **Sensitivity:** 40–60% in catarrhal stage; decreases to <5% after 4 weeks - **Specificity:** ~100% (culture is diagnostic) - **Timing:** This patient is at 3 weeks — still within the window for culture positivity ### Why Culture is Preferred Here | Investigation | Sensitivity | Specificity | Timing | Use | | --- | --- | --- | --- | --- | | **Culture** | 40–60% (catarrhal/early paroxysmal) | 100% | First 3 weeks | Gold standard; definitive | | **PCR** | 85–95% | 95–100% | Any stage | Faster, most sensitive; not always available | | **Serology (IgM/IgG)** | 60–90% | 90–95% | After 2–3 weeks | Late diagnosis; not useful acutely | | **CXR** | N/A | N/A | Any stage | Supportive; shows hyperinflation, atelectasis | | **CBC** | N/A | N/A | Any stage | Shows lymphocytosis; non-specific | **High-Yield:** In the first 3 weeks of illness, culture is the investigation of choice because sensitivity is still reasonable and specificity is absolute. Serology becomes useful only after 3 weeks when antibodies have developed. **Clinical Pearl:** The "whooping" sound and post-tussive vomiting are classic for pertussis, but these are clinical signs, not diagnostic. Confirmation requires microbiological or molecular evidence. ## Why Other Options Are Suboptimal - **Serology (IgM/IgG):** At 3 weeks, antibodies may be present, but serology is less specific than culture and is typically used for late-stage or epidemiological confirmation. It is not the first-line investigation in the paroxysmal stage. - **Chest X-ray:** Supportive imaging (hyperinflation, perihilar infiltrates, atelectasis) but not diagnostic. Used to rule out complications or alternative diagnoses. - **Complete blood count:** Lymphocytosis (often >15,000/µL) is suggestive but non-specific. Many viral infections cause lymphocytosis. **Mnemonic:** **BNPC** — **B**ordet–Gengou / **N**asopharyngeal swab / **P**aroxysmal stage / **C**ulture = gold standard.