Principles of staged diagnosis of iron metabolism
Laboratory search is based on a clear separation of hematological and biochemical parameters. When pathology develops, biochemical tests react first, signaling the depletion of reserves in stores, whereas hemoglobin and erythrocyte indices of the complete blood count decrease later, marking the manifestation of the disease.
Two successive phases of the pathological process are distinguished:
- Latent iron deficiency (LID): hemoglobin concentration remains within the age-related norm, however, indicators of the stored pool (serum ferritin) and circulating serum iron are already reduced.
- Overt iron deficiency anemia: hemoglobin drop below the age-related threshold combined with a drop in ferritin and serum iron.
Biochemical markers of iron metabolism
Assessment of storage and transport includes a set of mutually complementary parameters:
- Ferritin — a specialized protein for intracellular storage. A decrease in its concentration below 10 µg/L (by radioimmunoassay) or below 12 µg/L (according to WHO and UNICEF standards) serves as direct evidence of store depletion. It is important to consider that reference values for ferritin in children differ significantly from the adult population norms and depend on test systems. In addition, ferritin acts as an acute-phase protein, which can mask true deficiency in concomitant inflammation.
- Transferrin — the carrier of free iron cations. The transferrin saturation percentage decreases to 15% and below. The amount of free carrier protein is directly reflected by the total iron-binding capacity of serum (TIBC), which compensatorily increases above the normal 55–65 µmol/L.
- Soluble transferrin receptor — a protein indicator reflecting tissue starvation. Its concentration rises above 7 mg/L in pronounced cellular deficiency.
The WHO/UNICEF diagnostic triad includes three simultaneous signs: hemoglobin below the age-related norm, serum ferritin less than 12 µg/L, and soluble transferrin receptor above 7 mg/L.
Clinical and biochemical comparison of deficiency stages
Differential diagnosis is based on comparing the pool of circulating oxygen-binding protein and tissue reserves:
| Assessment parameter | Latent iron deficiency | Overt IDA |
|---|---|---|
| Hemoglobin (Hb) | Normal for age | Reduced below age-related norm |
| Serum iron | Reduced (<14.3 µmol/L) | Reduced (<14.3 µmol/L) |
| Serum ferritin | Decreased | Reduced (<12 µg/L) |
In the complete blood count, the overt process manifests as hypochromic microcytic anemia: a drop in erythrocyte indices (MCV, MCH, MCHC) and color index is recorded (e.g., a decrease from 0.9 to 0.7 with a hemoglobin drop from 120 to 75 g/L).
Modern markers of erythropoiesis and therapy monitoring
Latest-generation analyzers provide specific parameters of the functional state of the bone marrow germ line, free from the drawbacks of traditional acute-phase proteins:
- Ret-He (reticulocyte hemoglobin content): a value less than 30.6 pg serves as the criterion for deficiency.
- HYPO-He (percentage of hypochromic erythrocytes): exceeding the 5% level confirms functional insufficiency.
The key advantage of the Ret-He and HYPO-He parameters is their independence from gestation, uremia, and inflammatory reactions. During iron supplementation, ferritin reacts with a predictable delay of 2–3 weeks. In contrast, the increase in the Ret-He index outpaces both the classical reticulocyte crisis and the rise in systemic hemoglobin, being the earliest laboratory evidence of ferrotherapy efficacy.
The WHO triad for overt anemia is the "Three arrows" rule: hemoglobin drops below the age-related norm (↓), ferritin stores drop below 12 µg/L (↓), and soluble transferrin receptors of starved tissues compensatorily rise above 7 mg/L (↑).
Class and exam questions
Why does ferritin not rise immediately after starting iron supplementation?
Intracellular iron storage by ferritin occurs gradually. Its serum level rises with a physiological delay of two to three weeks from the start of therapy, which is why it is used for delayed monitoring of store accumulation.
How does latent deficiency fundamentally differ from overt anemia?
In the latent form, only biochemical stores are depleted (ferritin and serum iron are reduced), while hemoglobin remains within normal limits. In overt anemia, the hemoglobin level drops below the age-related boundary.
What is the advantage of Ret-He and HYPO-He parameters over the ferritin level?
Ferritin is an acute-phase protein and is falsely elevated in inflammation. The Ret-He and HYPO-He indices are resistant to concomitant inflammation, uremia, and pregnancy, and the increase in Ret-He is the first to confirm the response to iron preparations.
What else to study on this topic
- Features of interpreting reference intervals of pediatric ferritin depending on the test systems used
- Degree of transferrin saturation with iron as a calculated parameter of the transport pool
- Dynamics of the color index and MCV, MCH, MCHC indices in posthemorrhagic anemia
- Assessment of hematocrit, reticulocytes, and total red blood cell count in absolute iron deficiency
- Comparative informative value of radioimmunoassay and WHO/UNICEF standards in assessing ferritin
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