Master iron absorption, transport, storage, and lab assessment — serum iron, TIBC, ferritin — for ASCP MCQs.

ASCPprep.com | Hematology

Chapter 5 — IRON KINETICS & LABORATORY ASSESSMENT

ASCP Board Exam · Lab-Focused Study Guide
Iron Compartments · Absorption & Transport · Hepcidin · Ferritin · Iron Studies · sTfR · CHr · Thomas Plot · ZPP · Practice Questions

1 · Priority Table & Quick Foundation

Topic Priority Why It’s Tested
Iron studies: SI, TIBC, % saturation, ferritin — methods, reference intervals, compartment assessed HIGH The core of every anemia work-up question; % saturation calculation
Ferritin as storage marker AND acute phase reactant HIGH Classic trap: normal ferritin with inflammation doesn’t exclude iron deficiency
Absorption proteins: DMT1, DcytB, ferroportin, hephaestin, transferrin HIGH ‘Which protein…’ matching items
Hepcidin regulation of body iron (ferroportin inactivation) HIGH Explains iron deficiency vs anemia of inflammation vs hemochromatosis
No excretion mechanism — body iron regulated by absorption HIGH One-line recall item
Transferrin receptor cycle & soluble transferrin receptor (sTfR) HIGH sTfR is the test for equivocal iron studies
Iron compartments (functional, storage, transport) MEDIUM Which test measures which compartment
Prussian blue staining (gold standard for stores) MEDIUM Method principle; ferritin vs hemosiderin staining
Hemoglobin content of reticulocytes (CHr), sTfR/log ferritin, Thomas plot MEDIUM Newer assays; quadrant interpretation
Zinc protoporphyrin (ZPP) MEDIUM Lead poisoning and iron-deficient erythropoiesis
Dietary iron, bioavailability, enhancers/inhibitors MEDIUM Heme vs ionic iron; who needs supplements
Hepatocyte iron sensing (HFE, TfR2, HJV, BMP6, SMAD, matriptase-2) LOW Names only; mutations covered with iron overload disorders
Fenton reaction chemistry LOW Why free iron is toxic — concept only
🎯 KEY POINT — Where to spend your time

Focus: the four classic iron tests (what each measures, method, reference interval), ferritin’s acute-phase problem, % saturation math, hepcidin–ferroportin, and the absorption proteins in order. Skim: the hepatocyte sensing molecules (HFE, TfR2, HJV, BMP6, SMAD) beyond knowing they control hepcidin.

Quick Foundation — from zero

  • Iron carries oxygen (in hemoglobin) and powers energy production (in cytochromes). Most body iron is in hemoglobin.
  • Free iron is dangerous: ferrous iron + peroxide makes hydroxyl free radicals (Fenton reaction) that damage proteins, lipids and DNA. So iron is always carried or stored by proteins.
  • The body has no way to excrete iron (only small losses in shed skin, hair and gut cells). It controls iron by controlling absorption — and it recycles iron from old RBCs.
  • Iron is absorbed in the duodenum (DMT1 in, ferroportin out), carried in plasma on transferrin, taken into cells by the transferrin receptor, and stored as ferritin.
  • The liver hormone hepcidin is the master switch: high iron → more hepcidin → ferroportin blocked → less absorption and recycling.
  • Lab tests look at three compartments: transport (serum iron, TIBC, % saturation), storage (ferritin, Prussian blue), and functional iron in cells (sTfR, CHr, ZPP).
💡 Analogy

Think of body iron as money in a country with no exports. The only control is how much you import (absorption). Ferritin is the savings bank, transferrin is the delivery truck, the transferrin receptor is each shop’s loading dock, and hepcidin is the customs officer who closes the border (ferroportin) when the bank is full — or during a war (inflammation), even when the bank is empty.

🏥 Why this matters clinically

Iron deficiency is the world’s most common cause of anemia, and anemia of inflammation is the most common mimic. Choosing and interpreting the right iron tests separates them — and avoids giving iron to someone who can’t use it.

Concept Plain-English Explanation
Functional iron Iron doing a job: hemoglobin, myoglobin, cytochromes/enzymes
Storage iron Iron saved for later as ferritin and hemosiderin (macrophages, hepatocytes)
Transport iron Iron riding on transferrin in plasma — a tiny amount at any moment
Apotransferrin vs transferrin Apotransferrin = empty carrier; transferrin = carrier with iron (binds 2 Fe³⁺)
TIBC How much iron plasma could carry — an indirect measure of transferrin
% Transferrin saturation How full the carriers are (SI ÷ TIBC)
Hepcidin Liver hormone that shuts iron export (ferroportin) when iron is high or during inflammation
Acute phase reactant Protein that rises with inflammation or infection regardless of the condition being measured (ferritin is one)

2 · Core Content Tables

2.1 Iron Compartments

Compartment Form and Site % of Body Iron Typical Content
Functional Hemoglobin iron in blood ~68% 2.4 g
Functional Myoglobin iron in muscle ~10% 0.36 g
Functional Peroxidase, catalase, cytochromes, riboflavin enzymes (all cells) ~3% 0.12 g
Storage Ferritin and hemosiderin — mostly macrophages and hepatocytes; small amounts in all cells except mature RBCs ~18% 0.667 g
Transport Transferrin in plasma <1% 0.001 g

Rodak’s text rounds these as ~65% in hemoglobin, ~25% in storage and ~10% elsewhere; the table above is the detailed version. Either way: most iron is in hemoglobin, and very little is in transit.

2.2 Iron Chemistry & Systemic Regulation

Feature Details
Valence states Ferrous (Fe²⁺, reduced) ↔ ferric (Fe³⁺, oxidized) — this switching powers electron transport in cytochromes
Why iron is dangerous Fenton reaction: Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH• (hydroxyl radical) → damages proteins, lipids, nucleic acids
Excretion None — only minimal loss via exfoliated skin, hair and intestinal epithelium
Control point Body iron is regulated by absorption (plus recycling)
Master regulator Hepcidin (made by hepatocytes) binds ferroportin and inactivates it on enterocytes, macrophages and hepatocytes
Iron stores high Hepatocytes sense it → ↑ hepcidin → ferroportin inactivated → ↓ absorption and ↓ release from macrophages → plasma iron falls
Iron stores low ↓ hepcidin → ferroportin active → ↑ absorption and ↑ recycling → plasma iron and stores restored

2.3 Hepatocyte Iron Sensing (hepcidin control)

Feature Details
System 1 — transferrin sensing HFE (hemochromatosis protein) sits on TfR1. When diferric transferrin binds TfR1, HFE is released → binds TfR2 → signal → phosphorylates SMAD → ↑ hepcidin gene expression
System 2 — stores sensing High body iron → macrophages (Kupffer cells) secrete BMP6 → binds BMPR + coreceptor hemojuvelin (HJV) → SMAD → ↑ hepcidin
Brake Matriptase-2 inactivates HJV → ↓ hepcidin when iron is low
Clinical significance Mutations in these proteins cause iron overload (e.g., HFE hereditary hemochromatosis) or iron deficiency; molecular testing is growing (details in the iron disorders chapter)

2.4 Intestinal Absorption & Plasma Transport

Feature Details
Site Duodenum (small intestine enterocytes)
Reduction Dietary iron is mostly ferric; must be reduced to Fe²⁺ — helped by gastric acid, acidic foods (citrus) and duodenal cytochrome B (DcytB) on the luminal membrane
Into the enterocyte DMT1 (divalent metal transporter 1) carries Fe²⁺ across the luminal membrane
Out of the enterocyte Ferroportin on the basolateral membrane — the only known iron exporter in cells (also on macrophages and hepatocytes)
Oxidation for transport Hephaestin on the basolateral membrane oxidizes Fe²⁺ → Fe³⁺ as it exits
Plasma carrier Apotransferrin binds two Fe³⁺ → called transferrin (Tf)
Heme iron Absorbed more readily than ionic iron; the heme carrier is still being sought (the candidate carrier transports folate more efficiently)

2.5 Cellular Uptake — Transferrin Receptor Cycle

Feature Details
Receptor TfR1 — highest affinity for diferric transferrin at plasma pH (7.4)
Endocytosis TfR1–Tf complexes cluster → membrane invaginates (clathrin-coated pit) → endosome
Iron release Proton pump acidifies endosome (pH ~5.5) → iron released from Tf; TfR1 now prefers apotransferrin
Into cytoplasm Iron reduced and moved out of endosome by DMT1 → stored as ferritin or sent to mitochondria (mitoferrin) for heme/cytochromes
Recycling Endosome returns to the surface; at pH 7.4 TfR1 has very low affinity for apoTf → apoTf released to plasma; TfR1 reused
Cell regulation Iron-sensitive cytoplasmic proteins act on TfR1 mRNA: ↓ cell iron → ↑ TfR1; ↑ cell iron → ↓ TfR1
sTfR A truncated TfR1 is shed into plasma → soluble transferrin receptor proportional to total cellular TfR
RBC precursors Need far more iron; iron may bypass the cytoplasmic sensing system, moving from endosome directly to mitochondria

2.6 Storage: Ferritin & Hemosiderin

Feature Details
Ferritin Apoferritin cage storing Fe³⁺; one molecule holds >4,000 iron ions. Iron mobilized by lysosomal degradation of the protein
Hemosiderin Partially degraded ferritin; considered less available; stains with Prussian blue
Where All cells except mature RBCs; mostly macrophages and hepatocytes
Serum ferritin Secreted by macrophages in proportion to stores (reason unknown) → convenient storage marker

2.7 Iron Recycling

Feature Details
Main source Senescent RBCs ingested by splenic macrophages; Hb degraded, iron held as ferritin
Export Macrophage ferroportin → iron binds plasma apotransferrin, as if newly absorbed
Salvage proteins Haptoglobin (free Hb) and hemopexin (free heme) prevent urinary loss and return iron to the liver; hepatocytes also export via ferroportin
Link Hepcidin blocks macrophage ferroportin too → in inflammation iron is trapped in macrophages (anemia of inflammation)*

2.8 Dietary Iron, Bioavailability & Demand

Feature Details
Iron-rich foods Red meat, legumes, dark green leafy vegetables
Most bioavailable Heme iron from meat (myoglobin and hemoglobin)
Ionic (nonheme) iron Must be Fe²⁺ to enter via DMT1; dietary iron is mostly Fe³⁺, especially from plants
Inhibitors Oxalates, phytates, phosphates, calcium (bind iron)
Enhancers Gastric acid, acidic foods (citrus), DcytB
Daily numbers Typical U.S. diet 10–20 mg/day; only 1–2 mg absorbed — enough for most men
Higher need Menstruating women, pregnant and lactating women, growing children

2.9 Laboratory Assessment of Body Iron Status (Rodak Table 11-3, adult male)

Assay Reference Interval Compartment / Use
Serum iron (SI) 50–160 µg/dL Available transport iron
TIBC (transferrin) 250–400 µg/dL Indirect indicator of iron stores
% Transferrin saturation 20–55% Indirect indicator of stores, with transport iron
Serum ferritin 40–400 ng/mL Iron stores
Marrow/liver Prussian blue Normal stores visualized Visual, qualitative tissue stores (gold standard)
sTfR 1.15–2.75 mg/L Functional iron available in cells
sTfR/log ferritin 0.63–1.8 Functional iron available in cells
RBC zinc protoporphyrin (ZPP) <80 µg/dL RBCs Functional iron available in RBCs
Hb content of reticulocytes (CHr) 27–34 pg/cell Functional iron in developing RBCs

Reference intervals vary by lab, method, sex and age (a later review question in Rodak uses ferritin 12–400 ng/mL). Always use your laboratory’s intervals.

2.10 Serum Iron (SI)

Feature Details
Principle Acid releases iron from transferrin → chromogen (e.g., ferrozine) forms a colored complex → spectrophotometry
Limitation High within-day and between-day variability; rises after iron-containing foods or supplements — limited value alone
Collection practice Traditionally fasting, early morning (highest levels); questioned, but diurnal hepcidin variation may still support it
🧪 PRE-ANALYTICAL — Serum iron & TIBC
  • Fasting, morning specimen; stop iron supplements beforehand (lab policy, often ≥24 h)*
  • Serum (or heparin plasma per method) — not EDTA, which chelates iron*
  • Hemolysis falsely raises SI (RBC iron released)*
  • Use the same units for SI and TIBC when calculating % saturation

2.11 Total Iron-Binding Capacity (TIBC) & % Saturation

Feature Details
TIBC principle Add excess ferric iron to saturate transferrin → remove unbound iron with magnesium carbonate → measure iron by the SI method → iron detected = all binding sites
What it really measures An indirect measure of transferrin, reported as an iron value
% Saturation formula SI ÷ TIBC × 100 (units must match)
Rule of thumb About ⅓ of transferrin is normally saturated with iron
Worked example SI 63 µg/dL, TIBC 420 µg/dL → 63 ÷ 420 × 100 = 15% (low)

2.12 Prussian Blue Staining

Feature Details
Compound Prussian blue, Fe₇(CN)₁₈
Principle Acidic potassium ferrocyanide reacts with ferric iron in tissue → dark blue dots; graded semiquantitatively
Status Gold standard for body iron stores; done routinely when marrow or liver biopsies are taken for other reasons
What stains Hemosiderin stains; ferritin does not (iron protected inside the protein cage)
Limitation Invasive — why serum ferritin became the routine stores test
🧪 PRE-ANALYTICAL — Marrow iron stain*
  • Aspirate smear must contain spicules — iron is assessed within particles, not free cells
  • Decalcified core biopsies can leach iron → false low; aspirate preferred
  • Use iron-free glassware and positive control slides; stain precipitate can mimic granules

2.13 Serum Ferritin

Feature Details
Correlation Correlates highly with marrow stores on Prussian blue
Method Immunoassay
Big drawback Acute phase reactant — rises with inflammation/infection without more body iron (likely to sequester iron from bacteria)
Equivocal zone Ferritin 20–100 ng/mL is equivocal when inflammation is present
Predictive value A value within the reference interval has weak predictive value (could be masked deficiency); a low ferritin has high predictive value for iron deficiency (only low stores lower it)
🧪 PRE-ANALYTICAL — Ferritin
  • Serum/plasma immunoassay; interpret with an inflammation marker (e.g., CRP)* and clinical context
  • Liver injury and malignancy can also release ferritin → falsely high*
  • Very high ferritin can cause a high-dose hook effect in some immunoassays — dilute*

2.14 Soluble Transferrin Receptor (sTfR) & sTfR/log Ferritin

Feature Details
Principle Immunoassay for truncated TfR shed into plasma — proportional to total cellular TfR
↑ because of more TfR per cell Iron deficiency (cells starved of iron make more receptors)
↑ because of more cells Rapid erythropoiesis (e.g., hemolytic anemia) — normal TfR per cell but many more cells
Advantage* Not an acute phase reactant → helps when ferritin is falsely raised by inflammation
sTfR/log ferritin sTfR rises and ferritin falls in iron deficiency → ratio amplifies the signal; most useful when one test has changed but is still within its reference interval. Ref 0.63–1.8

2.15 Hemoglobin Content of Reticulocytes (CHr)

Feature Details
Definition Analyzer-reported Hb per reticulocyte — like an MCH for retics only (27–34 pg/cell)
Why useful Retics reflect the last ~24 hours of erythropoiesis → near real-time view of iron available for Hb production
Interpretation ↓ CHr = iron-restricted erythropoiesis (true or functional deficiency). Separate intervals for infants/children
🧪 PRE-ANALYTICAL — CHr / RET-He*
  • EDTA whole blood on analyzers that report it; analyze within a few hours — cells swell on storage and change indices
  • Low values also occur in thalassemia trait (small cells, not iron lack)

2.16 Thomas Plot

Feature Details
Axes sTfR/log ferritin (x-axis) vs hemoglobin content of reticulocytes (y-axis) → 4 quadrants
Upper left Normal iron status — both normal
Lower left Functional iron deficiency — normal sTfR/log ferritin (ferritin raised by inflammation) but ↓ CHr (stores exist but can’t be delivered) → e.g., anemia of chronic inflammation
Upper right Latent iron deficiency — ↑ sTfR/log ferritin, CHr still normal (retics protected first); stores depleting, no anemia yet
Lower right Iron deficiency — ↑ sTfR/log ferritin and ↓ CHr

2.17 Zinc Protoporphyrin (ZPP)

Feature Details
Mechanism When iron isn’t inserted into protoporphyrin IX, zinc binds instead → ZPP accumulates in RBCs
Method Easily detected by fluorescence (hematofluorometer)
Uses Rises in iron-deficient erythropoiesis, but most valuable when ferrochelatase is impaired — lead poisoning
Reference <80 µg/dL RBCs
🧪 PRE-ANALYTICAL — ZPP*
  • EDTA whole blood; protect from light; bilirubin and riboflavin fluoresce and can interfere with front-face hematofluorometers (washed-cell methods reduce this)
  • ZPP rises slowly (reflects RBCs made over the prior months) — not an acute marker

*From general hematology/laboratory practice, not stated in Rodak Ch. 11.

3 · Visual Memory Hooks

🧠 Absorption order
“Duodenal Doors: DcytB Reduces, DMT1 Delivers, Ferroportin Frees, Hephaestin Hands to Transferrin”.
🧠 DMT1 vs ferroportin
“DMT1 = IN, Ferroportin = OUT (the Only exit)”.
🧠 Iron charge along the path
“Ferric in food → Ferrous through the gut → Ferric on the truck” (Fe³⁺ → Fe²⁺ → Fe³⁺).
🧠 Hepcidin
“Hepcidin Hides the iron” → blocks ferroportin → iron stuck in gut cells and macrophages.
🧠 Transferrin load
“Trans-FER carries a PAIR” → 2 Fe³⁺ per apotransferrin.
🧠 TfR1 regulation
“Hungry cells open more doors” → ↓ cell iron → ↑ TfR → ↑ sTfR.
🧠 % Saturation
“Iron over Capacity, times 100” → SI/TIBC × 100; normal ≈ one-third.
🧠 Iron study reference
“50–160, 250–400, 20–55, 40–400” → SI, TIBC, % sat, ferritin (adult male).
🧠 Ferritin trap
“Low ferritin never lies; normal ferritin can hide” → inflammation raises it.
🧠 Prussian blue
“Hemosiderin shows, Ferritin hides” → cage protects ferritin iron from the stain.
🧠 Thomas plot quadrants
“Up-Left OK · Down-Left Locked (functional) · Up-Right Running low (latent) · Down-Right Deficient”.
🧠 ZPP
“Zinc Poses as iron — Lead Loves it” → ZPP ↑ with iron lack; best for lead poisoning.
🧠 Diet
“Meat beats Plants; Citrus helps, Calcium and Phytates block”.
🧠 Excretion
“Iron has no exit — only an entrance guard” → regulated at absorption.

Pattern Triggers

If you see… Think… Because…
↓ ferritin (any value below reference) Iron deficiency Only depleted stores lower ferritin — high predictive value
↓ SI, ↑ TIBC, ↓ % sat, ↓ ferritin Iron deficiency (classic pattern)* Stores empty → more transferrin made → carriers mostly empty
↓ SI, normal/↓ TIBC, normal/↑ ferritin, inflammation Anemia of inflammation (functional iron deficiency)* Hepcidin traps iron in macrophages; ferritin is acute phase
Ferritin 20–100 ng/mL with ↑ CRP Equivocal — can’t exclude iron deficiency Acute phase rise may mask low stores
↑ sTfR with normal ferritin in inflammation Coexisting iron deficiency* sTfR rises with cellular iron lack and isn’t an acute phase reactant
↑ sTfR with ↑ retics and hemolysis Expanded erythropoiesis, not iron deficiency More cells, normal TfR per cell
↓ CHr, normal sTfR/log ferritin Functional iron deficiency (Thomas lower-left) Stores present but not delivered
↑ sTfR/log ferritin, normal CHr, no anemia Latent iron deficiency (Thomas upper-right) Other cells starved before erythroid cells
↑ ZPP + basophilic stippling in a child Lead poisoning Lead inhibits ferrochelatase → zinc inserted
↑ % sat, ↑ ferritin, family history Iron overload / hereditary hemochromatosis* Low hepcidin → uncontrolled absorption
Regular blood donor with ↓ ferritin but normal Hb Depleted stores; ↑ absorption ↓ hepcidin → ferroportin active

Commonly Confused Pairs

Feature A B
DMT1 vs ferroportin Imports Fe²⁺ across luminal membrane (and out of endosome) Exports iron from enterocytes, macrophages, hepatocytes
Hephaestin vs DcytB Oxidizes Fe²⁺ → Fe³⁺ on exit (basolateral) Reduces Fe³⁺ → Fe²⁺ on entry (luminal)
Transferrin vs ferritin Plasma transport protein (2 Fe³⁺) Intracellular storage cage (>4,000 Fe)
Ferritin vs hemosiderin Soluble; not stained by Prussian blue Degraded ferritin; stains with Prussian blue
TIBC vs SI Carrying capacity (indirect transferrin) Iron actually carried
TfR1 vs TfR2 Iron uptake on all cells; binds HFE Hepatocyte sensor; binds freed HFE → hepcidin signal
sTfR ↑ in iron deficiency vs hemolysis More receptors per cell More cells, normal receptors each
Latent vs functional iron deficiency ↑ sTfR/log ferritin, normal CHr Normal sTfR/log ferritin, ↓ CHr
Hepcidin ↑ vs ↓ Iron overload, inflammation → ↓ absorption Iron deficiency, blood donation → ↑ absorption

Timeline Table — Development of Iron Deficiency*

Stage First Markers to Change Later Anemia? Thomas Plot
1 · Storage depletion ↓ Ferritin, ↓ marrow iron ↑ absorption (↓ hepcidin) No Moves right (↑ sTfR/log ferritin)
2 · Iron-deficient erythropoiesis ↓ SI, ↑ TIBC, ↓ % sat, ↑ sTfR ↓ CHr, ↑ ZPP Not yet / borderline Upper-right → lower-right
3 · Iron deficiency anemia ↓ Hb ↓ MCV, ↓ MCH (microcytic, hypochromic) Yes Lower-right
Test How fast it reflects change Why
CHr ~24 hours Retics show the last day of erythropoiesis
Serum iron Hours (meals, diurnal) High day-to-day variability
Ferritin Weeks (stores); days (acute phase) Tracks stores but jumps with inflammation
ZPP Weeks–months Accumulates in RBCs over their life span*
MCV Months Microcytic cells must replace older normocytic cells*

4 · Diagnostic Flowcharts

Flowchart 1 — Microcytic/hypochromic anemia: which iron picture?*

↓ MCV/MCH anemia → Order ferritin, SI, TIBC, % saturation (+ CRP)
↓ Ferritin → Iron deficiency anemia — find the source of loss
Normal/↑ ferritin, ↓ SI, normal/↓ TIBC, inflammation present → Anemia of inflammation (functional iron deficiency)
Ferritin equivocal (20–100) with inflammation → Add sTfR, sTfR/log ferritin, CHr → Thomas plot
Normal/↑ iron studies, basophilic stippling, ↑ ZPP → Consider lead poisoning; normal iron + target cells → thalassemia
⚠️ Common mistake: calling a normal ferritin ‘no iron deficiency’ in an inflamed patient — ferritin is an acute phase reactant.

Flowchart 2 — Equivocal classic iron studies: method selection

Classic iron studies equivocal → Is inflammation present?
Yes → Measure sTfR (not acute phase) → calculate sTfR/log ferritin
Need a real-time erythroid iron view → CHr (last 24 h of production)
Combine both → Thomas plot quadrant
Still unresolved and marrow being sampled anyway → Prussian blue marrow stain (gold standard)
⚠️ Common mistake: choosing ZPP or peripheral sideroblasts to resolve equivocal iron deficiency; sTfR is the recommended answer.

Flowchart 3 — Hepcidin logic

Body iron stores rise (or inflammation) → Hepatocytes sense (HFE/TfR2; BMP6/HJV → SMAD)
↑ Hepcidin → Ferroportin inactivated on enterocytes, macrophages, hepatocytes
Result → ↓ absorption, ↓ recycling → plasma iron falls
Stores fall (bleeding, donation, pregnancy) → ↓ hepcidin (matriptase-2 cleaves HJV)
Result → Ferroportin active → ↑ absorption (can double) and ↑ release
⚠️ Common mistake: thinking hepcidin acts on DMT1 or transferrin. Its target is ferroportin.

Flowchart 4 — Timing: a regular blood donor’s iron status

Repeated donations (~485 mL each, ~200–250 mg iron*) → Iron leaves the body with each unit
Weeks–months → Storage depletion → ↓ ferritin first; Hb still normal
↓ Hepcidin → Absorption rises (donors in Garry’s study absorbed >2× normal)
If losses continue → ↓ % saturation, ↑ sTfR, ↓ CHr → then anemia
⚠️ Common mistake: expecting Hb or MCV to fall first. They are late markers; ferritin falls first.

Flowchart 5 — Iron overload screen*

Fatigue, liver disease, diabetes, bronze skin, or family history → Measure % saturation and ferritin
↑ % saturation + ↑ ferritin → Suspect hereditary hemochromatosis (HFE) → molecular testing
↑ ferritin, normal % saturation → Consider inflammation, liver injury, malignancy (acute phase)
Confirm tissue iron → Liver biopsy Prussian blue / imaging
⚠️ Common mistake: diagnosing iron overload from ferritin alone — ferritin rises in inflammation; % saturation is needed.

5 · 25 High-Yield Facts

# Fact
1 Most body iron (about two-thirds) is in hemoglobin.
2 Storage iron as ferritin and hemosiderin is mainly in macrophages and hepatocytes.
3 Less than 1% of body iron is in transit on transferrin.
4 The body has no mechanism for active iron excretion; iron is regulated by absorption.
5 Free ferrous iron reacts with peroxide (Fenton reaction) to form damaging hydroxyl radicals.
6 Iron is absorbed in the duodenum after reduction to the ferrous form.
7 DMT1 transports ferrous iron across the luminal membrane of the enterocyte.
8 Ferroportin is the only known cellular iron exporter.
9 Hephaestin oxidizes iron to the ferric form as it leaves the enterocyte.
10 One apotransferrin molecule binds two ferric iron atoms.
11 Hepcidin, made by hepatocytes, inactivates ferroportin to decrease absorption and recycling.
12 Low body iron decreases hepcidin, increasing iron absorption and release.
13 Cells take up iron by receptor-mediated endocytosis of transferrin bound to TfR1.
14 Endosomal acidification releases iron from transferrin; TfR1 and apotransferrin are recycled.
15 Cells low in iron increase TfR1, raising the soluble transferrin receptor level.
16 One ferritin molecule can store more than 4,000 iron ions.
17 Heme iron from meat is the most bioavailable dietary iron.
18 Of 10–20 mg of dietary iron per day, only 1–2 mg is absorbed.
19 TIBC is an indirect measure of transferrin.
20 Percent transferrin saturation equals serum iron divided by TIBC times 100; about one-third is normal.
21 Prussian blue staining of bone marrow is the gold standard for iron stores; it stains hemosiderin but not ferritin.
22 Ferritin is an acute phase reactant, so a normal value doesn’t exclude iron deficiency in inflammation.
23 A low serum ferritin has high predictive value for iron deficiency.
24 Hemoglobin content of reticulocytes reflects iron available for erythropoiesis in the previous 24 hours.
25 Zinc protoporphyrin increases when iron isn’t incorporated into heme and is most useful in lead poisoning.
⭐ EXAM TIP — The single most testable pattern

Ferritin logic: low ferritin = iron deficiency (reliable). Normal or high ferritin with inflammation = unreliable → use sTfR (or sTfR/log ferritin, CHr, Thomas plot). And know the formula: % saturation = SI ÷ TIBC × 100.

Quick Interpretation Table

Lab Finding Clinical Interpretation
SI 63 µg/dL, TIBC 420 µg/dL (% sat 15%) Low saturation — iron-deficient transport
Ferritin 8 ng/mL Depleted stores — iron deficiency
Ferritin 90 ng/mL, CRP ↑, ↑ sTfR Iron deficiency masked by inflammation
↑ sTfR/log ferritin, normal CHr, normal Hb Latent iron deficiency
Normal sTfR/log ferritin, ↓ CHr, chronic inflammation Functional iron deficiency
↑ ZPP, basophilic stippling Lead poisoning (ferrochelatase inhibited)
↑ % saturation, ↑ ferritin Iron overload*

🎴 Flashcards — Active Recall Practice

How it works: tap the card to flip it. Then mark it ✓ Knew It or ✗ Review Again (buttons unlock after flipping). Use 🔀 Shuffle to randomize and 🏁 Finish anytime — your summary appears at the end, and you can drill only the cards you missed.

📝 Practice Quiz — Test Your Knowledge

How it works: tap an answer for each case. You’ll get instant feedback with the full explanation and the examiner’s trap. Your final score appears once all 10 are answered.

🪤 Trap Questions — Beat the Examiner

Harder on purpose. Each question targets a real misconception. After answering you’ll see the correct reasoning, the trap explained, why the examiner wrote it, and the wrong answer most students choose.

8 · Rapid Revision Sheet

📖 READ THIS 2 HOURS BEFORE YOUR EXAM

Compartments

  • Hb ~68% (2.4 g) · myoglobin ~10%
  • Enzymes ~3% · storage ~18%
  • Transport <1% (transferrin)
  • No excretion → regulated by absorption

Absorption Path

  • Diet Fe³⁺ → DcytB → Fe²⁺
  • DMT1 in (luminal)
  • Ferroportin out (only exporter)
  • Hephaestin → Fe³⁺ → apoTf (2 Fe)

Hepcidin

  • Liver hormone → blocks ferroportin
  • ↑ iron / inflammation → ↑ hepcidin
  • ↓ iron / bleeding → ↓ hepcidin
  • Sensing: HFE/TfR1→TfR2; BMP6/BMPR/HJV → SMAD
  • Matriptase-2 cleaves HJV

Cellular Iron

  • TfR1 binds diferric Tf (pH 7.4)
  • Endosome pH 5.5 releases Fe → DMT1 out
  • TfR1 + apoTf recycled
  • ↓ cell iron → ↑ TfR → ↑ sTfR
  • Ferritin >4,000 Fe · hemosiderin stains

Reference Intervals

  • SI 50–160 µg/dL
  • TIBC 250–400 µg/dL
  • % sat 20–55% (≈⅓)
  • Ferritin 40–400 ng/mL
  • sTfR 1.15–2.75 mg/L · ratio 0.63–1.8
  • ZPP <80 µg/dL · CHr 27–34 pg

Methods

  • SI: acid release + ferrozine
  • TIBC: excess Fe³⁺ + MgCO₃ → indirect Tf
  • % sat = SI/TIBC × 100
  • Prussian blue: acidic K ferrocyanide
  • ZPP: fluorescence

Ferritin Rules

  • Low = iron deficiency (reliable)
  • Acute phase reactant
  • 20–100 ng/mL equivocal in inflammation
  • Correlates with marrow iron

Thomas Plot

  • Up-left: normal
  • Down-left: functional (low CHr, normal ratio)
  • Up-right: latent (high ratio, normal CHr)
  • Down-right: iron deficiency

Confusing Pairs

  • DMT1 (in) ≠ ferroportin (out)
  • DcytB reduces ≠ hephaestin oxidizes
  • Transferrin (transport) ≠ ferritin (store)
  • sTfR ↑ iron def ≠ ↑ hemolysis
  • Transporter (ferroportin) ≠ regulator (hepcidin)

Diet & Demand

  • Heme iron most bioavailable
  • 10–20 mg eaten → 1–2 mg absorbed
  • Block: oxalate, phytate, phosphate, Ca
  • Help: gastric acid, citrus
  • ↑ need: menses, pregnancy, lactation, growth

9 · Final Memory Map

  • 🧲 IRON KINETICS & ASSESSMENT
    • Compartments
      • Functional: Hb, myoglobin, enzymes
      • Storage: ferritin/hemosiderin (macrophages, hepatocytes)
      • Transport: transferrin (<1%)
    • Absorption & Transport
      • DcytB → DMT1 → ferroportin → hephaestin
      • Apotransferrin binds 2 Fe³⁺
      • Heme iron most bioavailable
    • Regulation
      • No excretion · absorption controlled
      • Hepcidin blocks ferroportin
      • HFE/TfR2 and BMP6/HJV → SMAD → hepcidin
    • Cellular Iron
      • TfR1 endocytosis · endosome pH 5.5
      • TfR recycled · sTfR shed
      • Ferritin storage · mitochondria for heme
    • Recycling
      • Splenic macrophages → ferroportin → transferrin
      • Haptoglobin/hemopexin salvage
      • Hepatocyte export
    • Classic Tests
      • SI (ferrozine) · TIBC (indirect Tf) · % sat = SI/TIBC
      • Ferritin = stores but acute phase
      • Prussian blue = gold standard (hemosiderin)
    • Newer Tests
      • sTfR · sTfR/log ferritin
      • CHr (24-h view)
      • Thomas plot · ZPP (lead)



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