Capillary sample
Blood sample - capillary; Fingerstick; Heelstick
A capillary sample is a blood sample collected by pricking the skin. Capillaries are tiny blood vessels near the surface of the skin.
Red blood cells are the oxygen carriers. As they travel away from the heart, they traverse smaller and smaller arteries, finally arriving at the collections of microscopic blood vessels, known as capillaries. Here, they exchange nutrients and oxygen for cellular waste products. The exchange of oxygen and nutrients between the red blood cells and the surrounding tissues occurs through a process called diffusion. In diffusion, when capillaries contain a high concentration of oxygen and nutrients, while the surrounding tissues contain a lower concentration, oxygen and nutrients leave the capillaries and enters the tissues. Conversely, when body tissues contain high concentrations of carbon dioxide and metabolic waste, while the capillaries contain a lower concentration, the waste products diffuse from the tissues into the capillaries, and from there are carried by the venous system back toward the heart. The waste products are eventually eliminated from the bloodstream through the urinary and respiratory systems.
Blood is routinely drawn from newborn infants for testing. Blood is obtained by a heel stick and collected on a special blotter paper. Routine testing includes phenylketonuria and blood type. Many hospitals include other tests such as thyroid function, hemoglobin S (sickle cell disease), or may test for other blood disorders (hemoglobinopathies). Testing can be tailored to the local population, determining what routine testing should be done.
Blood is routinely drawn from newborn infants for testing. Blood is obtained by a heel stick and collected on a special blotter paper. Routine testing usually includes phenylketonuria, thyroid function, hemoglobin S (sickle cell disease), and may test for other disorders. Newborn screening programs vary from state to state. Testing can be tailored to the local population, determining what routine testing should be done.
A capillary sample of blood is obtained by pricking the skin's surface to obtain a drop or several drops of blood for laboratory testing. The usual location of capillary blood sampling is the skin of the finger or heel.
How the Test is Performed
The test is done in the following way:
- The area is cleansed with antiseptic.
- The skin of the finger, heel, or another area is 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 the Test will Feel
Some people feel moderate pain. 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 cells and a fluid called plasma. Plasma contains various dissolved substances. The cells are mainly red blood cells, white blood cells, and platelets.
Because blood has many functions, tests on the blood or its components provide valuable clues in the diagnosis of medical conditions.
Capillary blood sampling has several advantages over drawing blood from a vein:
- It is easy to obtain (it can be difficult to obtain blood from the veins, especially in infants).
- There are several collection sites on the body, and these sites can be rotated.
- Testing can be done at home and with little training. For example, persons with diabetes must check their blood sugar several times a day using capillary blood sampling.
Disadvantages to capillary blood sampling include:
- Only a limited amount of blood can be drawn using this method.
- The procedure has some risks (see below).
- Capillary blood sampling may result in inaccurate results, such as falsely elevated sugar, electrolyte, and blood count values.
What Abnormal Results Mean
Results vary depending on the test done. Your health care provider can tell you more.
Risks
Risks of this test may include any of the following:
- Excessive bleeding
- Fainting or feeling lightheaded
- 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
References
Rolon MEDV. Procedures. In: Anderson CC, Kapoor S, Mark TE, eds. The Harriet Lane Handbook. 23rd ed. Philadelphia, PA: Elsevier; 2024:chap 4.
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. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 31.
Version Info
Last reviewed on: 4/25/2023
Reviewed by: Charles I. Schwartz, MD, FAAP, Clinical Assistant Professor of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, General Pediatrician at PennCare for Kids, Phoenixville, PA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
