A capillary blood sample is collected by pricking the skin. Capillaries are small vessels formed by branching of arterioles. Blood flows from the heart to arteries, which divide into arterioles, and then further divide into capillaries. After the tissue has been perfused, capillaries then unite to become venules and then widen more to become veins, which return blood to the heart.
In the skin, many capillaries are found near the surface and can be accessed by puncturing the dermis layer of the skin.
A specimen from a dermal puncture will therefore be a mixture of arterial and venous blood along with interstitial and intracellular fluids.
Capillary blood sampling is used in collecting the specimen of choice for infants, very young children, elderly patients with fragile veins, and severely burned patients.
In infantsthe capillary blood sample is obtained by heel stick. In small children and adults the asmple is usually obtained from fingers.
After the prick, the specimen is then collected with a capillary tube or pipette, placed on a glass slide or a piece of filter paper or collected in vial.
Glucose testing at home using a glucometer is an example of capillary blood sampling.
Advantages of Capillary Blood Sample
- Draws small amount of blood. This is especially useful in patients where repeated tests are needed and blood loss would be greater with routine methods.
- Easy access – can be undertaken by most of healthcare workers.
- Less traumatic and painful
- There are several collection sites on the body, and these sites can be rotated.
- Patient can be taught to take the sample at home
Disadvantages of Capillary Blood Sampling
- Only a limited amount of blood can be drawn
- May result in inaccurate results, such as falsely elevated sugar, electrolyte, and blood count values.
- Not all tests can be run on capillary samples.
- The method can cause cell rupture leading to inaccurate results
- Calcified nodules can develop at the site of collection, especially in infants. The nodules usually resolve by themselves.
Method of Taking A Capillary Blood Sample
- The area is cleansed with antiseptic [skin of the finger, heel or another area]
- The desired area is pricked with a sharp needle or a lancet.
- The blood may be collected in a pipette (or small glass capillary 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.
Difference in Results of Capillary Blood Sample from Other Methods
The differences between capillary blood versus plasma in most cases are minimal.
But there may be discrepancies in hemoglobin and hematocrit values and in platelet counts.
There may be a difference between glucose levels.Venous blood glucose is generally slightly higher than capillary blood glucose measured at the same time