Capillary Puncture and Microcollection

Indications, Contraindications, and Blood Composition

Capillary puncture serves as an essential collection method when standard venipuncture cannot or should not be performed. In adult and pediatric patients, a phlebotomist turns to capillary puncture when venipuncture veins are inaccessible, severely burned, or fragile. In infants, capillary puncture is often appropriate for tests requiring a small blood volume, but venipuncture is needed when the ordered test or required specimen volume calls for it. Performing repetitive venipunctures on an infant risks drawing too much blood, which can directly cause excessive blood loss and iatrogenic anemia. Furthermore, dermal collection is commonly used across all patient populations for point-of-care testing, such as bedside blood glucose monitoring, which requires only a minimal blood volume for immediate bedside assessment.

Despite its clinical utility, capillary collection has distinct limitations and contraindications. Blood cultures should be collected by venipuncture using the required aseptic technique because capillary puncture generally cannot provide the required specimen volume or meet blood-culture collection requirements. It is also contraindicated for routine coagulation tests that require plasma, such as PT and aPTT. Phlebotomists must also evaluate the puncture site itself and the patient's systemic state. Capillary puncture must be avoided at edematous sites because excessive tissue fluid can contaminate the specimen and skew laboratory values. Similarly, poor peripheral perfusion, including that caused by severe dehydration, impairs blood circulation to the extremities and makes capillary collection unsuitable.

Understanding what makes up a capillary blood sample helps explain why collection technique is so vital. Capillary blood is not identical to pure venous blood; instead, it is a complex biological mixture of arterial blood, venous blood, capillary blood, and intracellular and interstitial fluids. Because of the naturally higher arterial pressure within the capillary bed, capillary blood more closely resembles arterial blood than venous blood. Phlebotomists must recognize that while they collect and process these samples, they do not diagnose medical conditions or interpret the resulting laboratory values.