CBC finding of scarlet fever
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Good hits. Let me pull the Quick Compendium of Clinical Pathology section (it's specifically CBC/lab-focused) and Andrews'/Fitzpatrick's clinical section for full detail.
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scarlet fever CBC findings leukocytosis eosinophilia
CBC Findings in Scarlet Fever
White blood cell count (leukocytosis):
- WBC typically elevated to 12,000-16,000/µL, with a differential showing up to 95% polymorphonuclear leukocytes (neutrophilia) early in the illness - reflecting the underlying group A streptococcal (Streptococcus pyogenes) infection.
Eosinophilia:
- During the second week of illness, eosinophilia develops and can reach as high as 20% of the differential count. This is a relatively distinctive feature of scarlet fever compared with other exanthems and is sometimes used as a supportive (though nonspecific) diagnostic clue - Quick Compendium of Clinical Pathology; Henry's Clinical Diagnosis and Management by Laboratory Methods (lists scarlet fever among nonparasitic infectious causes of eosinophilia).
Other supportive labs (not strictly CBC, but often obtained together):
- Elevated ESR and CRP (acute inflammatory response)
- Positive throat culture/rapid antigen detection test for group A strep
- Elevated antistreptolysin O (ASO) or anti-DNase B titers if cultures were not obtained early, confirming recent streptococcal infection - Andrews' Diseases of the Skin.
Summary pattern
| Timing | Finding |
|---|---|
| Early/acute | Leukocytosis (12,000-16,000/µL) with neutrophilia (up to 95% PMNs) |
| Second week | Eosinophilia (up to 20%) |
| Adjunct | Raised ESR/CRP, positive strep culture/RADT, rising ASO/anti-DNase B |
This pattern - neutrophilic leukocytosis early, followed by eosinophilia later in the illness - reflects the acute pyogenic streptococcal infection followed by a delayed hypersensitivity-type response to the erythrogenic (pyrogenic) exotoxin produced by S. pyogenes.
A quick recent-evidence check (PubMed, meta-analyses/systematic reviews, last 2 years) did not surface any studies that contradict or update this classic lab pattern - it remains a textbook-consistent finding without recent controversy.
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