Iron Tests

Definition

Iron tests consist of four assays performed on serum or plasma to aid in the diagnosis and treatment of iron deficiency or iron overload. These tests are serum iron, total iron binding capacity (TIBC), serum ferritin, and serum transferrin. Iron is an essential trace element needed for the production of hemoglobin as well as the function of cytochromes (compound molecules that are important in cell respiration) and certain enzymes. Iron in plasma is almost entirely bound to transport proteins. The total iron binding capacity (TIBC) is the maximum amount of iron that these proteins can bind. Transferrin, a beta globulin (molecular weight 75,000) is the principal transport protein for iron in plasma. Therefore, the TIBC is determined mainly by the concentration of serum transferrin. Iron is stored in the epithelial cells of the gastrointestinal tract and in the reticuloendothelial cells of the liver, spleen, and bone marrow. Ferritin is the principal form of storage iron. It consists of a protein (apoferritin) and iron in the form of ferric salts.

Purpose

Serum or plasma iron tests are used for the following purposes:

• To help in the differential diagnosis of anemias. Iron deficiency anemia is the most common form of anemia worldwide and is quite common in the United States—especially in multiparous females, young children, and persons with chronic intestinal bleeding.
• To assess the severity of anemia and monitor the treatment of patients with chronic anemia.
• To diagnose conditions of iron excess, including iron ingestion, thalassemia, hemosiderosis, and hemochromatosis. Hemosiderosis and hemochromatosis are conditions produced by excessive iron stores in the tissues. Hemosiderosis, which results from repeated blood transfusions, is not associated with tissue damage. Hemochromatosis, which is a disorder of iron absorption, can cause painful joints, skin bronzing, diabetes, and liver damage if the iron concentration in the body is not lowered. Hemachromatosis is still underdiagnosed because of its long latency period and lack of awareness on the part of medical professionals.

A serum iron test can be used without the others to evaluate cases of iron poisoning.

Precautions

Collection of blood samples

Patients should not have their blood tested for iron within four days of a blood transfusion or tests and treatments that use radioactive materials. Recent high stress levels or sleep deprivation are additional reasons for postponing iron tests. Clinicians should ask if patients are taking oral contraceptives or multivitamins, since these may alter results.

Blood samples for iron tests should be taken early in the morning because serum iron levels vary during the day, being higher in the morning and lower at night. This precaution is especially important in evaluating the results of iron replacement therapy.

Hemolysis must be avoided during collection of blood samples to prevent interference with test results from iron in the red blood cells.

Interpretation of test results

Some acute and chronic illnesses can increase the release of ferritin from the body stores, resulting in high serum levels. These disorders include infections, late-stage cancers, lymphomas, and severe inflammations. Alcoholics often have high ferritin levels owing to liver inflammation.

Medications and substances that can cause increased serum iron levels include chloramphenicol, estrogen preparations, dietary iron supplements, alcoholic beverages, methyldopa, and birth control pills. Medications that can cause decreased iron levels include aspirin, cholestyramine, cortisone, methicillin, and testosterone.

Medications and treatments that can cause increased ferritin levels include dietary iron supplements, oral contraceptives, theophylline, and x-ray therapy. Decreases in ferritin levels are seen with antithyroid therapy and high doses of ascorbic acid.

Medications that can cause increased transferrin levels include cortisone and cortisol. Those that can cause decreased transferrin levels include oral contra-ceptives and carbamazepine.

Description

Iron tests are performed on samples of the patient's blood, withdrawn from a vein into a vacuum tube. The amount of blood taken is between 6 mL and 10 mL (1/3 of a fluid ounce). The procedure, which is called a venipuncture, takes about five minutes.

Iron level test

The iron level test measures the amount of iron in the blood serum that is being carried by a protein (transferrin) in the blood plasma. Serum iron is most often measured by colorimetric analysis. Iron is deconjugated from the transferrin by adding dilute acid or guinidinium. The iron is reduced to Fe²⁺ by ascorbic acid. The reduced iron forms coordinate bonds with the nitrogen groups (a chromophore) forming a colored complex. The most common chromophore is FerroZine which reacts with Fe²⁺ to form a magenta-colored complex that is measured at 570 nm. Thiourea is added to prevent a reaction between FerroZine and copper.

Total iron-binding capacity (TIBC) test

The TIBC test measures the amount of iron that the blood would carry if the transferrin were fully saturated. Since transferrin is produced by the liver, the TIBC can be used to monitor liver function and nutrition.

Transferrin test

The transferrin test is a direct measurement of transferrin—which is also called siderophilin—levels in the blood. Transferrin is most often measured by rate immunophelometry. Some laboratories prefer this measurement to the TIBC. The saturation level of the transferrin can be calculated by dividing the serum iron level by the TIBC.

Ferritin test

The ferritin test measures the level of a protein in the blood that stores iron for later use by the body. Ferritin is most often measured by double antibody sandwich immunoassay. It is the most sensitive indicator of iron deficiency because a low serum level reflects depleted body stores. The body stores must be fully depleted before the serum iron becomes low or iron deficiency anemia develops. In persons with acute and chronic illness, however, ferritin levels may not reflect the status of the iron stores since more ferritin escapes into the circulation in these conditions.

Preparation

Iron absorption and metabolism are influenced by several factors. These should be identified prior to testing via a medical history that includes the following:

• prescription medications and multivitamins that affect iron levels, absorption, or storage
• blood transfusion within the last four days
• recent extreme stress or sleep deprivation
• recent eating habits

Blood collected for iron level or TIBC tests should be collected following a 12-hour fast. Fasting is not required for serum or plasma ferritin.

Aftercare

Aftercare consists of routine care of the area around the venipuncture.

Complications

The primary complication is the possibility of a bruise or swelling in the area of the venipuncture. The patient can apply moist warm compresses if there is any discomfort.

Results

Iron level test

Normal serum iron values are as follows:

• Adult males: 65-175 micrograms/dL.
• Adult females: 50-170 micrograms/dL.
• Children: 50-120 micrograms/dL.
• Infant: 40-100 micrograms/dL.
• Newborns: 100-250 micrograms/dL.

TIBC test

Normal TIBC values are as follows:

• Adult males: 300-400 micrograms/dL.
• Adult females: 300-450 micrograms/dL.

Transferrin test

Normal transferrin values are as follows:

• Adults: 200-400 mg/dL.
• Children: 203-360 mg/dL.
• Newborns: 130-275 mg/dL.

Normal transferrin saturation values are between 30% and 40%.

Ferritin test

Normal ferritin values are as follows:

• Adult males: 20-300 ng/mL.
• Adult females: 20-120 ng/mL.
• Children (one month): 200-600 ng/mL.
• Children (two to five months): 50-200 ng/mL.
• Children (six months to 15 years): 7-140 ng/mL.
• Newborns: 25-200 ng/mL.

Abnormal test results

Serum iron level is increased in thalassemia, hemochromatosis, severe hepatitis, liver disease, lead poisoning, acute leukemia, and kidney disease. It is also increased by multiple blood transfusions and intramuscular iron injections.

Iron levels above 350-500 micrograms/dL are considered toxic; levels over 1000 micrograms/dL indicate severe iron poisoning.

Serum iron level is decreased in iron deficiency anemia, chronic blood loss, chronic diseases (lupus, rheumatoid arthritis), late pregnancy, chronically heavy menstrual periods, and thyroid deficiency.

Abnormal TIBC test

The TIBC is increased in iron deficiency anemia, polycythemia vera, pregnancy, blood loss, severe hepatitis, and the use of birth control pills.

The TIBC is decreased in malnutrition, severe burns, hemochromatosis, anemia caused by infections and chronic diseases, cirrhosis of the liver, and kidney disease.

Abnormal transferrin test

Transferrin is increased in iron deficiency anemia, pregnancy, hormone replacement therapy (HRT), and the use of birth control pills.

Transferrin is decreased in protein deficiency, liver damage, malnutrition, severe burns, kidney disease, chronic infections, and certain genetic disorders.

Abnormal ferritin test

Ferritin is increased in liver disease, iron overload from hemochromatosis, certain types of anemia, acute leukemia, Hodgkin's disease, breast cancer, thalassemia, infections, inflammatory diseases, and hemosiderosis. Ferritin levels may be normal or slightly above normal in patients with kidney disease.

Ferritin is decreased in chronic iron deficiency and severe protein depletion.

KEY TERMS

Anemia— A disorder marked by low hemoglobin levels in red blood cells, which leads to a decrease in the oxygen carrying capacity of the blood.

Chromophore— Any chemical group that produces color in a compound.

Cytochrome— A compound molecule consisting of a protein and a porphyrin ring. Cytochromes participate in cell respiration by electron transfer.

Ferritin— A protein found in the liver, spleen, and bone marrow that stores iron. Ferritin consists of a protein called apoferritin and iron in the form of ferric salts.

Hemochromatosis— A disorder of iron absorption characterized by increased iron absorption and excess deposition of iron in the tissues. It can cause painful joints, pancreatic, heart and liver damage if the iron concentration is not lowered.

Hemolysis— The breakdown of red blood cells with liberation of hemoglobin.

Hemosiderosis— An overload of iron in the body resulting from repeated blood transfusions. Hemosiderosis occurs most often in patients with thalassemia.

Iron poisoning— A potentially fatal condition caused by swallowing large amounts of iron dietary supplements. Most cases occur in children who have taken adult-strength iron formulas. The symptoms of iron poisoning include vomiting, bloody diarrhea, convulsions, low blood pressure, and turning blue.

Plasma— The liquid part of blood.

Siderophilin— Another name for transferrin.

Thalassemia— A hereditary form of anemia that occurs most frequently in people of Mediterranean origin.

Transferrin— A protein in the plasma that carries iron derived from food intake to the liver, spleen, and bone marrow.

Health care team roles

Iron tests may be ordered by physicians or by nurse practitioners. Blood samples are usually drawn by nurses or phlebotomists. The samples are analyzed in the laboratory by medical laboratory technicians, with the results returned to the physician.

Patient education

Patients should be informed of any abnormal test results. Health care professionals may refer patients with iron deficiency to a dietitian to discuss nutrition therapy. With regard to excessive iron storage, all health care professionals should monitor patients for signs of hemochromatosis, which is easily treated but fatal if untreated.

Resources

BOOKS

Burtis, Carl A., and Edward R. Ashwood. Teitz Textbook of Clinical Chemistry, 3rd ed. Washington, DC: American Association of Clinical Chemistry (AACC) Press, 1999.

Fischbach, Frances Talaska. A Manual of Laboratory and Diagnostic Tests. Philadelphia and New York: Lippincott, 1996.

Mosby's Diagnostic and Laboratory Test Reference, 5th ed. Edited by Kathleen Deska Pagana and Timothy James Pagana. St. Louis, MO: Mosby-Year Book, Inc., 2000.

Springhouse Corporation. Everything You Need to Know About Medical Tests, edited by Michael Shaw et al. Springhouse, PA: Springhouse Corporation, 1996.

PERIODICALS

Powell, Lawrie W., MD, et al. "Diagnosis of Hemachromatosis." Annals of Internal Medicine 129 (December 1, 1998): 925-931.