The Clinical Approach
From a physiological state to a diagnosis — the definition at altitude, two ways to classify, the microcytic workup, and the iron gatekeeper.
Anemia is a state, not just a number
Anemia is a state of decreased tissue oxygenation from a low hemoglobin concentration — so the threshold that defines it must move with the physiology. At altitude, lower oxygen pressure pushes the “normal” range upward. Drag the slider from sea level to the ʿAsīr highlands and watch the cutoff shift.
| Reference ranges · sea level (WHO) | |
|---|---|
| Adult male | < 13 g/dL |
| Adult female (non-pregnant) | < 12 g/dL |
| Pregnant | < 11 g/dL |
True cutoff depends on age, sex, pregnancy, altitude, and smoking. WHO adds ~+0.9 g/dL per 1,000 m above ~1,000 m.
The probability game: who is the patient?
Pre-test probability should steer the workup. Of 100 patients with anemia, where you meet them changes everything. Toggle the setting and watch the population reshuffle.
In clinic, iron deficiency dominates. On the ward the picture flips — anemia of inflammation leads, followed by critical illness and blood drawn for tests.
The erythropoiesis production line
Red cells come off an assembly line. Ingredients feed a marrow “factory” that the kidney throttles with EPO in response to hypoxia. Anemia is either low output (missing ingredients or broken machinery) or product loss (bleeding or hemolysis).
↓ Low output
Missing ingredients (iron, B12, folate) or broken machinery (marrow failure, infiltration, no EPO). Reticulocytes low.
↗ Product loss
Cells made fine, then lost early — bleeding or hemolysis. The factory responds with high reticulocytes.
Is the factory responding?
One number sorts the entire differential in two: the reticulocyte count. It measures whether the marrow is compensating. Pick a value.
Factory working
The marrow is pumping out young cells as fast as it can. The problem is external to production:
- Hemolysis
- Acute bleeding
Factory failing
Output can’t keep up. The problem is internal to production:
- Deficiency (iron / B12)
- Marrow infiltration
- Renal failure (no EPO)
Clinical pearl — a marrow recovering after iron or B12 replacement also spikes reticulocytes transiently.
Sizing the problem with MCV
The second axis is cell size. Mean corpuscular volume splits the differential into three. Tap a size.
Iron deficiency
Thalassemia
Chronic disease
Sideroblastic
Acute blood loss
Renal failure
Early chronic disease
Endocrine
Megaloblastic
B12 / folate deficiency — a DNA-synthesis problem.
Non-megaloblastic
Alcohol, liver disease — a membrane problem.
The case of the shrinking cell
Reduced hemoglobin means extra cell divisions and smaller cells. Hemoglobin is built from parts — click a cause to see which part it breaks.
- δ-aminolevulinic acid (δ-ALA)
- Porphobilinogen
- Hydroxymethylbilane
- Coproporphyrinogen
- Protoporphyrin
- + Fe²⁺ → HEME
Hepcidin & the ferroportin door
Iron leaves a macrophage through one door — ferroportin. Hepcidin controls whether that door is open or locked. This single switch explains why ferritin is normal-to-high in chronic disease even as the patient is anemic. Flip the scenario.
IDA vs thalassemia minor
Two common microcytic anemias, told apart at the bench. The smear even shows two signature shapes.
| Iron deficiency (IDA) | Thalassemia minor | |
|---|---|---|
| Ferritin | Low — diagnostic | Normal or high |
| RDW | High — varied sizes | Normal — uniformly small |
| Smear | Pencil cells, anisocytosis | Prominent target cells |
| MCV vs anemia | Proportional to severity | Disproportionately low (MCV 60, Hb 10) |
Nuclear delays & membrane expansions
Big cells come from two very different mechanisms — and the neutrophil tells you which.
DNA problem · Megaloblastic
Nuclear–cytoplasmic dyssynchrony — the cytoplasm grows while the nucleus lags.
- B12 deficiency
- Folate deficiency
- Drugs
Membrane problem · Non-megaloblastic
Lipid deposition increases membrane surface area — the cell is round and large.
- Alcoholism
- Liver disease
- Hypothyroidism
The “why” behind the anemia
Anemia is often a readout of another organ. Hover or tap an organ.
No EPO
Renal failure removes the erythropoietin signal — a normocytic anemia of underproduction.
If hemoglobin, white cells, and platelets are all low, suspect bone-marrow failure (aplastic) or leukemia. Immediate referral.
Reading the body: physical signs
Three findings you can spot without a single lab.
Pallor
Pale creases and mucous membranes.
Koilonychia
Concave, spoon-shaped nail — chronic iron deficiency.
Glossitis
Smooth, red tongue — atrophy of the papillae.
The revolving door of iron deficiency
41-year-old woman, anemic. The lesson lives in the follow-up.
Diagnosis
41 yo female, anemia. Treated with IV iron.
3 months later
Hb normalised. Ferritin already dropping.
6 months later
Relapse — Hb 8 g/dL, ferritin low again.
The investigation
A detailed history reveals menorrhagia.
The cure
Treat the bleeding → anemia resolves permanently.
Lesson: iron replacement is temporary. Treat the source of blood loss.
The trap of normal ferritin
Microcytic anemia, but ferritin isn’t low. What’s locking the iron away?
Patient profile
45 yo female · rheumatic heart disease.
| Hb | 9.9 g/dL |
| MCV | 78 fL · microcytic |
| Ferritin | Normal |
The explanation
Chronic inflammation (heart disease) drives hepcidin ↑, which locks the macrophage door. Iron can’t reach the marrow — so it piles up in stores and ferritin reads normal/high while the marrow starves.
Diagnosis: anemia of inflammation.
The CBC is the physical exam of the blood
Context matters
Adjust “normal” for altitude, age, and pregnancy.
Check the factory
Reticulocytes separate failure from loss (kinetic).
Check the size
MCV narrows the differential — micro / normo / macro.
Treat the patient
Find the bleeding, the deficiency, or the disease — not just the number.
“Formulate a differential, confirm with labs, and treat the underlying cause.”
End of Part 1. Next: the red cell under attack — acquired hemolytic anemia.
Part 2 · Acquired hemolysis →