LABORATORY ASSESSMENT OF BODY IRON STATUS
Classic iron studies:
◻ Serum Iron (SI)
◻ Total Iron-Binding Capacity (TIBC)
◻ Percent Transferrin Saturation
◻ Prussian Blue Staining of Tissues
◻ Ferritin
Special Test
◻ Soluble transferrin receptor (sTfr)
◻ Hemoglobin content of reticulocyte (CHr)
◻ RBC zinc protoporhyrin level
Performed when results of routine assays are equivocal or too invasive
Serum Iron
◻ Represent the number of bound iron
◻ Spectrophotometry
Ferrozine as reagent
■ Subject to diurnal variation
•High within -day or between-day variability
■ Increases in recent ingestion of iron containing foods/supplement
•Fasting early morning specimen (?)
•Early morning specimen
Variation in hepcidin contributes to serum iron variability
TIBC
The amount of iron in plasma or serum will be limited by the amount of transferrin that is available to carry it.
◻Measure the availability of iron binding sites on transferrin
Add excess ferric iron to specimen
Unbound iron are precipitated w/ magnesium carbonate powder
❖ Result: amount of iron detected represents all the binding sites available for transferrin
Indirect measure of transferrin
■ 1g transferrin binds 1.4 mg of iron
■ Expressed as an iron value
Percent Transferrin Saturation
◻ Degree to which the available sites are occupied by iron
SI/TIBC X 100 = % Transferrin Saturation
■ It is important that SI & TIBC be expressed in the same units.
❖ Rule of thumb: 1/3 of transferrin is saturated with iron
Prussian Blue Staining of Tissues
◻ Gold standard for assessment of body iron
◻ Potassium ferrocyanide
Ferric ion in tissue reacts with reagent
■ Dark blue dots (microscopic)
■ scored semiquantitatively
■ bone marrow; liver
Ferritin
Assessment of stored body iron
Currently accepted assay for diagnosing stored iron (immunoassay)
Highly indicative of iron deficiency
■ Level of serum ferritin highly correlates with Prussian blue stain of BM
DRAWBACK:
Acute phase reactant protein
■ Falsely elevated in:
•Acute inflammation (e.g. bacterial infection)
VALUES:
■ Values between 20 – 100 ng/ml is equivocal
■ Low ferritin result shows high predictive value for iron deficiency
Soluble Transferrin Receptor (sTfR)
◻ The amount of TfR on membrane is based on the amount of intracellular iron
decreased intracellular iron = increase in expressed TfR
Increased sTfR
■ Increase TfR1 on individual cells
•iron deficiency
■ increase in number of cells, each with a normal number of TfRs
•rapid erythropoiesis
•Hemolytic anemia
Hemoglobin content of reticulocyte (CHr)
◻ Amount of hemoglobin in reticulocytes
Analogous to MCH (reticulocyte)
■ # of reticulocytes represents the status of erythropoiesis
■ CHr will decrease when iron for erythropoiesis is restricted
■ Effective early indicator of iron deficiency early in erythrocyte hemoglobinization.
❖ Increases if reticulocyte circulate longer in response to anemia
❖ Separate reference intervals may be provided for children and infants.
Soluble Transferrin Receptor/Log Ferritin
◻ Ratio of sTfR to ferritin or sTfR to log ferritin
Improves the identification of iron deficiency when values of ferritin & sTfR are equivocal E.g.
■ increased sTfR – iron deficiency
■ Decreased ferritin
■ Decreased log ferritin
❖ Ratio is useful when one of the parameters has changed but is not outside the reference interval
Thomas Plot
◻ The plot is based on the relation between the ratio of the soluble transferrin receptor value to the logarithm of the ferritin value (sTfR/log Ferr) and the reticulocyte hemoglobin equivalent (RET-He parameter )
Different assessment of iron status
◻ soluble transferrin receptor (sTfR)
◻ Ferritin
◻ log ferritin
◻ Ret-He
Can improve the identification of iron deficiency
Thomas Plot
sTfR- I
FeSrrTitiAn–TDUS
sTfR/log ferritin – H
CHr – L
PLOT – lower right quadrant
■ True Iron deficiency
Inflammation
◻ Ferritin – falsely H
◻ sTfR/log ferritin – N
◻ CHr – L
PLOT – lower left quadrant
Functional iron deficiency
■ Iron is adequate but not available for transport
Zinc Protoporphyrin (ZPP)
◻ Accumulate in RBCs when iron is NOT incorporated into heme and zinc binds instead to protoporphyrin IX
◻ Fluorescence
◻ Importance
elevated value is observed when the activity of ferrochelatase is impaired
•Lead poisoning
