The four data sources are applied in a fixed order because each one narrows the next. The reticulocyte count is read first and sets the kinetic branch: a low count means the marrow is not responding, so the problem lies in production; a high count means the marrow is responding, so the problem lies in loss or destruction. The MCV is read second and sets the morphologic category inside that branch, and the RDW states whether the red cell population is uniform or mixed. The smear is read third and must agree with the category while also supplying the specific feature that names a cause. The confirmatory test is chosen last, from the working category.
Consider a patient with hemoglobin 8.9 g/dL, MCV 71 fL, RDW 19%, and a reticulocyte count of 0.7%. The low reticulocyte count places the patient in the hypoproliferative branch, so hemolysis and acute blood loss are set aside. The MCV of 71 fL places the patient in the microcytic category, and the RDW of 19% indicates a heterogeneous population rather than a uniform one. Within the hypoproliferative microcytic category the two leading causes are iron deficiency and thalassemia trait. A high RDW with marked anisocytosis and pencil cells on the smear favors iron deficiency, while a normal RDW with prominent target cells favors thalassemia trait. Here the high RDW and the smear favor iron deficiency, so the confirmatory test is an iron studies panel. If the RDW had been normal with target cells, the confirmatory test would instead have been hemoglobin electrophoresis.
Changing any single input changes the branch and therefore the test. If the reticulocyte count were 6% instead of 0.7%, the patient would move to the loss or destruction branch, the microcytic category would become less central, and the confirmatory test would become a hemolysis panel rather than iron studies. The algorithm is not a checklist to be completed; it is a sequence in which the earliest result decides which later results are even relevant.