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PA13.2 | Approach to Anemia: Classification & Diagnostic Workup — SDL Guide
Learning Objectives
- Define anemia using WHO criteria adjusted for age, sex, pregnancy, and Indian population norms
- Classify anemia morphologically (MCV-based) and etiologically (production/destruction/loss)
- Apply the reticulocyte production index (RPI) to distinguish hypoproliferative from hyperproliferative anemia
- Grade anemia severity using WHO/CTCAE Hb cutoffs
- Construct a stepwise 5-step diagnostic workup algorithm for a patient presenting with pallor and low Hb
- Interpret a peripheral blood smear morphology in the context of the MCV-based decision tree
INSTRUCTIONS
Anemia is the most common hematological disorder in India — the NFHS-5 survey found 57% of women aged 15–49 years are anemic. As a physician, you will encounter anemia every single day, across every specialty. This module gives you the diagnostic thinking framework: how to classify what you are looking at, how to grade severity, and — critically — how to work backward from a blood count to a cause. The sibling module (SDL 2) will take you into the laboratory, where you will actually perform these tests. Reference back to Cluster H1 for your understanding of hematopoiesis, stem cell compartments, and specimen tube science, which underpin everything here.
References
- Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 14 — Diseases of Red Blood Cells (textbook)
- Harsh Mohan: Textbook of Pathology, 8th ed., Ch 12 — Diseases of Blood (textbook)
- Wintrobe's Clinical Hematology, 14th ed., Ch 22 — Classification and Laboratory Investigations of Anemia (textbook)
- WHO: Haemoglobin Concentrations for the Diagnosis of Anaemia, 2011 (WHO/NMH/NHD/MNM/11.1) (guideline)
- ICMR Guidelines on Anemia in India — National Iron Plus Initiative, 2013 (guideline)
Version 2.0 | NMC CBUC 2024
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
Approach to Anemia: Clinical Presentation, WHO Criteria, and Diagnostic Thinking
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.
WHO Hemoglobin Thresholds and MCV-Based Anemia Classification
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.
MCV Decision Tree for Morphological Classification of Anemia
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.
Etiological Classification of Anemia: Bone Marrow Response Framework