The human body possesses a complex system to manage bleeding and clotting, known as hemostasis. When this system is out of balance, it can lead to serious health issues, such as excessive bleeding or dangerous blood clots. Blood coagulation tests are essential diagnostic tools that help medical professionals assess how well a person’s blood clots, aiding in the diagnosis of various bleeding or clotting disorders. The accuracy of these tests relies heavily on proper blood collection, particularly the type of tube used.
Identifying the Correct Tube
For reliable coagulation test results, blood samples are collected in a light blue top tube, which contains liquid sodium citrate. The tube’s design is important, as it maintains a precise blood-to-anticoagulant ratio, usually 9 parts blood to 1 part sodium citrate. Many of these tubes operate using a vacuum system, which helps draw the correct volume of blood into the tube.
How the Anticoagulant Works
Sodium citrate functions as an anticoagulant by binding to calcium ions in the blood. Calcium is a necessary component for several steps in the blood coagulation cascade, the series of reactions that lead to clot formation. By chelating, or binding, these calcium ions, sodium citrate effectively prevents the clotting process from starting within the collection tube. This action ensures the blood sample remains in a liquid state, suitable for laboratory testing. In the laboratory, when coagulation tests are performed, calcium is added back to the sample. This reintroduces the essential ion, allowing the clotting cascade to reactivate under controlled conditions, enabling measurement of clotting times.
Understanding Coagulation Tests
Coagulation tests measure various aspects of the blood’s clotting ability. These tests help identify potential bleeding disorders, monitor the effectiveness of anticoagulant medications, and assess clotting risk. They provide insights into the different pathways involved in blood clotting.
Common coagulation tests include:
- Prothrombin Time (PT), which evaluates the extrinsic and common pathways of coagulation, often reported with an International Normalized Ratio (INR) for consistency across laboratories.
- Activated Partial Thromboplastin Time (aPTT), which assesses the intrinsic and common pathways.
- Fibrinogen, which measures the amount of a key protein needed for clot formation.
- D-dimer tests, which help detect the breakdown products of blood clots, indicating recent clotting activity.
Critical Factors for Accurate Results
Achieving accurate coagulation test results depends significantly on proper specimen collection and handling. The correct fill volume in the light blue top tube is important; under-filling or over-filling can alter the blood-to-anticoagulant ratio, leading to incorrect results. For instance, an under-filled tube increases the anticoagulant concentration relative to the blood, which can falsely prolong clotting times.
After collection, gentle inversion of the tube, 3-4 times, is necessary to ensure the sodium citrate and blood are thoroughly mixed without causing damage to blood cells. Vigorous shaking can lead to hemolysis, the breakdown of red blood cells, which can interfere with test results and cause specimen rejection. Following the established “order of draw” during blood collection also prevents contamination from additives in other tubes. The coagulation tube is collected second after a blood culture bottle to ensure sample integrity.