The light blue top tube is a standard, color-coded container used in phlebotomy, the process of drawing blood for testing. This tube is specifically designed to collect blood for tests that analyze the body’s ability to stop bleeding, known as coagulation studies. The distinctive color alerts healthcare professionals to the specific type of testing intended for the collected sample. This color system helps ensure the correct sample is processed for accurate laboratory results.
The Sodium Citrate Additive
The light blue top tube contains a liquid additive called sodium citrate, which functions as an anticoagulant to prevent the drawn blood from clotting inside the tube. This action is achieved because sodium citrate binds to calcium ions present in the blood sample. Calcium is a necessary component for the coagulation cascade, which leads to the formation of a blood clot. By effectively removing the available calcium, the sodium citrate temporarily halts the clotting process, preserving the blood in a liquid state for testing.
The concentration of sodium citrate is typically a 3.2% solution, calibrated to work effectively with a specific volume of blood. This results in a precise blood-to-anticoagulant ratio of nine parts blood to one part anticoagulant (9:1). Maintaining this exact ratio is essential for accurate test results because an imbalance can artificially alter the blood’s clotting time. The laboratory will add calcium back to the sample to initiate clotting for testing, and the precise amount of citrate ensures this process can be correctly reversed.
Tests for Coagulation Studies
The primary purpose of blood collected in the light blue top tube is for coagulation studies, which evaluate the function of various clotting factors in the blood. These tests help medical providers diagnose bleeding disorders or monitor patients who are receiving anticoagulant medications. The two most common tests performed are the Prothrombin Time (PT) and the Activated Partial Thromboplastin Time (aPTT).
Prothrombin Time (PT) measures how quickly blood clots through the extrinsic and common pathways of the coagulation cascade. This test is frequently used to monitor the effectiveness of anticoagulant medications such as warfarin. Warfarin is often prescribed to prevent blood clots in conditions like atrial fibrillation or deep vein thrombosis. The result is often reported as the International Normalized Ratio (INR), a standardized value that allows for consistent interpretation of PT results across different laboratories.
The Activated Partial Thromboplastin Time (aPTT or PTT) assesses the intrinsic and common pathways of the clotting cascade. This measurement helps monitor patients receiving unfractionated heparin therapy. Both the PT and aPTT are used together as a general screening to identify deficiencies in specific clotting factors, such as those indicating hemophilia. Other specialized tests performed with this tube include D-dimer and fibrinogen levels.
Importance of Proper Collection Technique
Due to the critical 9:1 ratio, the light blue top tube requires meticulous collection techniques to ensure the integrity of the sample. It is standard practice to follow a specific “Order of Draw” when multiple tubes are being collected during a single venipuncture. The light blue tube is typically drawn second, right after any blood culture bottles, to prevent contamination from additives in other tubes that could interfere with the coagulation results.
If a butterfly needle is used for the draw, a non-additive or discard tube must be drawn first, even if no other tests are ordered. This initial tube removes the air from the tubing, ensuring the light blue tube fills completely to the required volume. Underfilling the tube is the most common collection error, resulting in an excess of sodium citrate relative to the blood volume.
This imbalance dilutes the sample and artificially extends the clotting time. A falsely prolonged test result could impact patient diagnosis or treatment.
Once the tube is filled to the designated line, it must be gently inverted three to six times to thoroughly mix the blood with the sodium citrate. Vigorous shaking is avoided as it can damage the red blood cells, a condition known as hemolysis, which compromises the accuracy of the test results. Adhering to these strict collection guidelines is necessary to provide the laboratory with a quality specimen suitable for accurate coagulation analysis.