Preparation for blood tests depends on the requirements of each specific test. Physicians will advise patients beforehand on the preparations required for each type of blood test.

In most cases, minimal preparation is necessary. The patient may just need to reduce or stop the use of certain medications and/or vitamins at some point prior to the test. Additionally, food intake and exercise may be temporarily restricted or suspended, such as an overnight fast for some glucose tests. Alcohol and caffeine should also be avoided prior to a blood test.

On the day of the test, the procedure will be explained and patients will have the opportunity to ask questions. To determine if the patient is taking any medications that will interfere with the test’s accuracy or has any history of clotting problems, a physician or medical professional will also ask questions about the patient’s medical history prior to administering the test.

Having blood drawn for a blood test is an easy and nearly painless process for most people. If the test only requires a drop or two of blood, then just a prick of the finger, earlobe or heel (in the case of infants) can provide enough blood for testing.

A sterile, sharp lancet is used for pricking the patient’s skin. Then, the technician will gently squeeze the punctured area to extract drops of blood that are collected in tiny glass tubes. To stop the bleeding, sterile gauze is applied with light pressure to the punctured area. A bandage is usually not necessary since bleeding is minimal in most cases.

When a substantial amount of blood is required, it is usually drawn from a vein in a process called venipuncture. A needle is inserted into a vein, usually at the end of the elbow or on the back of the hand. The technician cleans the area around the puncture with rubbing alcohol and a wide elastic band or piece of latex tubing may be placed around the upper arm to slightly increase pressure in the vein. One end of a sterile, double–ended needle that has been attached to an open–ended syringe (which contains an empty test tube) is then inserted into the vein. Because the test tube contains a partial vacuum, blood flows directly from the vein through the double–ended needle and into the test tube.

The precise amount of blood to be drawn is determined by the type and number of tests scheduled. Usually, it is around 7 milliliters. The technician may change test tubes more than once during a venipuncture to either allow for more blood to be collected or to change the type of tubing being used. Tubes are marked with different color tops that indicate the way in which the collected blood will be preserved:

• A lavender top indicates that the tube contains an anticoagulant, which prevents the collected blood sample from clotting.
  
  
• A red top indicates that the tube contains no anticoagulants, allowing serum and the blood to form a blood clot.
  
  
• A gray top indicates that the tube contains a preservative, which keeps glucose from breaking down in the tube.

After the necessary amount of blood is drawn, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site. After several minutes, the cotton will be discarded or replaced, and a small bandage will be placed on the puncture wound. The whole process usually takes less than 10 minutes.

Despite the precautions taken to avoid bruising and soreness, it does sometimes occur. Typically, this is not a cause for great concern. If soreness is present, patients can immediately apply a warm compress to the puncture site and repeat the application every three hours until the discoloration or pain subsides.