Iron is very important mineral for body and has critical role in function of cells. It has very important role in transport of oxygen. Iron is present in enzymes like cytochrome system in mitochondria. The amount of iron required varies among cells. Deficiency of iron result in various clinical disorders like iron deficiency anemia, hypo-proliferative disorders etc. Here we will discuss about the diagnostic laboratory tests for iron deficiency states.
Serum Iron
Free iron is toxic to the cells, so iron absorbed from diet or released form stores cannot circulate in its free form. So, iron is bound to transport protein called transferrin. So, serum iron level represent the iron bound to transferrin circulating in body. Normal range of serum iron is 50-150 µg/dL.
Total iron binding capacity
It is the maximum amount of iron that can bind to the transferrin molecules in blood. Transferrin is blood plasma glycoprotein containing two lobes (N-lobe and C-lobe). Not all transferrin carries iron at the time. So total iron binding capacity is measurement of potential for iron binding or the total amount of iron that can be bound. Its normal value is 300µg/dL-360µg/dL.
Serum Iron vs. TIBC
| Serum Iron | TIBC |
| It is amount of circulating iron bound to the transferrin.It is measure of amount of iron bound to transferrin.Normal value:It means 50-150 µg/dL of iron is present in 1 deciliter of blood. | It is maximum amount of iron that can bind to the transferrin in blood.It is indirect measure of circulating transferrin.Normal value:It means 300-360 µg/dL of iron can be bound in 1 deciliter of blood. |
Transferrin saturation
It is the ratio of serum iron and total iron binding capacity expressed in percentage. It is normally 25-50%.
Transferrin saturation= Total serum iron×100 ÷Total iron binding capacity
Serum ferritin
As iron is transported or circulated by transferrin, it is stored as complex protein called ferritin. Hemosiderin is iron storage complex formed by accumulation of ferritin with in reticulo-endothelial cells. Under steady state conditions, serum ferritin co-relates with total body iron stores. Ferritin level is different according to age and gender.
| Normal range (µg/L) | |
| New born | 25-200 |
| Children 6 months – 15 years | 7-140 |
| Women | 18-160 |
| Men | 18-270 |
Bone marrow iron stores evaluation
Reticulo-endothelial iron stores can be estimated from iron stain of bone marrow aspirate biopsy. When the marrow smear is stained for iron, 20-40% of developing erythroblasts (sideroblasts) will have visible ferritin granules in their cytoplasm.
Histological grading for bone marrow iron status according to Gale et al (1076–82).
| Grade 0 | None | None No visible iron under high power magnification (1000) |
| Grade 1 | Very slight | Small iron particles just visible in few reticulum cells underhigh power magnification (1000) |
| Grade 2 | Slight | Numerous small iron particles present in reticulum cells |
| Grade 3 | Moderate | Numerous small iron particles present in reticulum cellsthroughout the marrow fragment (100) |
| Grade 4 | Moderate heavy | Larger iron particles throughout the fragment with tendencyto aggregate into clumps (100) |
| Grade 5 | Heavy | Dense, large clumps of iron throughout the fragment (100) |
| Grade 6 | Very heavy | Very large deposits of iron, both intra- and extra-cellular,obscuring cellular detail in the fragment (100) |
Serum level of transferrin receptor protein
Transferrin receptor protein is released by cells into circulation. Cells can regulate iron uptake by modulating the expression transferrin receptor on cell surface and storing excess iron as ferritin. If iron supply declines the expression of transferrin receptor increases. This test is being increasingly used along with serum ferritin for increase in diagnostic sensitivity and specificity for diagnosis of iron deficiency states. Normal values are 4-9 µg/L.
Erythrocyte protoporphyrin levels
protoporphyrin is an intermediate in pathway of hame synthesis. In low iron level in body, hame synthesis is impaired and the protoporphyrin accumulates in red cells.
Most common causes of increased erythrocyte protoporphyrin are iron deficiency states and lead poisoning. Normal values are less than 50 µg/dL for red cells. Values greater than 100 µg/dL can be seen in iron deficiency states.
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References
- Harrison’s Principles of Internal Medicine, 20th Edition.
- Harper’s Illustrated Biochemistry, 31st Edition.
- Gale E, Torrence J, Bothwell T. The quantitative estimation of total iron stores in human bone marrow. J Clin Invest 1963;42:1076–82
- Lippincott Illustrated Reviews: Biochemistry, 7th Edition.
- World Health Organization, Serum ferritin concentrations for the assessment of iron status and iron deficiency in populations.
- Neil A. Came, Quality of bone marrow iron assessment.
