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Iron-Status Indicators

In This Section

Ferritin, Iron, Total iron-binding capacity, Transferrin saturation, Protoporphyrin

Background

Iron functions as a component of proteins and enzymes. Almost two-thirds of the iron in the body (approximately 2.5 grams of iron) is found in hemoglobin, the protein in red blood cells that carries oxygen to tissues, and about 15 percent is in the myoglobin of muscle tissue. The average American diet provides 10–15 milligrams (mg) of iron daily in the form of heme and nonheme iron. Heme iron is found in animal foods that originally contained hemoglobin and myoglobin, such as red meat, fish, and poultry. Nonheme iron is found in plant foods, such as lentils and beans, and also is provided in iron-enriched and iron-fortified foods. Although heme iron is absorbed better than nonheme iron, most dietary iron is nonheme iron (Miret 2003). Each day the body absorbs approximately 1–2 mg of iron to compensate for the 1–2 mg of iron that the (nonmenstruating) body loses (Institute of Medicine 2001).

Transporting iron from one organ to another is accomplished by the reversible binding of iron to the transport protein, transferrin, which will then form a complex with a highly specific transferrin receptor (TfR) located on the plasma membrane surfaces of cells. Intracellular iron availability is regulated through the increased expression of cellular TfR concentration by iron-deficient cells. Ferritin is the major iron-storage compound: its production increases in cells as iron supplies increase. Although all cells are capable of storing iron, the liver, spleen, and bone marrow cells are primary iron-storage sites in people (Institute of Medicine 2001).

Iron deficiency and iron overload are the two major disorders of iron metabolism. Iron-deficiency anemia is the most severe form of iron deficiency. It is linked to many adverse consequences of iron deficiency, such as reduced physical capacity (Haas 2001) and poor pregnancy outcomes (Schorr 1994). Iron deficiency without anemia, however, has been linked to negative effects on cognitive development among infants and adolescents (Grantham-McGregor 2001; Beard 1999). Iron overload is the accumulation of excess iron in body tissues, and it usually occurs as a result of a genetic predisposition to absorb iron in excess of normal but can also be caused by excessive ingestion of iron supplements or multiple blood transfusions (Pietrangelo 2004). In advanced stages of iron overload disease (hemochromatosis), the iron accumulates in the parenchymal cells of several organs, but particularly the liver, followed by the heart and pancreas; this condition can lead to organ dysfunction and even death (Pietrangelo 2004).

The Recommended Dietary Allowance (RDA) for all age groups of men and postmenopausal women is 8 mg per day; the RDA for premenopausal women is 18 mg per day. The Tolerable Upper Uptake Level for adults is 45 mg per day of iron, a level based on gastrointestinal distress as an adverse effect (Institute of Medicine 2001).

Medical technologist places samples for ferritin measurement into a clinical analyzer. Clinical laboratories typically use conventional units for iron-status indicators: iron, total iron-binding capacity (TIBC), and erythrocyte protoporphyrin (EPP) are calculated in micrograms per deciliter (µg/dL), ferritin in nanograms per milliliter (ng/mL). Conversion factors to international system (SI) units are as follows: 1 µg/dL = 0.179 micromole per liter (µmol/L) for iron and TIBC, 1 µg/dL = 0.01777 µmol/L for EPP, and 1 ng/mL = 2.247 picomole (pmol)/L for ferritin.

Several methods are used to measure iron and related analytes. Serum iron concentration measures the amount of ferric iron (Fe³⁺) bound mainly to serum transferrin but does not include the divalent iron contained in serum as hemoglobin. Serum iron concentration is decreased in many people with iron-deficiency anemia and in people with chronic inflammatory disorders. Elevated concentrations of serum iron occur in iron-loading disorders such as hemochromatosis. Serum iron is not, however, a good indicator of iron stores and is not a sensitive measure of iron deficiency, partly because of daily fluctuations. For enhanced utility, serum iron measurements are used in conjunction with TIBC measurements. Normally, because only about one third of the iron-binding sites of transferrin are occupied by Fe³⁺, serum transferrin has considerable reserve iron-binding capacity. TIBC is a measurement of serum transferrin after saturation of all available binding sites with reagent iron. Concentrations of serum TIBC vary with the type of iron-metabolism disorder. For example, in iron deficiency TIBC is often increased, and in chronic inflammatory disorders, malignancies, and hemochromatosis, it is often decreased. The ratio of serum iron to TIBC is called transferrin saturation. Low iron values in conjunction with elevated TIBC values (or specifically measured transferrin concentrations), yielding less than 16 percent transferrin saturation, generally indicate iron-deficiency anemia (World Health Organization 2001). Transferrin saturation values in excess of 60 percent may be indicative of hemochromatosis or iron overload (World Health Organization 2001).

Ferritin is present in the blood in very low concentrations. Plasma ferritin is in equilibrium with body stores, and its concentration declines early in the development of iron deficiency. Low serum ferritin concentrations thus are sensitive indicators of iron deficiency. Ferritin is also an acute-phase protein; acute and chronic diseases can result in increased ferritin concentration, potentially masking an iron-deficiency diagnosis. The generally accepted cut-off level for serum ferritin below which iron stores are considered to be depleted is 15 ng/mL for people aged 5 years and older and 12 ng/mL for people younger than 5 years of age (World Health Organization 2001).

Finally, when iron delivery to the bone marrow is not sufficient for maintaining the incorporation of iron into newly synthesized globin and porphyrin protein, EPP concentrations increase. Yet EPP is not useful to distinguish iron deficiency from infection and also elevates in response to lead poisoning (Roels 1975). As a result, the measurement of EPP is most useful in settings where iron deficiency levels are common and where infections, lead poisoning, and other forms of anemia are rare. The generally accepted cut-off level for EPP is 80 µg/dL red blood cells for people aged 5 years and older and 70 µg/dL red blood cells for children younger than 5 years of age (World Health Organization 2001).

More information on iron–status indicators is available online:

Three national health objectives for Healthy People 2010 relate to iron deficiency reduction: Objective 19–12 (reduce iron deficiency among young children and females of childbearing age), Objective 19–13 (reduce anemia among low-income pregnant females in their third trimester), and Objective 19–14 (reduce iron deficiency among pregnant females) (U.S. Department of Health and Human Services 2000).

To address the changing epidemiology of iron deficiency in the United States, CDC staff, in consultation with outside experts, developed recommendations in 1998 for use by primary health-care providers to prevent, detect, and treat iron deficiency (U.S. Centers for Disease Control and Prevention 1998). Since the inception of NHANES in 1971, monitoring the iron status of the U.S. population has been an important component. To provide the best possible assessment of this element, each NHANES has included a battery of hematologic and biochemical indicators of iron status (Looker 1995). Since NHANES II (1976–1980), models that employ multiple biochemical iron-status indicators have been used to define iron deficiency in the population (Pilch 1984). The three-indicator model, using serum ferritin, transferrin saturation, and erythrocyte protoporphyrin, was developed in 1980 and applied to NHANES III (1988–1994) as well as to the other most recent surveys that became continuous beginning in 1999.

Reference data for hematologic and iron-related analytes were published for NHANES II (Fulwood 1982) and NHANES III (Hollowell 2005). Prevalence estimates of iron deficiency using the three-indicator model were similar in NHANES III (Looker 1997) and in NHANES 1999–2000 (Looker 2002). In NHANES 1999–2000, the estimated prevalence of iron deficiency was greatest among toddlers aged 1–2 years (7 percent) and adolescent and adult females aged 12–49 years (9 percent to 16 percent). The prevalence of iron deficiency was approximately two times higher among non-Hispanic black and Mexican-American females (19 percent to 22 percent) than among non-Hispanic white females (10 percent). Across all age and sex groups in the United States, iron-deficiency anemia has an estimated prevalence of less than 5 percent.

Selected Observations and Highlights

The following example observations and figures are taken from the tables of 1999–2002 data (for ferritin) and 1999–2000 data (for all other iron-status indicators) contained in this report. Statements about categorical differences between demographic groups noted below are based on non-overlapping confidence limits from univariate analysis without adjusting for demographic variables (e.g., age, sex, race/ethnicity) or other determinants of these blood concentrations (e.g., dietary intake, supplement usage, smoking, BMI). A multivariate analysis may alter the size and statistical significance of these categorical differences. Furthermore, additional significant differences of smaller magnitude may be present despite their lack of mention here (e.g., if confidence limits slightly overlap or if differences are not statistically significant before covariate adjustment has occurred). For a selection of citations of descriptive NHANES papers related to these biochemical indicators of diet and nutrition, see Appendix E.

General Observations

Among all age groups, 1–5 year-old children have the lowest ferritin concentrations. Children up to age 11 have lower transferrin saturation levels than do adolescents or adults.
Women 12 years and older are more likely to be defined as iron deficient than are men. These women have lower concentrations of serum ferritin, lower transferrin saturation, and higher EPP concentrations.
Mexican Americans have lower serum ferritin and higher EPP concentrations than do either non-Hispanic whites or non-Hispanic blacks.
Non-Hispanic blacks have lower serum transferrin saturation levels than do non-Hispanic whites.
Mexican-American and non-Hispanic white children (aged 1–5 years) have lower serum ferritin concentrations than do non-Hispanic black children.
Mexican-American children (aged 1–5 years) have higher EPP concentrations than do non-Hispanic black or non-Hispanic white children.
Mexican-American women of childbearing age (aged 20–39 years) have lower serum ferritin concentrations than do non-Hispanic white women. Concentrations for non-Hispanic black women of childbearing age fall between those of Mexican-American and non-Hispanic white women.
Non-Hispanic black women of childbearing age (aged 20–39 years) have lower serum transferrin saturation levels than do non-Hispanic white women. Serum transferrin saturation levels for Mexican-American women of childbearing age fall between levels for non-Hispanic white and Mexican-American women of childbearing age.

Highlights

Because children and women have lower serum ferritin and transferrin saturation levels than do men and older people (≥ 60 years), children and women are at greater risk for iron deficiency.

Two minority groups, non-Hispanic blacks and Mexican Americans, typically are at greater risk for iron deficiency than are non-Hispanic whites.

At least 5 percent of persons in each age group, except for older people (≥ 60 years), have low serum ferritin concentrations (< 12 ng/mL for children younger than 5 years and < 15 ng/mL for people aged 5 years and older) that are consistent with depleted iron storage.

At least 10 percent of persons in each age group have low transferrin saturation levels (< 16 percent), which are indicative of iron deficiency.

Tables

Serum ferritin

Serum iron

Serum total iron-binding capacity

Serum transferrin saturation

Erythrocyte protoporphyrin

References

Beard JL. Iron deficiency and neural development: an update. Arch Latinoam Nutr. 1999;49(3 Suppl 2):34S-9S.

Fulwood R, Johnson CL, Bryner JD. Hematological and nutritional biochemistry reference data for persons 6 months–74 years of age: United States, 1976–1980. National Center for Health Statistics, Vital Health Stat Series 11(232), 1982.

Grantham-McGregor S, Ani C. A review of studies on the effect of iron deficiency on cognitive development in children. J Nutr. 2001;131(2S-2):649S-6S.

Haas JD, Brownlie T 4th. Iron deficiency and reduced work capacity: a critical review of the research to determine a causal relationship. J Nutr. 2001;131:691S-6S.

Hollowell JG, Van Assendelft OW, Gunter EW. Hematological and iron-related analytes – Reference data for persons aged 1 year and over: United States, 1988–1994. National Center for Health Statistics, Vital Health Stat Series 11(247), 2005.

Institute of Medicine, Food and Nutrition Board. Dietary reference intakes: vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium and zinc. Washington, D.C.: National Academy Press; 2001.

Looker AC, Gunter EW, Johnson CL. Methods to assess iron status in various NHANES surveys. Nutr Rev. 1995;53:246-54.

Looker AC, Dallman PR, Carroll M, Gunter EW, Johnson CL. Prevalence of iron deficiency in the United States. JAMA. 1997;277:973-5.

Looker AC, Cogswell ME, Gunter EW. Iron deficiency, United States, 1999–2000. Morb Mortal Wkly Rep. 2002;51:897-9.

Miret S, Simpson RJ, McKie AT. Physiology and molecular biology of dietary iron absorption. Annu Rev Nutr. 2003;23:283-301.

Pietrangelo A. Hereditary hemochromatosis – a new look at an old disease. N Engl J Med. 2004;350:2382-97.

Pilch SM, Senti FR, editors. Assessment of iron nutritional status of the U.S. population based on data collected in the Second National Health and Nutrition Examination Survey, 1976–1980. Bethesda (MD): Federation of American Societies for Experimental Biology; 1984.

Roels HA, Lauwerys RR, Buchet JP, Vrelust M-Th. Response of free erythrocyte porphyrin and urinary δ-aminolevulinic acid in men and women moderately exposed to lead. Int Arch Arbeitsmed. 1975;34:97-108.

Schorr TO, Hediger ML. Anemia and iron-deficiency anemia: compilation of data on pregnancy outcome. Am J Clin Nutr. 1994;59(Suppl):492S-501S.

U.S. Centers for Disease Control and Prevention. Recommendations to prevent and control iron deficiency in the United States. Morb Mortal Wkly Rep. 1998;47(RR-3):1-36.

U.S. Department of Health and Human Services. Healthy people 2010: understanding and improving health. 2nd ed. Washington, D.C.: U.S. Government Printing Office; November 2000.

World Health Organization (WHO). Iron deficiency anaemia – Assessment, prevention, and control. A guide for programme managers. Geneva: World Health Organization; 2001 (WHO/NHD/01.3) [cited 2008]. Available from: http://www.who.int/nutrition/publications/en/ida_assessment_prevention_control.pdf

National Report on Biochemical Indicators of Diet and Nutrition in the U.S. Population 1999–2002