Testing Iron levels: Understanding iron blood tests and anaemia

Anaemia is the term used to describe an abnormally low number of red blood cells in the blood. There are many causes for anaemia, each with its own treatment. Since the specific treatment for anaemia depends on the cause, it is important to identify the precise cause of anaemia. The blood tests used to diagnose causes of anaemia are complex, even for many physicians. As you can imagine, it can be especially difficult for patients with anaemia to interpret their test results. On the other hand, a handful of routine blood tests can help diagnose a very precise cause of anaemia in most patients. 

The initial diagnosis of anaemia

The first step in diagnosing anemia is usually based on a routine test called a complete blood count or CBC. The complete blood count is so common, it is one of the tests that defines “routine blood work.”

Within the complete blood count, will be a count of the number of red blood cells. If this number is low—below 4.7 million cells per microliter (mcL) in men or 4.2 million cells/mcL in women—the patient has anemia of some type. 

The other common tests that are used to make an initial diagnosis of anemia are hemoglobin and hematocrit. Hemoglobin and hematocrit are measured together.

• The normal hemoglobin level in men is 14 to 18 g/dL and 12 to 16 g/dL in women. 
• The normal range for hematocrit is between 40 and 54% for men and between 36 to 48% in women.

As with red blood cell count, numbers below these values indicate anaemia. Unfortunately, it is impossible to tell which type of anemia a person has with certainty based on CBC, hemoglobin, or hematocrit. For this reason, some additional investigation is usually needed.

Mean corpuscular volume (MCV)

A big clue on the CBC is mean corpuscular volume (MCV). Mean corpuscular volume (MCV) is the average volume of red blood cells. The normal range for MCV is approximately 80 to 100.

Anemia with a low MCV is called microcytic anemia (microcytic means “small cell”). Likewise, anemia with normal MCV is normocytic and anemia with high MCV is macrocytic anemia. When the MCV is below 80, it generally means the patient has some problem with iron levels or metabolism. It does not necessarily mean the patient has iron deficiency, though this is usually the case.

Anaemia with a normal MCV could mean the patient has experienced blood loss through internal bleeding, hemorrhage, bone marrow suppression, sickle cell disease, renal insufficiency, or the red blood cells are breaking apart for some reason. 

The cause of normocytic anemia can often be found with additional tests such as creatinine (to diagnose renal insufficiency) or peripheral blood smear (for sickle cell anaemia).

When the MCV is above 100, the patient may have B12 deficiency, folate deficiency, or both. Macrocytic anaemia may also be caused by a myelodysplastic disorder, leukemia, liver disease, hypothyroidism, or is the effect of certain drugs.

A focus on microcytic anemia

If someone has microcytic anemia, i.e. and MCV below 80 fL, additional tests may be ordered to investigate how the body is handling iron. The three main tests ordered when microcytic anemia is diagnosed are serum iron, serum ferritin, and TIBC. These three values can help distinguish between various causes of microcytic anemia .

Iron deficiency anemia is usually associated with low serum iron, low serum ferritin, and high TIBC levels. TIBC stands for total iron-binding capacity, and is equivalent to transferrin. Importantly, transferrin is different than transferrin saturation. Transferrin saturation is the ratio of serum iron to TIBC, multiplied by 100.

In iron deficiency anemia, transferrin (TIBC) is high while transferrin saturation is very low. This is because serum iron is the numerator (top number) of the ratio and TIBC is the bottom number (denominator). Since the top number of the ratio, serum iron, is lower than normal and the bottom number, TIBC, is higher than normal, the ratio becomes much lower than normal.

Unsaturated iron-binding capacity or UIBC is used less frequently to determine the cause of anemia. UIBC is simply TIBC minus serum iron.

Iron deficiency

Iron deficiency is the most common nutritional disorder on the globe. About 50% of all cases of anemia are due to iron deficiency. Most people who are deficient and have a low serum iron will also be anemic, since iron is required for the proper function of haemoglobin and the normal production of red blood cells. Nevertheless, some people may have iron deficiency without anemia. 

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