Pencil Cells and Teardrop Cells with Microcytic Hypochromic Anemia: Primary Causes
The presence of pencil cells (elliptocytes) and teardrop cells (dacrocytes) in a peripheral smear with microcytic hypochromic anemia most commonly indicates iron deficiency anemia (IDA), though thalassemia trait and bone marrow infiltrative disorders must be considered in the differential diagnosis.
Understanding the Morphologic Findings
Pencil-shaped cells (elongated elliptocytes) are characteristic of severe iron deficiency, representing red blood cells that have become deformed due to inadequate hemoglobin content. Teardrop cells, while classically associated with myelofibrosis and bone marrow infiltration, are also commonly found in hemolytic anemias 1. Research demonstrates that dacrocytes appear in 89% of autoimmune hemolytic anemia cases and 91% of microangiopathic hemolytic anemia cases, though these typically present as normocytic rather than microcytic anemia 1.
Diagnostic Algorithm for Microcytic Hypochromic Anemia
Step 1: Assess Iron Studies
- Serum ferritin <15 μg/L: Highly specific for iron deficiency (specificity 0.99) 2
- Serum ferritin <45 μg/L: Reasonable cutoff with 92% specificity, especially with chronic inflammation 2
- Serum ferritin >150 μg/L: Essentially excludes absolute iron deficiency even with inflammation 2
Step 2: Differentiate IDA from Thalassemia Trait
When iron studies are equivocal or normal with persistent microcytosis:
- MCV disproportionately low (often <70 fL) relative to degree of anemia → suspect thalassemia 2
- Mentzer Index (MCV/RBC count): <13 suggests thalassemia trait with 100% sensitivity and 69.4% specificity 3
- Green and King Index: 90% sensitivity and 82% specificity for distinguishing IDA from thalassemia 4
- RDW Index: 90% sensitivity and 77% specificity 4
Hemoglobin electrophoresis is mandatory in patients with microcytosis and normal iron studies, particularly those of Mediterranean, African, Middle Eastern, or Southeast Asian descent 2.
Step 3: Consider Genetic Disorders of Iron Metabolism
If ferritin is elevated or transferrin saturation is abnormal despite microcytic anemia:
- Elevated ferritin + elevated transferrin saturation: Consider genetic iron metabolism disorders (TMPRSS6, SLC11A2 defects) 5
- Low transferrin saturation + low-normal ferritin (>20 mg/L): Suggests genetic heme synthesis disorders 5
- Sideroblastic anemia: Consider SLC25A38, STEAP3, ALAS2 defects, particularly in children with severe microcytic anemia 5
Critical Pitfalls to Avoid
Do not dismiss microcytosis with "normal" ferritin in inflammatory states: Use the 45 μg/L cutoff rather than laboratory reference ranges 2
Do not assume all microcytic anemia is iron deficiency: Thalassemia trait affects 13.5% of children with microcytic anemia, and 7% have both conditions simultaneously 3
Do not overlook bone marrow pathology: While teardrop cells classically indicate myelofibrosis, this typically presents with leukoerythroblastic picture and splenomegaly, not isolated microcytic anemia
Therapeutic trial confirms diagnosis: A hemoglobin rise ≥10 g/L within 2 weeks of iron therapy strongly confirms absolute iron deficiency, even with equivocal iron studies 2
Most Likely Diagnosis
In clinical practice, iron deficiency anemia accounts for 77% of microcytic anemia cases 3, making it the overwhelming favorite when pencil cells and teardrop cells are present together. The combination of these morphologic findings with microcytic hypochromic indices points to severe, prolonged iron deficiency affecting red cell membrane integrity and hemoglobinization.
Measure serum ferritin immediately. If <45 μg/L, initiate iron replacement and monitor response. If ferritin is normal or elevated, proceed with hemoglobin electrophoresis to exclude thalassemia trait before considering rarer genetic disorders of iron metabolism 5, 2.