What it is
The absolute eosinophil count (AEC) is the actual number of eosinophils per microlitre of blood — a far more reliable figure than the eosinophil percentage alone, because a percentage moves with the total white count. Eosinophilia is graded, and treatment urgency tracked, off the absolute number, not the percent.
Formula
$$ AEC,(/\mu L) = WBC,(\times10^9/L) \times \frac{eos,%}{100} \times 1000 $$
The x1000 converts the WBC from x10⁹/L (thousands per µL) into cells per µL. (Equivalently: WBC in cells/µL x eos%.)
Grading
- < 500 /µL → normal
- 500–1499 /µL → mild eosinophilia
- 1500–4999 /µL → moderate eosinophilia
- >= 5000 /µL → severe eosinophilia
Hypereosinophilia is defined as >= 1500 /µL on two occasions (>= ~1 month apart) or significant tissue eosinophilia.
When to use
Any time the differential flags eosinophils and you need to know whether it is trivial atopy or a count that demands a work-up. Converting the percentage to an absolute count is the first triage step before chasing causes.
Worked example
WBC 12 x10⁹/L, eosinophils 8%:
- AEC = 12 x (8 / 100) x 1000 = 960 /µL
960/µL falls in the mild eosinophilia (500–1499/µL) band — consistent with common benign causes; reassess with the clinical picture rather than launching an exhaustive work-up.
Pitfalls
- Percentage hides the truth. A high eosinophil percent with a low total WBC can give a normal absolute count — and vice versa. Always grade off the absolute number.
- Common paediatric causes first. Atopy (eczema, asthma, allergic rhinitis), parasitic/helminth infection, and drug reactions dominate; in endemic regions parasites are a leading driver. Don’t jump to rare clonal disease for a mild count.
- Persistent moderate-to-severe elevation (especially the >= 1500/µL hypereosinophilia threshold sustained over two readings) warrants a structured work-up for end-organ damage (cardiac, pulmonary, GI, neurological) and consideration of clonal/idiopathic hypereosinophilic syndrome.
- Steroids and diurnal variation transiently suppress eosinophils — a single normal count under steroids does not exclude eosinophilia.
- Tissue eosinophilia can exist with a normal blood count — a normal AEC does not rule out an eosinophilic organ process.
Run it: Absolute Eosinophil Count (AEC)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.