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PA13.2-4 | Approach to Anemia: Classification & Diagnostic Workup — Part 1

CLINICAL SCENARIO

A 28-year-old woman, eight months post-delivery, walks into your Medicine OPD. She is pale, easily tired, and says she has been breathless climbing one flight of stairs. Her Hb comes back at 7.8 g/dL. Where do you start? Do you write 'iron deficiency anemia' and prescribe ferrous sulfate — or do you think? Her MCV is 94 fL. Suddenly 'iron deficiency' is the wrong reflex. This module is about building the thinking, not the reflex.

WHY THIS MATTERS

PA13.2 is a high-yield competency in the NMC CBUC 2024 framework — it appears in clinical postings, MBBS professional exams, and residency entrance exams (NEET-PG, INI-CET). More importantly, anemia evaluation is a core clinical skill you will use in Surgery, OBG, Pediatrics, Internal Medicine, and rural health settings. The classification and workup framework in this module is the scaffolding on which the next five hematology SDLs will build: microcytic, macrocytic, hemolytic, and aplastic anemia each branch off from the decision tree you will master here.

RECALL

Recall from H1 (Hematopoiesis & Blood Specimen Basics): erythropoiesis begins in the yolk sac, shifts to the liver/spleen in the fetus, and settles in the red marrow by birth. EPO (erythropoietin) from the peritubular cells of the kidney drives RBC production in response to hypoxia. The reticulocyte — the last nucleate-free precursor — still contains ribosomal RNA and can be counted as a proxy for marrow output. RBCs collected in EDTA tubes (purple-top) are essential for a valid CBC and MCV; serum tubes (red/yellow) are for ferritin, B12, and folate. These concepts anchor today's workup logic.

Defining Anemia: WHO Criteria & Indian Context

Anemia is defined as a reduction in the total circulating red blood cell mass, operationally measured as a fall in hemoglobin (Hb), hematocrit (PCV), or red cell count below established thresholds.

WHO diagnostic cut-offs (sea-level, venous blood):

Group	Hb threshold (g/dL)
Children 6 months – 4 years	< 11.0
Children 5 – 11 years	< 11.5
Children 12 – 14 years	< 12.0
Non-pregnant women ≥15 years	< 12.0
Pregnant women	< 11.0 (< 10.5 in 2nd trimester)
Men ≥15 years	< 13.0

Indian-context adjustment: ICMR data show that Indian adults have Hb values 0.5–1.0 g/dL lower than Western reference ranges on average, partly due to nutritional patterns and altitude distribution. Some national programs use 11.0 g/dL as the cut-off for non-pregnant women of reproductive age in field settings. In clinical practice, use the WHO thresholds but interpret borderline values against the clinical picture.

High-altitude correction: For every 1,000 m above sea level, the Hb threshold increases by approximately 0.2 g/dL (Leh and Shimla residents have higher 'normal' Hb than Chennai residents).

Plasma volume caveat: Hb and PCV reflect concentration, not absolute mass. Pregnancy, IV fluid loading, and splenomegaly cause dilutional pseudoanemia (low Hb, normal RBC mass). Dehydration causes hemoconcentration that can mask true anemia. Always interpret Hb alongside the clinical volume status.

Medical infographic showing WHO hemoglobin cut-offs by demographic group and MCV-based anemia classification flowchart with differential diagnoses. — **Panel A:** WHO hemoglobin cut-off values by demographic group with color coding and Indian context adjustments. **Panel B:** MCV decision tree showing three main branches based on red cell size (microcytic, normocytic, macrocytic). **Panel C:** Differential diagnoses organized by MCV category with key distinguishing features for each anemia type.

Morphological Classification: The MCV Decision Tree

The mean corpuscular volume (MCV) is your first branch point. It tells you the size of the red cell and immediately narrows the differential diagnosis to a manageable shortlist.

Normal MCV range: 80–100 fL (femtoliters). Values below 80 = microcytic; above 100 = macrocytic.

MCV branch logic:

``Anemia confirmed (Hb below threshold) | ▼ MCV / | \ <80 80-100 >100 | | | Micro Normo Macro``

Microcytic anemia (MCV < 80 fL) — problem with hemoglobin synthesis:
- Iron deficiency anemia (IDA) — most common worldwide
- Thalassemia (α or β) — suspect when MCV < 70 and target cells on smear
- Anemia of chronic disease (ACD) — usually normocytic, can be mildly microcytic
- Sideroblastic anemia — ring sideroblasts on Prussian blue stain

Normocytic anemia (MCV 80–100 fL) — diverse causes:
- Acute blood loss (early phase)
- Anemia of chronic disease (commonest normocytic)
- Hemolytic anemia (often normocytic unless chronic)
- Aplastic anemia, renal anemia
- Mixed deficiency (iron + B12/folate — microcytic + macrocytic cells average to normocytic!)

Macrocytic anemia (MCV > 100 fL) — impaired DNA synthesis or increased reticulocyte output:
- Megaloblastic: B12 deficiency, folate deficiency (hypersegmented neutrophils on smear)
- Non-megaloblastic: liver disease, hypothyroidism, alcohol, drugs (hydroxyurea, methotrexate)
- Reticulocytosis: young RBCs are larger → high retic count from hemolysis or recovery raises MCV

Clinical trap: A mixed deficiency (IDA + B12) can produce a 'normal' MCV. The red cell distribution width (RDW) will be elevated (anisocytosis), and the smear will show a dimorphic population. Always check RDW alongside MCV.

Anemia classification flowchart showing branching tree from root node by MCV values into microcytic, normocytic, and macrocytic categories with specific causes listed under each branch. — **Panel A:** Central anemia decision tree with root node, three MCV-based branches (microcytic <80, normocytic 80-100, macrocytic >100), specific causes under each branch (iron deficiency, thalassemia, B12/folate deficiency, etc.), and clinical pearl note box about mixed deficiency.

CLINICAL PEARL

A NORMAL MCV does NOT exclude iron deficiency anemia. When IDA coexists with B12 or folate deficiency (common in vegetarian Indian women who have both nutritional gaps), microcytic and macrocytic cells cancel each other out arithmetically, producing a falsely normal MCV. The smear will show a DIMORPHIC picture (two populations of cells), and the RDW will be elevated. Always look at the smear, not just the MCV number.

Etiological Classification: The Bone Marrow Response Framework

Morphological classification (MCV) tells you what the cell looks like. Etiological classification asks: why is there not enough hemoglobin? The answer always falls into one of three buckets:

1. Decreased production (hypoproliferative)
The marrow is failing to make enough RBCs. Causes:
- Nutritional deficiency (iron, B12, folate — substrate shortage)
- Bone marrow failure (aplastic anemia, infiltration by leukemia/myeloma)
- Anemia of chronic disease (cytokine suppression + iron sequestration)
- Renal failure (EPO deficiency — recall H1: EPO is made by peritubular kidney cells)
- Hypothyroidism, endocrine causes

2. Increased destruction (hemolytic)
The marrow is making RBCs at an accelerated rate, but they are being destroyed faster. Causes:
- Intrinsic RBC defects: hereditary spherocytosis, G6PD deficiency, sickle cell disease, thalassemia
- Extrinsic/immune: autoimmune hemolytic anemia (AIHA), transfusion reactions, malaria, microangiopathic (TTP, HUS)

3. Blood loss
- Acute: trauma, GI bleed, obstetric hemorrhage → initially normocytic, retic count rises in 3–5 days as marrow responds
- Chronic: hookworm, menorrhagia → body loses iron → eventually becomes IDA (microcytic)

The discriminator: Reticulocyte count
The reticulocyte response tells you whether the marrow is trying to compensate:
- High retic (> 2% or RPI > 2): marrow is responding appropriately → the problem is destruction or loss
- Low retic (< 0.5% or RPI < 1): marrow is NOT responding → the problem is production failure

This is the single most important early branch point after MCV.

Three-bucket diagram showing anemia classification by mechanism with reticulocyte count as the discriminating test between hypoproliferative and hyperproliferative causes. — **Panel A:** Three mechanism buckets (Decreased Production, Blood Loss, Increased Destruction) with example diseases, reticulocyte count decision point, and diagnostic flow arrows connecting hypoproliferative vs hyperproliferative categories.