Haemolytic anaemia: diagnostic approach: Viva

Opening question

What is your diagnostic approach to this child, and what single test do you most need next? ^[2]

This child has haemolytic anaemia: the falling haemoglobin with a reticulocyte count of 11 per cent, undetectable haptoglobin, raised lactate dehydrogenase and raised unconjugated bilirubin confirm red cell destruction outstripping marrow production. The spherocytes on the film narrow the differential to warm autoimmune haemolytic anaemia or hereditary spherocytosis. The single test I most need is the direct antiglobulin test, because it is the master branch point: a positive result proves an immune process and points to warm autoimmune haemolytic anaemia, while a negative result redirects me to the inherited membrane disorders. ^[2]