Serum Iron Testing
When is serum iron testing indicated?
Serum iron tests are indicated when the results from a complete blood count (CBC) indicate that your pet is anemic (decreased red blood cell numbers and/or decreased hemoglobin) and that the red blood cells are microcytic (smaller than usual) and hypochromic (contain less hemoglobin than usual).
Why do these changes suggest iron deficiency?
Microcytic red blood cells are suggestive of iron deficiency because iron is needed for the production of hemoglobin. When less iron is available, less hemoglobin is made and the maturation sequence of red blood cells in the bone marrow is affected, resulting in smaller red blood cells that contain less hemoglobin.
What causes iron deficiency anemia?
"...chronic external blood loss can lead to iron deficiency anemia."
Because blood is a rich source of iron, chronic external blood loss can lead to iron deficiency anemia. Causes of chronic blood loss include large burdens of intestinal parasites such as hookworms, and external parasites such as fleas. Chronic blood loss from the gastrointestinal tract or from the urinary tract may also lead to iron deficiency anemia.
Young animals on a predominantly milk based diet may also experience a transient, mild iron deficiency anemia because milk is a poor source of iron. This condition does not usually require treatment in puppies and kittens that are otherwise healthy.
What specific blood tests are available to document iron deficiency anemia?
These tests require a single blood sample that is sent to a veterinary referral laboratory.
The amount of iron in serum can be determined. With iron deficiency, the serum iron concentration is expected to be low. However, serum iron can also be low with other illnesses, so this finding is not specific for iron deficiency anemia.
The total iron binding capacity (TIBC) of a serum sample can be determined. The TIBC does not specifically measure iron concentration, but rather measures the total amount of transferrin (a blood protein that transports iron) that is available for binding to and transporting iron. In many species, transferrin concentrations are increased in iron deficiency anemia. Although transferrin concentrations are not increased in iron deficiency anemia in the dog, TIBC measurement is still useful in the dog because by measuring the serum iron concentration and the TIBC, we can determine the % transferrin saturation.
The % transferrin saturation is essentially the amount of the iron transport protein that is currently being occupied by iron. The amount of binding or % saturation is decreased in iron deficiency anemia.
Serum ferritin determination can also be used to assess the iron status. Ferritin is a storage form of iron. A small amount of ferritin is normally found in the circulation. The circulating form is thought to be a good indicator of total body iron stores. Until recently, serum ferritin concentrations were available only for research purposes but certain referral laboratories are now offering this test.
Another less commonly used method of evaluating iron deficiency anemia is the measurement of erythrocyte protoporphyrin. Protoporphyrins are intermediate compounds formed during hemoglobin production. Protoporphyrin concentrations increase with iron deficiency.
"The most accurate, but most invasive, method of assessing iron stores is to look at the bone marrow (except in the cat)."
The most accurate, but most invasive, method of assessing iron stores is to look at the bone marrow (except in the cat). This test requires that a sample of the marrow be taken using a special bone marrow needle. The bone marrow sample can then be spread onto glass slides, or if a core of marrow tissue is obtained, then sections of this material can be examined. Either method requires that the sample be submitted to a referral laboratory where a veterinary pathologist examines both the stages of red blood cells present in the sample and the amount of stored iron. In order to assess the amount of iron present in a sample, special stains are used to highlight the iron stores. Little to no stainable iron is found in animals with iron deficiency.
Why may bone marrow evaluation not be useful in the cat?
Even healthy cats do not have an appreciable amount of stainable iron stores in their bone marrow. In spite of this, the pattern of red blood cell development in a bone marrow sample may still be useful in evaluating the possible cause of an anemia.
What tests can be done to try to determine the cause of the anemia?
A serum biochemistry profile, urinalysis, and fecal evaluation are often used as screening tests to determine the cause or source of the chronic blood loss.
The biochemistry profile may indicate low serum proteins; this finding confirms an external loss of blood. The biochemistry profile may indicate that there is ongoing intestinal bleeding by an elevation in a substance called urea.
A urinalysis (urine sample) can be evaluated for the presence of red blood cells. As well, an examination of the cellular components of a urine sample (termed urine sediment examination) may alert us to the presence of unusual cells that might suggest an underlying bladder tumor as the source of the blood loss.
A fecal sample can be analyzed for the presence of intestinal parasites that may be contributing to blood loss via the gastrointestinal tract.
Depending upon what is found with these screening tests, further diagnostic testing may be required to pinpoint the source of chronic blood loss.
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