Before and during the blood test

Preparation for blood tests varies according to the requirements of each specific test. In most cases minimal preparation is necessary. The patient may need to reduce or stop certain medications at some point prior to the test. Additionally, food intake and exercise may be temporarily restricted or suspended. Alcohol and caffeine should 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. The medical professional will also obtain the patient’s medical history before the test to determine if the patient is taking any medications that will interfere with the test’s accuracy or has any history of clotting problems.

Drawing blood for a blood test is an easy and typically painless process for most people. If the physician needs only a drop or two of blood, then a simple prick of the finger, earlobe or heel (in the case of a baby) can provide enough blood for testing.

The technician will use a sterile, sharp lancet to prick the skin. The technician will then gently squeeze the puncture area to produce drops of blood that are collected in tiny glass tubes. Light pressure and sterile gauze are then applied to the puncture site to stop the bleeding. A bandage is not usually necessary.

If a substantial amount of blood is needed, it is usually drawn from a vein in a process called venipuncture. Blood samples may be more difficult to obtain from:

• Infants or young children
• Overweight people whose veins are difficult to find
• Elderly people whose veins tend to roll away
• Patients with scarred or collapsed veins due to multiple transfusions or drug use

During a venipuncture, a needle is inserted into a vein – usually at the inside of the elbow or on the back of the hand. The area around the puncture site is cleaned with rubbing alcohol and a wide elastic band or piece of latex tubing may be placed around the upper arm to slightly increase the 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 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 to be done. It is usually around 7 milliliters. The technician may change test tubes once or twice during the venipuncture to either allow for more blood to be collected or to change the type of tube being used. Tubes are marked with different colored tops that indicate the way in which the collected blood will be preserved:

• Lavender tops indicate that the tube contains an anticoagulant, which prevents the collected blood from clotting.
  
  
• Red tops indicate that the tube contains no anticoagulants, allowing serum to separate and the blood to form a blood clot.
  
  
• Gray tops indicate that the tube contains a preservative, which keeps glucose (blood sugar) 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 entire process 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. To minimize soreness, patients may immediately apply a warm compress to the puncture site and repeat this application every three hours until the discoloration or pain subsides.

Blood tests can be performed on different parts of the blood, including:

• Whole blood (to which an anticoagulant has been added in the test tube to prevent clotting)
  
  
• Blood serum, the term for plasma (the liquid part of blood) that has had the clotting agents removed
  
  
• Blood plasma, the whitish yellow liquid that remains in unclotted blood once the blood cells have settled out to the bottom of the test tube
  
  
• Blood cells, the individual red, white and platelet cells

If blood must be taken from an artery instead of a vein, (e.g., during an arterial blood gas study), it is usually drawn from a small artery located on the inside or the top side of the wrist.