According to CLSI guidelines, which type of tube should be placed upright as soon as they are mixed?

The correct choice is the nonanticoagulant gel tubes. These tubes contain a gel that serves as a barrier once the sample is centrifuged, separating serum or plasma from the cellular components of the blood. According to Clinical and Laboratory Standards Institute (CLSI) guidelines, after mixing the sample in these tubes, they should be placed upright promptly to allow the gel to create a proper barrier and prevent any contamination between the serum or plasma and the cellular components. This upright positioning is essential for achieving accurate laboratory results, as an improper separation can lead to issues with analyte levels.

Choosing nonanticoagulant gel tubes specifically indicates that these tubes are designed to work in conjunction with the absence of any anticoagulants, which could interfere with the formation of the gel barrier. Other types of tubes like anticoagulant tubes or vacuum blood collection tubes have different requirements for handling and mixing that do not necessitate the immediate upright position for optimal results.