Penn State Health expert explains how blood work works

Reviewed by James Ives, M.Psych. (Editor)Jul 19 2019

Blood tests can diagnose disease, determine organ function and help your doctor see how well treatments are working.

But when it's time to get blood work done, some patients wonder whether it's really a big deal if they eat before coming to the lab for fasting tests and why technicians need to draw so many samples.

Carole Andrews, supervisor of quality, compliance and point-of-care testing in the Department of Pathology and Laboratory Medicine at Penn State Health, said some tests require the blood to be clear of nutrients such as fats and sugars in order to get a good baseline.

Glucose and lipid testing are the most common types of fasting blood work.

The amount of fats and glucose [sugars] will increase in the blood if a person has recently eaten. This will affect the results of these specific tests."

Carole Andrews, Department of Pathology and Laboratory Medicine, Penn State Health

When it comes to collecting blood, patients often question why the phlebotomist pulls so many tubes for just a few tests.

Vials may contain different anti-coagulant liquids at the bottom or freeze-dried powder along the inside walls that keeps blood from clotting before the lab can test it. The technicians color code the tubes so they know which is which. For instance, a purple-colored vial is needed for a blood count and a green colored vial may be needed for a chemistry profile.

The number and type of tests a doctor orders -; and how many can be done from a single sample -; determine how many tubes must be taken.

Although the volume of blood in a person's body varies by weight and gender, most people have between 4,500 to 5,700 milliliters. Patients getting blood drawn for two routine testing panels -; a complete blood count, or CBC, and a comprehensive metabolic panel, or CMP -; can expect to lose maybe 10 milliliters.

"Even if you had 10 tubes of blood taken, that's less than 60 milliliters," Andrews said. "It's not going to make an impact because your body is designed to replace what is lost."

The collection process itself is also important to the quality of results.

Once drawn, the blood must immediately be mixed with the anticoagulant in the tubes by being gently inverted several times. "Even a tiny micro clot can give abnormal results," Andrews said.

Blood for chemistry profiles must be placed in a centrifuge for 10 minutes to separate red and white blood cells from plasma.

Patients can make the process easier by drinking plenty of fluids leading up to a draw. When you are dehydrated, the vein quality isn't as good and it's harder for the phlebotomist to get a good venipuncture.

"Also, it is easier if the patient is relaxed and comes in with a good attitude," Andrews said. "If they tense up too much, it can make the venipuncture difficult."

In rare cases, patients might be called back to repeat a test. "Sometimes the doctor wants additional tests, or sometimes they need to do confirmatory testing," Andrews said.

What many patients don't realize is how highly regulated the instruments and processes are in medical laboratories.

Every eight hours, labs run quality controls with material that has already been tested and has a standard value assigned to it. That confirms that equipment and processes are working correctly.

"Some controls are run at low, medium and high levels to ensure we're getting the whole range of values," Andrews said. "Then you know you're getting accurate results."