Capillary blood sampling is becoming a common way to minimize the amount of blood drawn from a patient. The 10 or 20 microliters can be used to look for anemia, check blood sugar or even to evaluate thyroid function or drug concentration levels in a person's system. The procedure is easier and less painful than traditional venipuncture which draws blood from a vein.
In order to collect a capillary blood sample, 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 for capillary blood sampling?
Finger - For finger-stick blood collection, the third or fourth finger is typically 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 lancet 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 puncture site is used for newborns and infants, less than one year old, because their fingers are too tiny for finger-stick blood collection. 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 potentially 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 fingertip or the heel. The ear lobe can still be used if no other site is available.
Great toe (Big 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 (fleshy mound at base of thumb) and hypothenar (fleshy mound at base of fourth finger) eminences have capillary circulation equivalent to the fingertip. Some people sample from the "heel" of the hand, though this may not produce enough sample volume.
Other capillary puncture sites may be used (i.e., the upper arm, abdomen, and thigh) in clinical trials and research studies, but these sites are still being evaluated.