Microcytic Anemia with Elevated RDW: Iron Deficiency Anemia
This CBC pattern—elevated RBC count (5.60), low MCV (78), low MCH (23.8), low MCHC (30.4), and elevated RDW (16.6)—is diagnostic of iron deficiency anemia, and you should immediately order serum ferritin and transferrin saturation to confirm the diagnosis before initiating iron supplementation. 1
Understanding the Laboratory Pattern
Your patient demonstrates the classic evolution of iron-deficient erythropoiesis:
The low MCH (23.8 pg) with elevated RDW (16.6%) is highly suggestive of iron deficiency, with 72% sensitivity for detecting iron deficiency even before frank anemia develops. 1 The bone marrow produces progressively smaller, more hypochromic red cells as iron stores deplete, creating a mixed population of normal older cells and microcytic newer cells—hence the elevated RDW. 1
The elevated RBC count (5.60) occurs commonly in iron deficiency anemia, particularly in mild-to-moderate cases, with frequencies reaching 61% in mild anemia. 2 This represents a compensatory marrow response attempting to maintain oxygen-carrying capacity despite reduced hemoglobin per cell. 2
The low MCHC (30.4) indicates hypochromia and represents a later stage of iron deficiency, typically occurring when hemoglobin falls below 9 g/dL and transferrin saturation drops below 16%. 3 Your patient's hemoglobin of 13.3 g/dL suggests earlier-stage deficiency. 3
Microcytosis (MCV < 80 fL) and anisocytosis (elevated RDW) are the first hematological abnormalities in iron deficiency, appearing before hemoglobin declines and when transferrin saturation falls below 32%. 3
Immediate Diagnostic Workup
Order these tests now to confirm iron deficiency and guide treatment:
Serum ferritin < 30 μg/L confirms iron deficiency in the absence of inflammation. 1 Ferritin is the single most useful marker but requires clinical context, as it rises falsely with inflammation, chronic disease, malignancy, or liver disease. 1
Transferrin saturation < 15-16% supports iron deficiency and is less affected by inflammation than ferritin. 1 This test helps differentiate true iron deficiency from anemia of chronic disease. 1
Measure C-reactive protein (CRP) to interpret ferritin values correctly, because ferritin rises as an acute-phase reactant during inflammation. 1 Without CRP, you risk missing iron deficiency when ferritin appears falsely normal or elevated. 1
Obtain a reticulocyte count to evaluate bone marrow response. 1 A low or inappropriately normal reticulocyte count in anemia indicates inadequate marrow response, consistent with iron deficiency. 1
Excluding Thalassemia Trait
While the elevated RBC count might suggest thalassemia trait, several features argue against it:
Thalassemia trait typically produces MCV ≤ 73.8 fL and MCH ≤ 21.9 pg with > 90% sensitivity and > 80% specificity. 4 Your patient's MCV of 78 fL and MCH of 23.8 pg fall above these thresholds. 4
The elevated RDW (16.6%) strongly favors iron deficiency over thalassemia trait. 5 Thalassemia trait characteristically shows low or normal RDW due to a homogeneous population of uniformly small cells. 6
If iron studies are normal, then order hemoglobin electrophoresis to evaluate for hemoglobinopathies, particularly in patients of Mediterranean, African, Middle Eastern, or Southeast Asian descent. 1
Investigating the Underlying Cause
Once iron deficiency is confirmed, you must identify the source:
All adult men and postmenopausal women with confirmed iron deficiency require endoscopic evaluation to exclude gastrointestinal malignancy, regardless of anemia severity. 1 Do not delay this evaluation even if hemoglobin appears "acceptable." 1
In premenopausal women, assess menstrual blood loss as the primary cause, but maintain a low threshold for GI evaluation if menstrual losses seem insufficient to explain the deficiency. 1
Evaluate for conditions impairing iron absorption: celiac disease (obtain tissue transglutaminase antibodies), inflammatory bowel disease, Helicobacter pylori infection, and prior gastric surgery. 1
Treatment Approach
After confirming iron deficiency with ferritin and transferrin saturation:
Initiate oral iron supplementation (e.g., ferrous sulfate 325 mg daily to three times daily) and monitor response. 7 A rise in hemoglobin after therapeutic iron supplementation confirms the diagnosis of iron-deficiency anemia. 7
Monitor iron status during treatment to assess response and detect potential iron overload. 1 Recheck CBC and ferritin after 4-6 weeks of therapy. 1
Continue iron supplementation for 3-6 months after hemoglobin normalizes to replenish iron stores. 1
Critical Pitfalls to Avoid
Do not empirically treat with iron based solely on CBC findings without confirming iron deficiency with ferritin and transferrin saturation. 6 Patients with thalassemia trait should never receive iron supplementation unless concurrent deficiency is documented. 1
Do not assume normal hemoglobin excludes significant iron deficiency. 1 Your patient's hemoglobin of 13.3 g/dL may appear "normal," but the low MCH and elevated RDW indicate evolving iron deficiency that will progress to overt anemia if untreated. 1
Mixed micro- and macrocytosis can neutralize the MCV, yielding a normal MCV but elevated RDW. 1 Always order iron studies even when MCV appears borderline or normal. 1
Race-specific considerations exist: the CDC notes that applying uniform anemia criteria results in higher false-positive rates for iron deficiency in Black patients. 7 Consider using additional iron status tests (ferritin, transferrin saturation) rather than relying solely on hemoglobin cutoffs. 7