TAILS & FAT RBC
Anemia Causes by MCV
Two memory aids — TAILS for microcytic, FAT RBC for macrocytic.
Memory aids matter when a CBC drops on the desk and MCV alone has to narrow the differential. TAILS captures microcytic anemia (MCV under 80): Thalassemia, Anemia of chronic disease, Iron deficiency, Lead poisoning, Sideroblastic. FAT RBC captures macrocytic (MCV over 100): Folate, Alcohol, Thyroid (hypo), Reticulocytosis, B12, Cirrhosis (and drugs like methotrexate, hydroxyurea). Normocytic sits between. AANP exam vignettes give an MCV plus one history clue — vegan diet, methotrexate, recent GI bleed — and ask the cause. Iron studies, B12/folate, and reticulocyte count are the recurring next-step right answers; pair the right test to the right MCV bucket.
- TThalassemiaMICROCYTIC. Inherited hemoglobinopathy; low MCV with normal iron studies.
- AAnemia of chronic diseaseMICROCYTIC or normocytic. Inflammation-driven iron sequestration; ferritin is normal or high.
- IIron deficiencyMICROCYTIC. Most common worldwide; low ferritin, high TIBC, high RDW.
- LLead poisoningMICROCYTIC. Basophilic stippling on smear; risk in old homes, occupational exposure.
- SSideroblasticMICROCYTIC. Ringed sideroblasts on marrow; genetic or from alcohol/isoniazid/lead.
- FFolate deficiencyMACROCYTIC (megaloblastic). Alcoholics, pregnancy, phenytoin, methotrexate.
- AAlcoholismMACROCYTIC. Direct marrow toxicity independent of folate deficiency.
- TThyroid (hypo)MACROCYTIC. Hypothyroidism slows RBC maturation — check TSH on a macrocytic anemia workup.
- RReticulocytosisMACROCYTIC (non-megaloblastic). Young RBCs are larger; seen in hemolysis or recovery from blood loss.
- BB12 deficiencyMACROCYTIC (megaloblastic). Pernicious anemia, vegans, ileal disease (Crohn, resection). Can cause irreversible neuro damage.
- CCirrhosis / liver diseaseMACROCYTIC. Abnormal lipid metabolism alters RBC membranes.
Clinical Context
The MCV tells you where to look. <80 = microcytic → think TAILS. 80-100 = normocytic (acute blood loss, hemolysis, chronic disease). >100 = macrocytic → think FAT RBC.
Microcytic workup starts with ferritin: low ferritin = iron deficiency; normal/high ferritin with low iron saturation = anemia of chronic disease. Hemoglobin electrophoresis catches thalassemia.
Macrocytic workup distinguishes megaloblastic (B12, folate, drugs) from non-megaloblastic (alcohol, liver, hypothyroidism, reticulocytosis). Check B12 and folate; supplement what's low. CRITICAL: macrocytic anemia with neuro symptoms is B12 deficiency until proven otherwise — giving folate alone can mask the blood changes without fixing (or even worsening) the neurologic damage.
Sources
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