What Is a Blood Test?

By taking and testing a small sample of a person’s blood, doctors can check for many kinds of diseases and conditions. Blood tests help doctors check how the body’s organs are working and see if medical treatments are helpful.

To help your child get ready for a blood test, find out if they need to fast (not eat or drink) or should stop taking medicines before the test. Explain what to expect during the test. If your child is anxious about it, work together on ways to stay calm.

Why Are Basic Blood Chemistry Tests Done?

Doctors use basic blood chemistry tests to check many conditions and learn how the body’s organs, like  the kidneys, are working.

Tests for Electrolytes

Basic blood chemistry tests check electrolytes. These minerals help keep the body's fluid levels in balance, which helps the muscles, heart, and other organs work properly. 

Typically, tests for electrolytes (also called an electrolyte panel) measure levels of sodium, potassium, chloride, and bicarbonate in the body.

Sodium helps regulate the amount of water in the body. Sodium’s passage in and out of cells is important to many body functions, like sending electrical signals in the brain and in the muscles. Testing sodium levels can show if blood has the right balance of sodium and liquid needed to carry out those functions.

In a child who is dehydrated (from vomiting, diarrhea, or other causes), sodium levels can be too high or low, which can cause confusion, weakness, lethargy, and even seizures.

Potassium is essential to regulating how the heart beats. Levels that are too high or too low can increase the risk of an abnormal heartbeat (an arrhythmia). Low potassium levels are also linked to muscle weakness and cramps.

Chloride, like sodium, helps body fluids stay balanced. Some medical problems — like dehydration, heart disease, kidney disease — can disrupt the balance of chloride.  

Bicarbonate prevents the body's tissues from getting too much or too little acid. The kidneys and lungs balance bicarbonate levels. So if bicarbonate levels are too high or too low, it might indicate a problem with those organs.

What Else Does a Basic Blood Chemistry Test Measure?

Other blood substances measured in the basic blood chemistry test include:

Blood urea nitrogen (BUN): This is a measure of how well the kidneys are working. Urea (yuh-REE-uh) is a nitrogen-containing waste product created when the body breaks down protein. If the kidneys aren’t working well, BUN levels build up in the blood. Dehydration, bleeding in the digestive tract, and severe infection leading to shock also can raise BUN levels.

Creatinine levels: High levels of creatinine (kree-AT-uh-neen) can be a sign of a kidney problem. The kidneys filter and excrete creatinine. So if they're not working as they should, creatinine can build up in the bloodstream. Both dehydration and muscle damage also can raise creatinine levels.

Glucose: This is the main type of sugar in the blood. It comes from the foods we eat and is the major source of energy needed to fuel the body's functions. Glucose levels that are too high or too low can cause problems. Diabetes is the main cause of high blood glucose levels. Other medical conditions and some medicines also can cause high blood glucose.

What if I Have Questions?

If you have questions about the basic blood chemistry test, or what the test results mean, talk to your doctor.