RBC Count in Iron Deficiency Anemia
In iron deficiency anemia with an MCV of 73 fL and hematocrit of 0.39, the RBC count is typically elevated or normal-to-high (often >4.5-5.0 × 10¹²/L), creating a characteristic pattern where microcytic cells are produced in increased numbers to compensate for their reduced hemoglobin content.
Understanding the RBC Response in Iron Deficiency
The bone marrow responds to iron deficiency by producing more red blood cells, but each cell contains less hemoglobin and is smaller in size. This compensatory mechanism results in:
- Increased or preserved RBC count despite low hemoglobin and hematocrit, as the marrow attempts to maintain oxygen-carrying capacity through quantity rather than quality 1
- Disproportionate microcytosis where the MCV drops more dramatically than the RBC count rises, creating the characteristic pattern 2
- Progressive changes where anisocytosis and microcytosis appear before the RBC count significantly changes 3
Expected Laboratory Pattern
In this specific case (Hct 0.39, MCV 73 fL):
- RBC count likely 4.5-5.5 × 10¹²/L or higher: The mathematical relationship between hematocrit, MCV, and RBC count (Hct = RBC × MCV ÷ 10) suggests RBC = 39 ÷ 7.3 = approximately 5.3 × 10¹²/L 4
- This elevated RBC count with low MCV distinguishes iron deficiency from thalassemia trait, where RBC counts are typically even higher (>5.5 × 10¹²/L) with similar or lower MCV values 2
- The MCV/RBC ratio >14 supports iron deficiency rather than thalassemia trait, which shows ratios <14 2
Diagnostic Implications
The RBC count provides critical discriminatory information:
- Iron deficiency typically shows: Low MCV (73 fL), normal-to-elevated RBC count (>4.5 × 10¹²/L), elevated RDW (>14%), and low MCH 1, 3
- Thalassemia trait shows: Similar or lower MCV, but disproportionately higher RBC count (often >5.5-6.0 × 10¹²/L), normal RDW (≤14%), and target cells on smear 1
- The RBC count elevation reflects the marrow's compensatory response before severe anemia develops, and may remain elevated even as hemoglobin falls 3, 4
Stages of Iron Deficiency Progression
Understanding where this patient falls in the spectrum:
- Stage 1 (Early): Anisocytosis and increased microcytic cells appear first, with normal hemoglobin and RBC count, transferrin saturation <32% 3
- Stage 2 (Moderate): MCV and MCH decline, hemoglobin generally subnormal but >9 g/dL, RBC count often elevated as compensation, transferrin saturation <16% 3
- Stage 3 (Severe): MCHC falls, hemoglobin <9 g/dL, RBC count may normalize or decrease as marrow compensation fails, transferrin saturation <16% 3
Critical Caveat
Do not rely on RBC count alone or assume "normal" RBC count excludes iron deficiency—many patients with early-to-moderate iron deficiency maintain normal or elevated RBC counts while developing microcytosis and anemia 5, 4. Always confirm with serum ferritin (<30 μg/L confirms deficiency without inflammation) and transferrin saturation (<15-16%) 6.