The anemia differential is organized along three axes that answer separate questions. The kinetic axis asks whether the marrow is responding appropriately: a high reticulocyte count means the marrow is producing and the problem is loss or destruction, while a low count means production is inadequate. The morphologic axis asks what the red cells look like, using the mean corpuscular volume (MCV) to sort anemia into microcytic, normocytic, and macrocytic categories. The etiologic axis asks what the underlying cause is, such as iron deficiency, chronic kidney disease, or a hemoglobinopathy. The axes are complementary rather than competing: a single patient is described on all three at once. A patient with a high reticulocyte count and a normocytic MCV is being described kinetically as increased production and morphologically as normocytic, and the etiologic axis then asks which hemolytic process is responsible.
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Three Axes, Three Different Questions
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Look at the three columns. The kinetic axis asks whether the marrow is responding, and the reticulocyte count answers it. The morphologic axis asks what the red cells look like, and the MCV answers it. The etiologic axis asks what the underlying cause is, and that is answered by the full workup. Notice that the same patient appears in all three columns at once. A patient with a high reticulocyte count and a normocytic MCV is not three different patients; that is one patient described on three axes. The axes are not competing systems you choose between. They are three questions you ask about the same person.
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