Capillary blood is the small volume of blood obtained from the microscopic vessels closest to the skin’s surface, typically collected via a finger or heel prick. This minimally invasive sampling method allows for quick analysis using only a drop or two of blood. It serves as a practical alternative to traditional venous blood draws, especially when frequent testing or portability is necessary, expanding the accessibility of health monitoring into homes and point-of-care settings.
The Capillary Network: Anatomy and Function
The blood collected in a finger prick originates from the capillary network, a dense web of the body’s smallest blood vessels. These vessels form the connection between the arterial system, which carries oxygenated blood away from the heart, and the venous system, which returns deoxygenated blood. Specifically, capillaries receive blood from tiny arteries called arterioles and deliver it to small veins known as venules.
Capillaries are remarkably thin, often only 5 to 10 micrometers in diameter, which is just wide enough for red blood cells to pass through in single file. This minimal wall thickness, composed of a single layer of endothelial cells, facilitates their primary function: the exchange of materials. Oxygen and nutrients diffuse out of the blood and into the surrounding tissues, while carbon dioxide and other waste products move from the tissues into the bloodstream to be carried away.
Unique Composition of Capillary Blood
Capillary blood is frequently described as “mixed blood” because its composition is not purely arterial or purely venous. The sample collected contains a blend of blood from the arterioles, the capillaries themselves, and the venules, along with a small amount of interstitial fluid from the surrounding tissue. This mixing results from the skin puncture technique accessing the entire microcirculation bed.
The presence of interstitial fluid and the arterial-venous mix leads to slight but measurable analytical differences compared to a purely venous sample. For example, capillary blood glucose levels are considered a closer reflection of current arterial blood glucose, often showing higher concentrations than venous blood, especially after a meal. Conversely, components like potassium, sodium, and urea nitrogen may show lower concentrations in capillary plasma compared to venous plasma.
This unique composition means that for certain tests, such as blood gas analysis, capillary blood provides results that are a closer match to arterial blood than a venous sample. However, the inclusion of tissue fluid can slightly dilute the sample, potentially leading to lower measurements for components like hematocrit and hemoglobin if collection is not performed correctly. Clinicians must account for these slight variations when interpreting results.
Practical Collection Methods and Diagnostic Uses
The collection of capillary blood is performed using a quick skin puncture with a sterile lancet. For adults and older children, the fingertip is the most common site, typically the third or fourth finger, avoiding the center pad. In infants, the heel is the preferred site for collection due to the thickness of the skin.
To ensure a high-quality sample, the collection site is often gently warmed before the puncture to increase blood flow, a process called arterialization. The puncture must be deep enough to ensure a free flow of blood, but not so deep as to risk injury, often penetrating less than 2.4 mm in adults. Standard practice is to wipe away the very first drop of blood, which is contaminated with tissue fluid and surface debris, and then collect the second drop into a specialized container.
Capillary sampling is the foundation for many common diagnostic applications because it is minimally invasive and requires only a small volume of blood.
Diagnostic Applications
Capillary blood is used extensively for rapid results in various settings, including:
- At-home glucose monitoring for individuals with diabetes.
- Point-of-care testing for measurements such as hemoglobin, cholesterol, and international normalized ratio (INR) for blood clotting.
- Newborn screening tests, which check for genetic and metabolic disorders like phenylketonuria and sickle cell disease, collected via a heel stick shortly after birth.