A sodium citrate tube is a specialized blood collection device used primarily in diagnostic medicine to assess a patient’s ability to form blood clots. It is identified by its light blue stopper, which signifies the presence of the sodium citrate additive. The tube collects whole blood samples required for coagulation studies. Sodium citrate temporarily prevents the blood from clotting after it has been drawn.
The tube ensures the blood remains liquid until analysis. This temporary preservation is necessary because coagulation testing requires the clotting factors to be available and functional, not consumed by clotting within the tube. Using this specific tube is foundational for accurately diagnosing bleeding and clotting disorders.
The Function of Sodium Citrate
Sodium citrate serves as an anticoagulant by interfering with the blood’s natural clotting process through calcium chelation. Blood coagulation is a cascade of reactions requiring calcium ions as co-factors. Calcium is necessary for activating various coagulation factors needed to form a stable clot.
When blood is collected, the citrate ions immediately bind to the free calcium ions in the plasma. This chelation removes calcium from the sample, halting the coagulation cascade. Sodium citrate ensures the blood sample remains liquid and its clotting factors remain stable for testing.
An advantage of sodium citrate is that its effect is reversible. In the laboratory, scientists can add a controlled amount of calcium back into the plasma sample to restart the clotting process. This allows for the precise measurement of clotting time under controlled conditions, which is the basis of most coagulation tests. This reversible binding makes sodium citrate the anticoagulant of choice for measuring clotting function.
Specific Coagulation Tests
The sodium citrate tube facilitates tests that evaluate the body’s ability to stop bleeding and manage clot formation. These diagnostic procedures are often grouped as hemostasis or coagulation testing. Results are used to monitor patients on anticoagulant medications and to diagnose inherited or acquired blood disorders.
The Prothrombin Time (PT), often reported with the International Normalized Ratio (INR), is a frequently ordered test. The PT measures the extrinsic and common pathways of the coagulation cascade, assessing clotting factors like Factor VII. This test is routinely used to monitor the therapeutic effect of the drug warfarin.
The Activated Partial Thromboplastin Time (aPTT) also relies on the sodium citrate tube. The aPTT evaluates the intrinsic and common pathways, involving factors such as Factor VIII, IX, and XI. This measurement is used for monitoring patients receiving unfractionated heparin therapy.
The D-dimer test measures a specific protein fragment released when a blood clot breaks down. An elevated D-dimer level suggests significant clot formation and breakdown. This test is used to help diagnose conditions like deep vein thrombosis or pulmonary embolism.
Ensuring Sample Integrity
The integrity of a sample collected in a sodium citrate tube depends on a specific, mandatory dilution ratio. To ensure accurate results, the tube must maintain a precise 9:1 ratio of whole blood to liquid anticoagulant solution. This ratio is necessary to achieve the correct concentration of citrate to bind the calcium in the blood volume.
The most common collection error is underfilling the tube, meaning the blood volume does not reach the indicated fill line. Underfilling results in a disproportionate excess of sodium citrate relative to the blood. This excess citrate binds too much calcium, artificially prolonging the clotting time.
A falsely prolonged clotting time can lead to inaccurate clinical interpretations, such as an incorrect diagnosis of a bleeding disorder. Because of this sensitivity, laboratories have strict rejection criteria; tubes not filled to at least 90% of the target volume are discarded. Proper collection technique, including filling the tube completely and gently inverting it to mix, ensures reliable test results.