The sodium citrate tube is a specialized blood collection device identified by its light blue stopper. Its primary function is to collect whole blood samples that must remain in a liquid, non-clotted state for specific laboratory analysis. The tube contains a measured solution of sodium citrate, which acts as an anticoagulant to preserve the sample’s integrity for testing the body’s ability to form a clot. This tube is used for studies that measure the function of the coagulation system, ensuring clotting is only initiated under controlled laboratory conditions.
The Anticoagulation Mechanism
Sodium citrate prevents blood from clotting through a chemical process known as chelation. The coagulation cascade, which is the body’s natural clotting pathway, requires ionized calcium to activate many of its protein factors. Calcium ions are a necessary cofactor for the biochemical reactions that lead to the formation of a stable blood clot.
When blood enters the tube, the sodium citrate solution immediately binds to and removes the free calcium ions from the plasma. This binding, or chelation, halts the entire clotting cascade by making the calcium unavailable to the coagulation factors. Because the clotting process is stopped at its earliest stages, the blood sample remains liquid. This anticoagulant effect is reversible, meaning technologists can add calcium back to the sample to deliberately start the clotting process under controlled conditions.
Primary Laboratory Applications
The main purpose of the sodium citrate tube is to provide a preserved sample for a range of coagulation tests that assess a patient’s hemostasis. These tests are performed by adding a substance to the plasma that triggers the clotting process, with the time taken to form a clot being precisely measured.
Prothrombin Time (PT)
The Prothrombin Time (PT) test is one of the most common applications, used to evaluate the extrinsic and common pathways of the coagulation cascade. The PT is frequently used to monitor patients taking the anticoagulant medication warfarin.
Activated Partial Thromboplastin Time (aPTT)
Another widely performed test is the Activated Partial Thromboplastin Time (aPTT), which specifically measures the integrity of the intrinsic and common clotting pathways. The aPTT is primarily used to monitor patients receiving unfractionated heparin therapy.
Other Assays
Other significant applications include the measurement of Fibrinogen, a protein that is the direct precursor to the final fibrin clot, and D-dimer testing. The D-dimer test measures a product of fibrin degradation, and elevated levels suggest that a significant clot has formed somewhere in the body and is being broken down.
The Critical Importance of Sample Integrity
Accurate coagulation testing depends on a precise blood-to-anticoagulant ratio, fixed at nine parts whole blood to one part sodium citrate solution. This 9:1 proportion ensures the correct balance of anticoagulant is present to chelate the calcium without excessively diluting the sample. The tube is manufactured with a vacuum to pull the exact volume of blood required to achieve this ratio.
A technical error known as under-filling the tube is a common pre-analytical problem that can severely compromise results. When too little blood is collected, the concentration of sodium citrate becomes proportionally too high, which excessively dilutes the plasma and can falsely prolong clotting times. This could lead to a misdiagnosis of a bleeding disorder or an incorrect adjustment to a patient’s medication dose.
Conversely, over-filling the tube results in too little anticoagulant relative to the blood volume, which may not chelate all the calcium and can allow the blood to partially clot before testing. Proper handling also includes mixing the tube gently by inversion immediately after collection, typically three to five times, to ensure the additive is fully dispersed. Furthermore, the sodium citrate tube must be drawn early in the sequence of tubes, generally second, to prevent contamination from other tube additives.