Light-blue-topped tubes are used primarily in the coagulation section of the laboratory. The primary additive or anticoagulant in the light-blue-topped tube is sodium citrate. Sodium citrate binds calcium, which is needed for coagula­tion, thus preventing coagulation. In order to have accurate laboratory results, this tube must be filled to its draw capacity and all vacuum must be exhausted. In most cases, a line is found on the tube to designate the proper fill level. Anything less than full could change the results of the blood test ordered, pos­sibly changing the patient’s treatment. Light-blue-topped tubes have a rela­tively large amount of additive, a 9:1 ratio of blood to sodium citrate. This ratio of blood to additive is more important than in other tubes with additives. The tube should be inverted several times immediately after collection to prevent clotting. Commonly performed coagulation tests include prothrombin time (PT), activated partial thromboplastin time (APTT) or partial thromboplastin time (PTT), fibrinogen, and D-dimers. Collecting the light-blue-topped tube in the incorrect order of draw may alter the results of these tests.

Serum separator tubes (SSTs) are used in the laboratory’s chemistry section. Rubber-stopper SSTs have a speckled look, with black mixed into the red por­tion of the top. Plastic splashguard SSTs are gold-topped. SSTs contain a clot activator and a thixotropic separator gel. The gel separates the serum from the blood cells by settling between the clot and the serum during centrifuga­tion. This separation process helps prevent chemical changes caused by cel­lular metabolism and fibrinolysis (dissolving of the clot). Even after leaving the body, cells continue to consume glucose and create waste products. The changes in these substances over time would invalidate test results if the gel were not present to keep the cells away from the serum or plasma. The gel sep­arator makes it easier for laboratory personnel to obtain the serum for testing; with the gel in place, the cells will not mix with the serum, making it easier to aliquot the liquid for testing. Aliquoting is the process of removing small samples from the original specimen and placing them in another container. Tests commonly performed in chemistry that may require blood collection in an SST include those involving electrolytes, enzymes, glucose, hormones, lip­ids, and proteins.