Neutropenia and neutrophil disorders: Viva

Branch 1: The first move and the definition

The candidate should immediately reframe the picture: an incidental isolated neutropenia in a well infant two weeks after a viral illness, most likely a benign transient post-viral dip, but to be confirmed. The first move is to repeat the full blood count on a fresh sample and to examine the peripheral film, because ethylenediaminetetraacetic-acid clumping and transient post-viral dips are common, and a single low count is never a diagnosis. The candidate should state the thresholds aloud: neutropenia is an absolute neutrophil count under 1.5 times ten to the ninth per litre, graded as mild 1.0 to 1.5, moderate 0.5 to 1.0, and severe under 0.5, with the absolute neutrophil count calculated as the white cell count times the percentage of segmented neutrophils plus bands divided by one hundred. ^[1]

The examiner will probe why the count is calculated and why the bands matter. The candidate should explain that the circulating pool is the fraction sampled by a full blood count, that the marginated pool is roughly equal in size, and that including the band cells in the absolute neutrophil count matters because they are the immediate neutrophil reserve. The examiner may ask what a low count in a child of African ancestry means. The candidate should name benign ethnic neutropenia, caused by the Duffy-null regulatory variant in the ACKR1 gene, with a normal marginated pool and normal function and no infection excess, and stress that recognising it prevents over-investigation. ^[11]