When you have low hemoglobin and hematocrit on a complete blood count (CBC); when your healthcare practitioner suspects that you may have too little iron (deficiency) or too much iron (overload) in your body
A blood sample drawn from a vein in your arm
You may be instructed to have your blood drawn in the morning and/or fast for 12 hours before sample collection; in this case, only water is permitted. Follow any instructions from your healthcare practitioner and/or from the laboratory performing the test.
Iron is an essential nutrient that, among other functions, is required for the production of healthy red blood cells (RBCs). It is a critical part of hemoglobin, the protein in RBCs that binds oxygen in the lungs and releases it as blood circulates to other parts of the body. The serum iron test measures the amount of iron in the liquid portion of blood.
Serum iron is almost always measured with other iron tests, such as serum ferritin, transferrin and total iron-binding capacity (TIBC). These tests are often ordered at the same time and the results interpreted together to help diagnose and/or monitor iron deficiency or iron overload.
The body cannot produce iron and must absorb it from the foods we eat or from supplements. Once absorbed, it is transported throughout the body by binding to transferrin, a protein produced by the liver.
In healthy people, most of the iron absorbed is incorporated into the hemoglobin inside RBCs. The remainder is stored in the tissues as ferritin or hemosiderin, with additional small amounts used to produce other proteins such as myoglobin and some enzymes.
When the level of iron is insufficient to meet the body's needs, the iron level in the blood drops and iron stores are depleted. This may occur because:
- Not enough iron is consumed (either foods or supplements)
- The body is unable to absorb iron from the foods eaten in conditions such as celiac disease
- There is an increased need for iron, such as during pregnancy or childhood, or due to a condition that causes chronic blood loss (e.g., peptic ulcer, colon cancer)
Insufficient levels of circulating and stored iron may eventually lead to iron deficiency anemia (decreased hemoglobin). In the early stage of iron deficiency, no physical effects are usually seen and the amount of iron stored may be significantly depleted before any signs or symptoms of iron deficiency develop. If a person is otherwise healthy and anemia develops over a long period of time, symptoms seldom appear before the hemoglobin in the blood drops below the lower limit of normal.
However, as the iron-deficiency progresses, symptoms eventually begin to develop. The most common symptoms of anemia include fatigue, weakness, dizziness, headaches and pale skin. (Read the article on Anemia to learn more.)
On the other hand, too much iron can be toxic to the body. Iron blood levels and iron storage increase when more iron is absorbed than the body needs. Absorbing too much iron can lead to progressive accumulation and damage to organs such as the liver, heart, and pancreas. An example of this is hemochromatosis, a rare genetic disease in which the body absorbs too much iron, even on a normal diet. Additionally, iron overdose can occur when someone consumes more than the recommended amount of iron.
How is it used?
The serum iron test is used to measure the amount of iron that is in transit in the body – the iron that is bound to transferrin in the blood. Along with other tests, it is used to help detect and diagnose iron deficiency or iron overload. Testing may also be used to help differentiate various causes of anemia.
The amount of iron present in the blood will vary throughout the day and from day to day. For this reason, serum iron is almost always measured with other iron tests, including ferritin, transferrin, and calculated total iron-binding capacity (TIBC) and transferrin saturation.
When is it ordered?
Serum iron tests may be ordered as follow-up tests when results from a complete blood count (CBC) show that a person's hemoglobin and hematocrit are low and their red blood cells are smaller and paler than normal (microcytic and hypochromic), suggesting iron deficiency anemia even though other clinical symptoms may not have developed yet.
- Chronic fatigue/tiredness
Testing may be ordered when iron overload is suspected. Signs and symptoms will vary from person to person and tend to worsen over time. They are associated with iron accumulation in the blood and tissues. These may include:
- Joint pain
- Fatigue, weakness
- Lack of energy
- Abdominal pain
- Loss of sex drive
- Organ damage, such as in the heart and liver
When a child is suspected to have ingested iron tablets, a serum iron test is ordered to detect and help assess the severity of the poisoning.
Iron tests may also be ordered periodically when iron deficiency or overload is being treated to evaluate the effectiveness of treatment.
What does the test result mean?
Serum iron levels are often evaluated in conjunction with other iron tests. A summary of the changes in iron tests seen in various diseases of iron status is shown in the table below.
Disease Iron TIBC/Transferrin UIBC Ferritin Iron Deficiency Low High High Low Low Hemochromatosis/Hemosiderosis High Low Low High High Chronic Illness Low Low/Normal Low/Normal Low/Normal High/Normal Hemolytic Anemia High Normal/Low Low/Normal High High Sideroblastic Anemia Normal/High Normal/Low Low/Normal High High Iron Poisoning High Normal Low High Normal
The early stage of iron deficiency is the slow depletion of iron stores. This means there is still enough iron to make red cells but the stores are being used up without adequate replacement. The serum iron level may be normal in this stage, but the ferritin level will be low.
As iron deficiency continues, all of the stored iron is used and the body tries to compensate by producing more transferrin to increase iron transport. The serum iron level continues to decrease and transferrin and TIBC and UIBC increase. As this stage progresses, fewer and smaller red blood cells are produced, eventually resulting in iron deficiency anemia.
If the iron level is high, the TIBC, UIBC and ferritin are normal and the person has a clinical history consistent with iron overdose, then it is likely that the person has iron poisoning. Iron poisoning occurs when a large dose of iron is taken all at once or over a short period of time. Iron poisoning in children is almost always acute, occurring in children who ingest their parents' iron supplements. In some cases, acute iron poisoning can be fatal.
A person who has mutations in the HFE gene is diagnosed with hereditary hemochromatosis. However, while many people who have hemochromatosis will have no symptoms for their entire life, others will start to develop symptoms such as joint pain, abdominal pain, and weakness in their 30's or 40's. Men are affected more often than women because women lose blood during their reproductive years through menstruation.
Iron overload may also occur in people who have hemosiderosis and in those who have had repeated transfusions. This may occur with sickle cell anemia, thalassemia major, or other forms of transfusion-dependent anemia. The iron from each transfused unit of blood stays in the body, eventually causing a large buildup in the tissues. Some people with alcoholism and with chronic liver disease also develop iron overload.
Is there anything else I should know?
Recent consumption of iron-rich foods or iron supplements can affect test results, as can recent blood transfusions.
Normal iron levels are maintained by a balance between the amount of iron taken into the body and the amount of iron lost. Normally, a small amount of iron is lost each day, so if too little iron is taken in, a deficiency will eventually develop. Unless a person has a poor diet, there is usually enough iron to prevent iron deficiency and/or iron deficiency anemia in healthy people.
In certain situations, there is an increased need for iron. Persons with chronic bleeding from the digestive tract (usually from ulcers or tumors such as colorectal cancer) or women with heavy menstrual periods will lose more iron than normal and can develop iron deficiency. Women who are pregnant or breast feeding lose iron to their baby and can develop iron deficiency if not enough extra iron is taken in. Children, especially during times of rapid growth, may need extra iron and can develop iron deficiency. Iron deficiency can also be seen in malabsorption diseases such as celiac disease.
Low serum iron can also occur in states where the body cannot mobilize and use storage iron properly. In many chronic inflammatory conditions, especially in cancers, autoimmune diseases, and with chronic inflammations or chronic infections (including AIDS), the body cannot properly use iron to make more red cells. Under these conditions, production of transferrin decreases, and serum iron is low because little iron is being absorbed from the gut and storage iron can't get mobilized, and ferritin increases.
Do all people with iron deficiency have anemia?
Iron deficiency refers to a decrease in the amount of iron stored in the body, while iron deficiency anemia refers to a drop in the number of red blood cells (RBCs), hemoglobin and hematocrit caused by not having enough stored iron (there are many other causes of anemia). It typically takes several weeks after iron stores are depleted for the hemoglobin level to decrease and production of RBCs to be affected and for anemia to develop. There usually are few symptoms seen in the early stages of iron deficiency, but as the condition worsens and blood levels of hemoglobin and RBCs decrease, ongoing weakness, pallor, and fatigue can eventually develop.
As your iron continues to be depleted, you may have shortness of breath and dizziness. If the anemia is severe, chest pain, headaches, and leg pains may occur. Children may develop learning (cognitive) disabilities. Besides the general symptoms of anemia, there are certain symptoms that are characteristic of iron deficiency. These include dysphagia, pica (cravings for specific substances, such as ice, corn starch, licorice, chalk, dirt, or clay), a burning sensation in the tongue or a smooth tongue, sores at the corners of the mouth, and spoon-shaped fingernails and toenails.
What foods can I eat to increase the iron in my diet?
The form of iron that is easiest for the body to absorb is found in meats and eggs. Other iron-rich sources include: green leafy vegetables (such as spinach, collard greens, and kale), wheat germ, whole grain breads and cereals, raisins, and molasses.
Who needs iron supplements?
The people who typically need iron supplements are pregnant women and those with documented iron deficiency. People should not take iron supplements before talking to their healthcare practitioner as excess iron can cause chronic iron overload. An overdose of iron pills can be toxic, especially to children.
Does anemia due to iron deficiency happen quickly or does it take a long time?
Iron deficiency anemia comes on gradually. When your rate of iron loss exceeds the amount of iron you absorb from the foods you eat or supplements, iron stores are slowly used up. At this stage, ferritin will be low, but serum iron and TIBC are usually normal and there is no anemia. As the iron deficiency worsens, serum iron levels fall, TIBC and transferrin rise, and anemia starts to develop. With prolonged or severe iron deficiency, the red cells become small and pale due to decreased hemoglobin levels. Reticulocyte count also decreases.
Does donating blood affect my iron level?
Yes. Every time you donate a pint of blood, your body loses some iron. The level of ferritin, which is a reflection of the total amount of storage iron in the body, drops with each donation and then returns to normal over time. Other tests, such as serum iron and TIBC, are not as affected by blood donation.
My doctor instructed me to take my iron supplement and then have an iron test done. Why would that be?
Your doctor may suspect that you are not absorbing the iron you need from your supplements and your diet, so may ask that you have your iron level checked shortly after you take your iron supplement. If you take iron and then have an abnormally low test result, you may have an underlying condition affecting the absorption of iron. You may need to be treated for the condition causing the malabsorption for your iron levels to return to normal.