Which additive does a light blue top tube contain, and what is its primary use in testing?

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Light blue tubes contain sodium citrate, an anticoagulant that binds calcium to keep blood from clotting. This preserves the clotting factors so that coagulation tests can be run on plasma rather than serum. The key purpose is to measure how long it takes for blood to clot, using tests like PT and aPTT, which rely on factors present in plasma after centrifugation.

For accurate results, the blood-to-anticoagulant ratio must be correct (about 9:1), and the sample should be gently inverted to mix—too little or too much citrate distorts clotting times. This tube is specifically for coagulation studies; other tubes are used for different purposes: EDTA tubes are common for complete blood counts, heparin tubes for certain plasma chemistries, and tubes without additive are used to obtain serum after the blood clots.