A capillary sample is a blood sample collected by pricking the skin. Capillaries are tiny blood vessels near the surface of the skin.
Blood sample - capillary; Fingerstick; Heelstick
How the test is performed:
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.
How to prepare for the test:
How the test will feel:
Some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
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.
What abnormal results mean:
Results vary depending on the test performed.
What the risks are:
- 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.
|Review Date: 5/2/2009|
Reviewed By: Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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