Absolute Eosinophil Count
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Direct answer
One cubic millimetre of blood normally carries only 40-440 eosinophils, and counting them directly — rather than extrapolating from a differential — is the absolute eosinophil count (AEC). The manual method dilutes EDTA blood in an eosin-based diluent (commonly phloxine or eosin in propylene glycol or saline) that haemolyses red cells while staining eosinophil granules brilliant red; the stained cells are counted in a Fuchs-Rosenthal or improved Neubauer counting chamber and converted to cells per cubic millimetre by chamber arithmetic. Today most laboratories derive AEC from automated analysers, but the manual count remains the exam-standard method and the backup for instrument flags. An AEC above roughly 440 per cubic millimetre signals atopy, drug reaction or — the Indian classic — helminthic or filarial infection.
What you must remember
- Method: direct chamber count; diluent lyses red cells and stains eosinophil granules; Fuchs-Rosenthal (preferred, larger volume) or improved Neubauer chamber; count promptly before granule staining fades.
- Calculation: cells counted, corrected for area counted, depth and dilution factor — with the standard dilution, multiply cells in the counted squares by the dilution and divide by the volume counted, reporting per microlitre (cubic millimetre).
- Normal range: 40-440 per cubic millimetre (0.04-0.44 x 10^9/L); diurnal variation is real — counts are lowest in the morning and rise towards midnight, mirroring cortisol rhythm.
- Physiological rise: allergic disorders (asthma, hay fever, atopic dermatitis) commonly AEC 500-1,000; parasitic infestations higher, and tissue-invasive parasites (filaria, Strongyloides, toxocariasis, schistosomiasis) drive the highest counts.
- Drug causes: sulphonamides, penicillins, NSAIDs, anticonvulsants — a eosinophilia appearing after a new drug is a stop-the-drug clue.
- High-interest diseases: Loffler syndrome (pulmonary eosinophilia with migrating infiltrates), hypereosinophilic syndrome (AEC persistently above 1,500 with organ damage), eosinophilic leukaemia; Cushing syndrome and acute bacterial infections suppress eosinophils.
- Steroid logic: corticosteroids and ACTH lower the AEC — Cushing syndrome and acute bacterial infection both show eosinopenia, and steroids can mask parasitic eosinophilia.
Doing the count at the bench
Take 20 microlitres of well-mixed EDTA blood into the eosin diluent to the marked line (a 1:10 dilution in most set-ups), mix gently for the recommended half-minute or so, and charge the chamber without bubbles. Under low power with the condenser lowered, eosinophils shine as refractile red-granulated cells against a clean background — nothing else should be counting. Count all stained cells in the ruled area (both sides of the chamber, average them), then apply the arithmetic: cells x dilution factor / volume of area counted. The Fuchs-Rosenthal chamber's larger volume changes the multiplication — mixing up the two chambers' formulae is a classic numerical error.
Interpretation follows thresholds: 440 is the upper fence; 500-1,000 points to allergy or enteric-helminth colonisation; beyond 1,000-1,500, especially with weight loss, cough or urticaria, chase tissue-invasive parasites — in endemic Indian districts, filariasis serology or a night blood film for microfilariae comes first. A persistently high AEC with no parasitological explanation deserves a peripheral smear for blast cells and, if sustained above 1,500 with organ signs, referral for hypereosinophilic syndrome evaluation.
The examiner's angle
Questions cluster on three points. Why a direct count rather than percentage-derived: the differential percentage is compressed by relative neutrophilia, so an "eosinophils 3 per cent" in a patient with a high total leucocyte count may hide a frankly raised AEC — direct counting or analyser multiplication is the only honest answer. Which chamber: the Fuchs-Rosenthal, deeper and larger-volumed, was designed for low-frequency cells; using Neubauer total-leucocyte arithmetic on it produces a wrong answer by a factor. And the diurnal question: because eosinophils follow the inverse of the cortisol curve, an 8 am and an 11 pm sample from one patient can look like two patients, so serial counts are compared at the same hour.
Frequently asked questions
What is the normal range of the absolute eosinophil count?
40-440 eosinophils per cubic millimetre of blood (0.04-0.44 x 10^9 per litre) in adults, with physiological diurnal variation from morning lows toward midnight highs.
Which diluent and chamber are used for the manual AEC?
An eosin-based diluent (phloxine or eosin in propylene glycol or saline) that haemolyses red cells and stains granules, counted in a Fuchs-Rosenthal or improved Neubauer chamber.
Why is a direct AEC preferred over a percentage from the differential?
Percentages are distorted by changes in other cell lines; a direct count (or analyser-derived absolute value) reflects true eosinophil mass regardless of neutrophil or lymphocyte shifts.
What clinical conditions commonly raise the AEC in India?
Allergic asthma and atopy, helminthic and filarial infections (the commonest Indian context), drug reactions, Loffler syndrome and, at extreme values, hypereosinophilic syndrome.
Why can a morning and midnight AEC differ in the same patient?
Eosinophil counts follow the inverse of the cortisol rhythm — lowest in early morning, highest around midnight — so serial counts should be drawn at the same time of day.