Why must light blue-top tubes be completely filled when collecting coagulation studies?

Prepare for the CAMLPR Specimen Collection Competency Test. Practice with diverse question sets including multiple-choice and flashcards. Each question includes hints and explanations to help you become proficient before the exam.

A precise blood-to-anticoagulant ratio is essential for coagulation tests because the anticoagulant in the tube (citrate) must be present in a standard proportion to the blood. Light blue-top tubes contain sodium citrate that binds calcium to prevent in vitro clotting, and the assays for PT and aPTT assume a fixed citrate-to-blood ratio (about 1 part citrate to 9 parts blood). If the tube isn’t filled to the marked line, the citrate concentration relative to blood changes, causing too much calcium binding and artificially prolonging clotting times. Filling completely ensures the expected ratio and therefore accurate, standardized PT and aPTT results. The other options don’t address why the ratio matters for assay accuracy.