An 8-week-old infant has 2 weeks of worsening paroxysmal cough followed by a whoop and post-tussive vomiting. Total leukocyte count is 28,000/mm3 with 75% small mature lymphocytes. Chest X-ray is normal. Which laboratory finding is most characteristic of this disease?

• A Absolute lymphocytosis caused by pertussis toxin ✓
• B Atypical lymphocytosis with positive Monospot test
• C Neutrophil predominance with left shift
• D Eosinophilia exceeding 20% of the differential count

Explanation

Bordetella pertussis produces pertussis toxin, which causes a striking absolute lymphocytosis, often above 20,000/mm3, out of proportion to the mild clinical findings. This distinguishes it from viral illnesses that produce atypical lymphocytes. The infant is in the paroxysmal stage, when culture yield on Bordet-Gengou agar is already low, so serology or PCR is preferred.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.