How do cancer blood tests work?

In combination with other tests, blood work can give your provider important information about:

Your healthcare provider will give you specific instructions on how to prepare for your blood test. These guidelines may differ depending on the type of test you need.

In some cases, you might have to fast for eight to 12 hours prior to the test. That means you shouldn’t eat or drink anything besides water. You also may need to avoid certain medications beforehand.

Collecting a sample of your blood should only take a few minutes. Your healthcare provider will:

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Wrap a tight band (tourniquet) around your upper arm.

Clean an area of your skin, usually on the inside of your elbow area or on your hand.

Insert a needle into that area, which may cause a stinging or pinching feeling.

Remove the tourniquet.

Attach a tube to the needle and fill the tube with blood.

Remove the band and the needle.

Put pressure on the puncture wound to help stop any bleeding.

Place a bandage on your skin.

After a cancer blood test, you should keep the bandage on for a few hours. You may also need to avoid exercise for a few hours afterward, if advised by your healthcare provider. (Elevating your heart rate can cause increased bleeding.)

Blood tests are safe and involve very little risk. You might have a small bruise where the needle went into your skin. Rarely, the vein may swell. In these cases, applying a warm compress (like a warm washcloth or heating pad) can help.