Final answer:
Sodium citrate is the additive found in a blood collection tube meant to be filled at a 9:1 blood to anticoagulant ratio for coagulation studies.
Step-by-step explanation:
The additive found inside a tube that should be filled to a 9:1 ratio is sodium citrate. This anticoagulant is commonly used in blood collection tubes, primarily for coagulation studies. It works by binding the calcium ions present in blood, which are necessary for blood clotting. When tubes containing sodium citrate are filled to the recommended ratio, typically 9 parts blood to 1 part anticoagulant, they provide the correct balance to prevent clotting while preserving the blood sample for accurate testing.