A capillary sample is a blood sample collected by pricking the skin. Capillaries are tiny blood vessels near the surface of the skin.

Some blood tests are performed on blood obtained by pricking the skin of the finger, heel, or other areas and collecting a drop (or a few drops) of blood on a test strip or into a small container.

The area is cleansed with antiseptic and pricked with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

Some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Blood transports oxygen, food, waste products, and other materials within the body. It also helps regulate body temperature. Blood is made up of a fluid called plasma and cells. Plasma contains various dissolved substances. The cellular portion consists primarily of red blood cells, but also includes white blood cells and platelets.

Because blood has multiple functions, tests on the blood or its components can provide valuable clues in the diagnosis of a multitude of medical conditions.

Capillary blood sampling offers several advantages:

• It is relatively easy to obtain (it can be difficult to obtain blood from the veins, especially in infants).
• There are several collection sites on the body (heel, fingertips, etc.) and these can be rotated.
• Testing can be performed at home and with minimal training. For example, diabetics must check their blood sugar several times a day using capillary blood sampling.

There are some disadvantages to capillary blood sampling.

• Only a limited amount of blood can be obtained using this method.
• There are some risks associated with the procedure (see below).
• Capillary blood sampling may provide inaccurate results, such as falsely elevated sugar, electrolyte, and blood count values.

See the specific test.

Results vary depending on the test performed.

• Excessive bleeding
• Fainting or feeling light-headed
• Infection (a slight risk any time the skin is broken)
• Scarring (occurs when there have been multiple punctures in the same area)
• Calcified nodules (sometimes occurs in infants, but usually disappear by 30 months of age)
• Damage to blood cells from this method of collection can sometimes cause inaccurate test results (and the need to repeat the test with blood drawn from a vein)

Capillary blood sampling is very useful when very small amounts of blood are needed or when blood is very difficult to obtain, such as in infants. Many times, however, when a larger sample is needed, the blood must be obtained from a vein.

Khan MI, Weinstock RS. Carbohydrates. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2006:chap 16.

Vajpayee N, Graham SS, Bem S. Basic examination of blood and bone marrow. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2006:chap 29.