What should be done immediately after a blood sample is collected and before labeling?

After a blood sample is collected, it is crucial to gently mix the tube to ensure uniformity of the sample without introducing bubbles or causing hemolysis that can occur from violent shaking. This step ensures that the anticoagulant, if present, is thoroughly mixed with the blood, thus preventing clot formation in cases where a serum or plasma sample is required. Mixing also helps preserve the integrity of the analytes being measured, leading to more accurate results.

The other options do not address the need for immediate and appropriate handling of the sample before labeling. Allowing the sample to rest for an hour could compromise the integrity of the sample, especially if clotting occurs. Storing the sample in a refrigerator prior to mixing is also inappropriate, as temperature fluctuations can affect the stability of certain components in the blood. Lastly, labeling while still in the puncture site is not ideal, as it doesn't guarantee accurate sample identification and can lead to errors. Therefore, gently mixing the blood sample in the collection tube is the correct procedure to follow immediately after collection.