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Blood Draws: Hemoglobin A1c for CFRD testing

One of the blood tests that helps determine the concentration of glucose (blood sugar) in the body is the hemoglobin a1c. This test goes by a variety of names including HbA1c, glycated hemoglobin, or glycosylated hemoglobin. It is usually part of the annually scheduled lab tests for patients with cystic fibrosis or cystic fibrosis related diabetes (CFRD).

Red blood cells (RBCs) have a life span of up to 180 days. The lab technician calculates how much sugar is "sticking" to the RBCs obtained from a blood draw. The results of the test tell the doctor the average concentration of glucose in the bloodstream over the last two to three months.

How the Sugar Sticks to Hemoglobin

Hemoglobin are proteins whose function is to carry oxygen in the blood. As these proteins move throughout the body, they inevitably will come into contact with glucose. The glucose can bind to the hemoglobin A, which is a specific and prevalent type of hemoglobin. When bonding occurs, the hemoglobin is said to be glycosylated. Naturally, when there is more glucose in the blood, more of the sugar will bind to the hemoglobin.

Think of it as putting a spoonful of sugar into a bowl of Cheerios. The more sugar you sprinkle in, the greater number of granules will likely land on each individual "O."

What is the test like?

There are two ways the blood sample can be obtained. One is by accessing the patient's vein and drawing the blood into a test tube. The other is by using a lancet--sometimes called a finger stick--to pierce the skin on the tip of a finger and squeeze out a small amount of blood. If other blood work is being done at the same time, venipuncture is the most likely method of retrieval. If the HbA1c is being done in conjunction with a fasting Oral Glucose Tolerance Test, a small puncture may be all that's required.

Most HbA1c tests are conducted in a laboratory, but home tests are also available by prescription. There is some debate as to whether the two are equally accurate. The home test may not be available to every one depending on their individual health care coverage.

Interpreting the Results

The results of the HbA1c test are expressed as percentage points. Each percent corresponds to a concentration of blood sugar equal to 30mg/dL (1.67 mmol/L). A person with normal, steady levels of glucose in their blood will have a HbA1c equal to 6%. This is equivalent to 135mg/dL or 7.5mmol/L. As blood sugar levels increase, so does the measured percentage. Levels between 6% and 6.5% are acceptably normal. Someone with impaired glucose tolerance or CFRD may have a test result of greater than 8%.

It is not uncommon for a person with impaired glucose tolerance to have slightly elevated HbA1c levels during periods of illness or infection.

- Lauren Beyenhof