Like HowStuffWorks on Facebook!

Blood Chemistry Tests

Your doctor may test a small sample of your blood to determine your potassium, sodium, creatinine, fasting glucose, total cholesterol, and HDL cholesterol levels. The results can help your doctor find organ damage or other risk factors for coronary heart disease.

How Is This Test Done?

To take a blood sample from a vein on your forearm or hand, your doctor or nurse will follow these steps.

  • Clean the skin over your vein with an antiseptic.
  • Wrap a strong rubber tube, called a tourniquet, around your upper arm. Doing this enlarges the veins in your lower arm by restricting blood flow through them. Enlarged veins are easier to take blood from.
  • Gently insert a small needle into your vein.
  • Collect your blood in a syringe or vial.
  • Remove the tourniquet.
  • Withdraw the needle.
  • Cover the puncture site with a bandage and apply pressure to stop any bleeding.