A clotted blood sample has partially or completely solidified inside the collection tube, forming a gelatinous mass instead of remaining a liquid. This occurs when the body’s natural defense mechanism, designed to stop bleeding, is mistakenly activated in the sample container. Drawing blood removes it from the vein and exposes it to foreign surfaces, triggering coagulation. If clotting is not successfully stopped, the sample becomes unusable for laboratory analysis, requiring a repeat blood draw.
The Body’s Natural Clotting Mechanism
The body’s natural response to vessel damage is hemostasis, a complex process that forms a blood clot. This begins when blood is exposed to substances outside the vessel lining, such as collagen, signaling injury. Specialized cell fragments called platelets rush to the site and stick together, forming a temporary plug.
This plug activates a cascade of proteins known as clotting factors. These factors convert the soluble protein fibrinogen into fibrin, an insoluble, sticky mesh. The fibrin network wraps around the platelet plug and traps circulating blood cells, stabilizing the mass into a solid clot. Since drawing blood simulates injury, the sample will inherently try to clot unless actively prevented.
Preventing Clotting in Sample Tubes
To counteract the blood’s natural clotting impulse, collection tubes contain specific chemical additives called anticoagulants. These substances interrupt the coagulation cascade, keeping the blood liquid for accurate testing. Different tests require different anticoagulants, which is why collection tubes are color-coded.
Ethylenediaminetetraacetic acid (EDTA), found in lavender-top tubes, prevents clotting by binding to calcium ions, which are necessary for coagulation factors to function. Sodium citrate, used for coagulation studies in light blue-top tubes, also binds calcium. Heparin, commonly in green-top tubes, prevents clotting by enhancing an enzyme inhibitor that inactivates several clotting factors.
Common Causes of Sample Clotting
Despite the presence of anticoagulants, samples can still clot due to procedural and technical errors during or immediately after the blood draw. The most frequent cause is insufficient or delayed mixing of the sample with the anticoagulant inside the tube. Since the additive is a small amount of powder or liquid, the blood must be gently inverted eight to ten times immediately after collection to ensure thorough contact.
Technical Errors Leading to Clotting
Technical errors account for the majority of clotted samples:
- A difficult or traumatic draw can prematurely activate the clotting process. If blood flow is sluggish or the phlebotomist probes repeatedly, coagulation may begin near the needle tip before the blood reaches the anticoagulant.
- Delayed transfer of blood drawn into a syringe allows micro-clots to form outside the influence of the anticoagulant.
- Using the wrong type of tube for a specific test, such as using a tube with no anticoagulant when one is required.
- Filling the tube with an incorrect volume of blood disrupts the calibrated ratio of blood to anticoagulant, rendering the chemical ineffective.
While patient factors like severe dehydration or certain clotting disorders can contribute to faster clotting time, these technical errors are the primary cause.
Why Clotting Invalidates Test Results
When a blood sample clots, its internal composition is fundamentally altered, making the remaining liquid unrepresentative of the patient’s physiological state. Clotting consumes platelets and specific clotting factors, trapping them within the fibrin mesh. This consumption means that any test designed to measure these components, such as a Complete Blood Count (CBC) or a coagulation assay, will produce artificially low and unreliable results.
The clot also sequesters red and white blood cells within the solid mass. This trapping skews the concentration of the liquid portion (plasma or serum), preventing a laboratory analyzer from accurately counting or measuring components. Since the clot formation process uses up and redistributes components, testing the remaining liquid would be diagnostically misleading and potentially dangerous for patient care. Consequently, a clotted sample must be rejected, and a new sample collected to ensure accurate medical diagnosis.