Skin puncture is a method of obtaining small blood samples, commonly used for infants, children, or adults with fragile veins when venipuncture is difficult or undesirable. The process is governed by strict protocols to ensure accurate laboratory testing. A primary rule involves the collection order: specimens requiring the anticoagulant Ethylenediaminetetraacetic acid (EDTA) must be collected first. Following this specific sequence is necessary to maintain reliable diagnostic results.
The Unique Conditions of Capillary Collection
A skin puncture creates a specimen fundamentally different from a venous blood draw. The lancet incision causes trauma, rupturing capillaries, arterioles, and venules, resulting in a mixture of blood with interstitial fluid. The presence of this tissue fluid can alter the concentration of blood components, potentially leading to inaccurate test results.
The slower, drop-by-drop collection method exposes the blood to air and injured tissue for a longer duration than rapid venipuncture. Collecting the small drops often involves scraping the skin, which accelerates the body’s natural clotting process. This increased exposure and trauma heighten the risk of specimen alteration.
Minimizing Platelet Activation and Clumping
The primary reason for collecting the EDTA specimen first is to counteract the immediate, localized activation of the coagulation cascade. When the skin is punctured, the trauma instantly triggers the formation of a platelet plug and the initiation of clotting. This reaction happens rapidly in capillary blood.
The EDTA tube is specifically designated for hematology testing, such as a Complete Blood Count (CBC), which requires the analysis of red blood cells, white blood cells, and platelets. For these tests, the platelets must remain in their individual, unactivated state for accurate counting and size analysis. If the platelets begin to clump together due to the trauma, the laboratory instrument will fail to count them as individual cells, leading to a falsely low platelet count.
The additive EDTA is a potent anticoagulant that works by binding to calcium ions in the blood. Since calcium is necessary for the coagulation cascade to proceed, EDTA effectively halts the clotting process and preserves the integrity of the blood cells. By drawing the EDTA tube first, the freshest blood is immediately mixed with this powerful anti-clumping agent. This procedural step ensures that the sample intended for cell analysis is collected before significant micro-clot formation can occur, ensuring a reliable result.
Consequences of Drawing Tubes Out of Order
Violating the strict capillary order of draw introduces two major types of analytical errors that can compromise patient care. The first is the contamination of subsequent tubes with the EDTA additive itself. EDTA contains high concentrations of potassium salts, and its function is to chelate, or bind, calcium and other metal ions.
If a tube containing EDTA is drawn before a serum or heparin tube, trace amounts of the additive can carry over into the next tube. This carryover will artificially raise the potassium level, leading to a misdiagnosis of hyperkalemia. Conversely, the potent binding of calcium by the carried-over EDTA will artificially lower the calcium level, resulting in false hypocalcemia.
The second consequence occurs when the EDTA tube is drawn after a non-additive tube. By the time the blood reaches the EDTA tube, the coagulation process may have already begun due to the trauma of the skin puncture. This delay allows for the formation of micro-clots, which will render the hematology sample unusable because the platelet and white blood cell counts will be inaccurate. The laboratory would then need to reject the sample and request a recollection, delaying diagnosis and treatment.
The Complete Capillary Order of Draw Sequence
The Clinical and Laboratory Standards Institute (CLSI) provides a standardized sequence for capillary collection to mitigate the risks of clotting and additive contamination. The primary consideration is to collect the most sensitive samples first, which begins with the EDTA microcollection container. This step secures the integrity of the hematology specimen by immediately preventing platelet activation.
The complete sequence minimizes EDTA carryover into tubes intended for chemistry analysis:
- EDTA microcollection containers.
- Other tubes containing chemical additives, such as heparin (lithium, sodium, or ammonium).
- Tubes that contain no additive or those that require the blood to clot naturally, such as serum tubes.