Anemia And RBC Disorders

Classifying Anemia By MCV

Anemia is a reduction in hemoglobin, hematocrit, or red cell mass below the reference interval. The MLS exam expects the MCV-based morphologic classification as the first triage step:

MCV = (Hct x 10) / RBC. MCH = (Hgb x 10) / RBC. MCHC = (Hgb x 100) / Hct. Be ready to calculate any of these from a given CBC. A worked index example: with Hgb 12 g/dL, Hct 36%, and RBC 4.0 x10^12/L, MCV = (36 x 10)/4.0 = 90 fL, MCH = (12 x 10)/4.0 = 30 pg, and MCHC = (12 x 100)/36 = 33.3 g/dL - all normal, a normocytic normochromic picture. Memorize the units that go with each index (fL, pg, g/dL) because the exam will list distractor answers with the wrong magnitude.

Beyond MCV, the reticulocyte count is the second triage axis. A high reticulocyte response in an anemic patient says the marrow is working and the problem is loss or destruction (hemorrhage or hemolysis). A low or inappropriately normal reticulocyte count says the marrow is the problem (deficiency, aplasia, infiltration). The corrected reticulocyte count = reticulocyte% x (patient Hct / 45) adjusts for the degree of anemia, and the reticulocyte production index further corrects for premature release of shift reticulocytes. Pairing MCV with the reticulocyte response sorts most anemia stems before you ever read the special tests.

Microcytic Anemias And Iron Studies

The most testable distinction is iron deficiency anemia (IDA) versus anemia of chronic disease (ACD) versus thalassemia trait:

A classic exam discriminator: thalassemia trait shows a normal or elevated RBC count with low MCV (the Mentzer index, MCV/RBC, is < 13), whereas IDA shows a low RBC count and Mentzer index > 13. RDW rises early and high in IDA but stays normal in thalassemia trait.

Iron deficiency progresses in stages, and the exam may test the order in which tests turn abnormal: ferritin falls first (storage iron depletes), then serum iron drops and TIBC rises, then microcytosis and hypochromia appear, and the RDW widens as the population becomes heterogeneous. Ferritin is the single most useful test for iron stores but is also an acute-phase reactant, so in an inflamed patient a normal-appearing ferritin can mask true deficiency.

Beta-thalassemia minor classically shows an elevated hemoglobin A2 (>3.5%) on electrophoresis, the confirmatory test, whereas alpha-thalassemia trait has a normal A2 and is diagnosed by exclusion or DNA analysis. Sideroblastic anemia is a third microcytic cause: iron is present but cannot be incorporated into heme, producing ring sideroblasts (iron-laden mitochondria around the nucleus) on a Prussian blue marrow stain, with high serum iron and high ferritin - the opposite iron pattern from IDA.

Macrocytic / Megaloblastic Anemias

Megaloblastic anemia results from impaired DNA synthesis, usually vitamin B12 (cobalamin) or folate deficiency. Hallmarks: MCV often > 110 fL, hypersegmented neutrophils (>5 lobes), oval macrocytes, and pancytopenia in severe cases. B12 deficiency adds neurologic signs (folate does not). Pernicious anemia is B12 deficiency from loss of intrinsic factor; the Schilling test (historical) and intrinsic-factor antibodies confirm it.

Hemolytic Anemias

All hemolytic anemias share a laboratory signature: increased reticulocytes, increased LDH, increased indirect (unconjugated) bilirubin, and decreased haptoglobin. Distinguish by morphology and special tests:

• Hereditary spherocytosis: spherocytes, increased MCHC, positive osmotic fragility, negative DAT.
• G6PD deficiency: bite cells and Heinz bodies after oxidant stress; X-linked.
• Sickle cell anemia: drepanocytes, target cells; confirmed by hemoglobin electrophoresis showing HbS (and screened by solubility test).
• Autoimmune hemolytic anemia: spherocytes with a positive direct antiglobulin test (DAT/Coombs) - the feature that separates it from hereditary spherocytosis.
• Microangiopathic (TTP, DIC, HUS): schistocytes (helmet cells, fragments).
• Paroxysmal nocturnal hemoglobinuria (PNH): complement-mediated intravascular hemolysis from loss of CD55/CD59; flow cytometry confirms.

A second useful split is intravascular versus extravascular hemolysis. Intravascular destruction (PNH, transfusion reactions, microangiopathy, severe G6PD crisis) releases free hemoglobin into plasma, producing hemoglobinemia, hemoglobinuria, and hemosiderinuria and a sharply low haptoglobin. Extravascular destruction (hereditary spherocytosis, warm autoimmune hemolysis) removes cells in the spleen and liver, producing splenomegaly and indirect hyperbilirubinemia without gross hemoglobinuria.

The exam often hides the answer in this distinction - dark urine and a plasma free-hemoglobin clue point intravascular, while splenomegaly and spherocytes point extravascular.

Worked Example

A patient has Hgb 9 g/dL, MCV 72 fL, ferritin 6 ng/mL, TIBC elevated, and RDW 18%. Low ferritin is the most specific marker of depleted iron stores, so this is iron deficiency anemia, not chronic disease (which would show high ferritin) or thalassemia (normal RDW, normal RBC count).

Common Traps

• Calling every microcytic anemia iron deficiency - check ferritin and RBC count to exclude thalassemia.
• Forgetting that a high reticulocyte count can push MCV into the macrocytic range without B12/folate deficiency.
• Confusing DAT-positive (autoimmune) with DAT-negative (hereditary) spherocytosis.