Numerous techniques are used to diagnose hypereosinophilic syndrome, of which the most important is blood testing. In HES, the eosinophil count is greater than 1.5 × 109/L. On some smears the eosinophils may appear normal in appearance, but morphologic abnormalities, such as a lowering of granule numbers and size, can be observed. Roughly 50% of patients with HES also have anaemia.
