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The light blue evacuated tube is specifically designed for coagulation testing because it contains sodium citrate as an anticoagulant. This anticoagulant binds calcium ions in the blood, which is crucial for the coagulation process. By preventing clotting, the sample can be tested for various coagulation factors and tests such as PT (prothrombin time) and aPTT (activated partial thromboplastin time). The 9:1 ratio of blood to anticoagulant in the light blue tube is balanced to ensure accurate results for these tests, making it the preferred choice in the laboratory for evaluating clotting ability.

Other colored tubes have different additives intended for various types of testing; for instance, red tubes typically do not contain any anticoagulants and are used for serum tests, while green tubes contain heparin, which would interfere with coagulation tests. The purple tube is primarily utilized for hematology tests. Hence, the light blue tube's specific formulation and intended use establish it as the most suitable option for coagulation testing.