Capillary blood sampling is becoming a common way to minimize the amount of blood drawn from a patient. The 10–300 microliters can be used to look for anemia, check blood sugar or even to evaluate thyroid function. The procedure is easier and less painful than traditional venipuncture which draws blood from a vein.
In order to collect blood with a capillary tube, the appropriate site must be cleaned and punctured with a lancet so that a drop of blood can be gently expressed.
But what is an appropriate site?
- Finger — Usually the third or fourth finger is preferred in adults and children. The thumb has a pulse and is likely to bleed excessively. The index finger can be calloused or sensitive and the little finger does not have enough tissue to prevent hitting the bone with the lancet. The puncture is done to the left or right of the midline of the palmar surface of the fingertip, staying away from the fingernail.
- Heel — This site is used for infants, less than one year old, whose fingers are too tiny. The puncture is done on the farthest lateral or medial aspect of the plantar surface of the heel, not on the bottom. Punctures done on the plantar surface can damage cartilage or bone.
- Ear lobe — The ear lobe has been used for capillary blood sampling, but is no longer recommended. Studies have shown that the blood flow in the ear lobe is less than that in either the finger tip or the heel. The ear lobe can still be used if no other site is available.
- Great toe — The big toe can be used if necessary in infants who are not yet walking. Callous formation after a child starts walking can interfere with blood collection and is a contraindication. The site should be lateral to the midline of the plantar surface of the toe. Only the great toe has the amount of tissue necessary to protect the bone from injury.
- Palm — There are two areas on the palm that can be used if necessary. Especially in diabetics who test their own blood sugars frequently, alternate sites give some relief from reusing the same sites over and over. The thenar and hypothenar eminences have capillary circulation equivalent to the fingertip.
Other sites such as the abdomen or thigh have been used, but are not generally recommended.
