Blood eosinophil count is now a treatment-selection biomarker in both asthma and COPD, and the commonest way it gets misread is timing. Systemic corticosteroids suppress the count within hours, so a sample drawn during or just after an exacerbation course records the treatment rather than the phenotype.
The fix is simple and rarely done: take the count in a stable outpatient interval, at least four weeks clear of any systemic steroid course, and write the date and the steroid status next to the number in the notes. A count of 90 cells per microlitre means one thing on day three of prednisolone and something entirely different eight weeks later.
Build the habit of collecting more than one. Counts vary within a patient over time, and the treatment decision is better served by two or three values across a year than by whichever one happens to be most recent - which, in practice, is usually the one taken in hospital.
- Draw the count when stable and at least four weeks clear of systemic steroids
- Write the date and steroid status beside every eosinophil count in the notes
- Collect two or three counts over a year before ruling a patient in or out of a biologic
- Do not use an inpatient count from an exacerbation admission to make the decision
- Remember parasitic infection and drug reactions as causes of a high count before calling it a phenotype
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