Light Blue Top Tube

The additive is buffered sodium citrate at 3.2 percent concentration. Citrate binds calcium reversibly, which means the lab can recalcify the sample later to run the test. EDTA cannot do this; it binds calcium too tightly. Citrate is the right additive for coagulation testing because of that reversibility.

The 9-to-1 ratio is non-negotiable. Nine parts blood to one part citrate. Most light blue tubes hold 2.7 mL or 4.5 mL, with the citrate already pre-loaded. The tube is calibrated to fill to the line at exactly the right ratio.

Underfilling shifts the ratio. With less blood and the same citrate, there is more anticoagulant per blood volume than there should be. The PT and aPTT come back falsely prolonged. Most labs reject any citrate tube under 90 percent of the fill line.

PT and INR (prothrombin time, international normalized ratio) for monitoring warfarin anticoagulation. aPTT for monitoring heparin therapy and screening for clotting factor deficiencies.

Fibrinogen and D-dimer for evaluating clot formation and breakdown. Both depend on the same citrate tube.

Specialty coagulation panels (factor assays, lupus anticoagulant, von Willebrand panels) all start with a light blue tube. Some send-out labs require a frozen aliquot drawn into a dedicated light blue.

Fill the tube to the indicator line. Most tubes have a fill arrow on the label; tilt the tube and watch the meniscus reach the line. Stopping short by even a few mL invalidates the test.

Invert 3 to 4 times. Citrate dissolves quickly into the blood; you do not need 8 to 10 inversions. More than 4 inversions risks micro-clot formation from over-mixing.

When using a butterfly (winged infusion set) and the light blue is the first tube, draw a discard tube first. The butterfly tubing holds 0.5 to 1 mL of air; that air enters the citrate tube and breaks the fill volume. The discard tube purges the dead space.