Capillary puncture is a collection method used to obtain small volumes of blood, often referred to as a fingerstick or heel stick. This technique involves puncturing the skin to access the capillaries, the smallest blood vessels, and is commonly used for patients whose veins are difficult to access, such as infants, or when only a limited blood sample is necessary. The process requires strict adherence to a specific collection sequence, known as the order of draw, to prevent contamination from additives in the collection devices, which could otherwise lead to inaccurate laboratory results. Following the correct order is essential for maintaining specimen integrity and ensuring the reliability of diagnostic testing.
Preparing for a Capillary Collection
Proper preparation begins with selecting the appropriate site for the puncture. In adults and older children, the ring or middle finger is typically chosen, specifically the fleshy pad toward the side of the fingertip. For infants under six months old, the heel is the preferred site, with the puncture limited to the most lateral or medial parts of the plantar surface to avoid bone damage.
Before the puncture, the chosen area is cleaned with an antiseptic, such as alcohol, which must be allowed to air dry completely to prevent sample dilution or interference with test results. Warming the site, often with a commercial warmer, can increase blood flow and help ensure an adequate sample volume. This step is especially important for heel sticks in infants.
After the skin is punctured with a sterile lancet, the very first drop of blood that appears must be wiped away using clean gauze. This initial drop contains excess tissue fluid, or interstitial fluid, which can dilute the blood sample and skew test results. Discarding this first drop also removes any residual antiseptic or skin contaminants, ensuring the subsequent blood is a more accurate representation of the patient’s circulating blood.
The Specific Order of Draw
The order of draw for capillary collection differs significantly from venipuncture due to the rapid clotting that occurs in blood collected via skin puncture. The sequence is specifically designed to manage this clotting tendency and minimize additive carryover between the tiny microcollection containers, often called microtainers. This sequence is governed by established laboratory standards to ensure the highest quality of the small volume samples.
EDTA tubes are collected early in the sequence to ensure that the blood-to-additive ratio is correct for hematology tests, which are highly sensitive to clotting and platelet aggregation. Capillary blood is particularly prone to clumping platelets, and collecting the EDTA tube first ensures the best possible sample for a complete blood count.
The standard order of draw is:
- Specimens for blood gas analysis (CBGs), collected first to minimize the effect of air exposure on the gases being measured.
- Blood smear slides, prepared immediately to prevent changes in cell morphology due to anticoagulants.
- Anticoagulant Ethylenediaminetetraacetic acid (EDTA) tubes (lavender or purple top).
- Other additive tubes, such as heparin (green top) or sodium fluoride/potassium oxalate (gray top).
- Non-additive tubes used to collect serum (red top).
The final tubes in the capillary order of draw are the non-additive tubes, which are used to collect serum. Since these tubes do not contain an anticoagulant, they are collected last to reduce the risk of any anticoagulant, especially potassium from the EDTA, contaminating the serum sample. The entire collection process must be done quickly and with gentle handling to maintain sample integrity.
Ensuring Sample Integrity
The reversed order of draw in capillary collection, where EDTA is collected before heparin, is a deliberate strategy to combat the unique challenges of small-volume sampling. The primary concern is the phenomenon of platelet aggregation, where platelets clump together rapidly at the puncture site. Collecting the EDTA tube first ensures that the platelet count is as accurate as possible before significant clumping occurs.
Additive carryover represents another major threat to sample integrity, which the specific order is designed to mitigate. If the EDTA, which contains potassium, were collected after a serum tube, trace amounts of the EDTA could contaminate the serum, leading to falsely elevated potassium levels in chemistry tests. This false elevation, known as pseudohyperkalemia, could lead to incorrect patient diagnoses and treatments.
Furthermore, the presence of tissue fluid in capillary samples means that blood composition is not identical to venous blood, making the avoidance of further contamination even more important. The small size of microcollection containers means that even minute amounts of contaminating additive can significantly alter the overall concentration in the sample. The mandatory sequence is therefore a procedural safeguard against common collection errors that would otherwise render the laboratory results unreliable.