There are four types of blood glucose tests: a fasting blood glucose test, a post-prandial blood glucose test, an oral glucose tolerance test and an A1C test. These tests calculate the concentration of glucose in the blood. Glucose levels are measured to determine if a person has pre-diabetes or diabetes, or another metabolic disorder that raises glucose levels above normal. Other causes of hyperglycemia include medications such as corticosteroids, hyperthyroidism, infection and stress. For a glucose test, a phlebotomist or nurse takes a blood sample and trained laboratory personnel calculate the glucose concentration in the blood. The results are sent back to a health professional for reporting to a patient.
Blood Sample Collection
Step 1
Put on sterile gloves.
Step 2
Prepare blood collection containers and syringes.
Step 3
Clean the site for blood collection with an antiseptic wipe.
Step 4
Wrap an elastic band around the upper arm of the person from which blood is being drawn so that the vein swells with blood.
Step 5
Insert the needle into the vein inside the elbow or on the back of the hand. Collect blood from a person who has fasted for at least eight hours prior to a fasting blood glucose test or an oral glucose tolerance test, also known as an OGTT. For a post-prandial blood glucose test collect blood from a vein of person who has consumed a meal two hours prior.
Step 6
Apply pressure briefly and bandage the area where the needle was inserted.
Step 7
Have the person drink a drink that contains 75g of glucose for an OGTT.
Step 8
Draw blood samples every 30 to 60 minutes for two hours from the person who consumed the high glucose drink for an OGTT.
Step 9
Send the blood samples to the laboratory.
Calculation of Glucose Concentration
Step 1
Centrifuge the blood sample to separate the red blood cells from the serum. This step and the following ones are performed by trained laboratory personnel.
Step 2
Measure the glucose concentration in the serum with an enzyme assay that yields a colored compound.
Step 3
Use a device called a spectrophotometer to measure the light intensity of the colored compound in solution. The intensity of the color or its absorbance is proportional to the glucose concentration.
Step 4
Convert the absorbance reading to milligrams per deciliter of blood from a standard chart. Sometimes blood glucose levels are reported as millimoles per liter. To convert millimoles per liter to milligrams per deciliter, multiply by 18.
Step 5
Send report of glucose concentrations to the physician who ordered the tests.
Tips and Warnings
- The American Diabetes Association, or ADA, reports that normal fasting blood glucose levels are under 100 mg/dL. In a person with pre-diabetes, levels are between 100 and 125 mg/dL, and diabetes is indicated if levels are above 125 mg/dL on more than one occasion. In some people who have altered glucose homeostasis, their fasting levels are normal but after eating a meal glucose levels are higher than normal. The International Diabetes Federation states that after eating a meal, blood glucose levels should typically not increase above 145 mg/dL. The ADA reports that if blood glucose levels during an OGTT exceed 200 mg/dL a person does not have a normal tolerance to glucose.
Things You'll Need
- Sterile gloves
- Syringe
- Needle
- Antiseptic wipes
- Elastic band
- Blood collection vials
- Bandage
- Glucose assay
- Spectrophotometer
