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
Serum or plasma iron tests are used for the following purposes:
A serum iron test can be used without the others to evaluate cases of iron poisoning.
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.
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 contraceptives and carbamazepine.
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.
Normal serum iron values are as follows:
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.
Normal transferrin values are as follows:
Normal transferrin saturation values are between 30% and 40%.
Normal ferritin values are as follows:
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Author Info: Jane E. Phillips, The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Nursing and Allied Health, 2002 |