Which color-coded tube should be used when collecting blood if serum is required?

The correct tube for collecting blood when serum is required is the red tube. This tube contains no additives, allowing the blood to clot naturally. Once the blood has clotted, it can be centrifuged to separate the serum from the cellular components. The resulting serum is then available for various laboratory tests, making the red tube the ideal choice for serum collection.

Using the other options would not yield the desired serum. The green tube typically contains heparin, which prevents clotting and is ideal for plasma collections. The blue tube contains sodium citrate, which also prevents clotting and is commonly used for coagulation tests, not for serum. The gray tube often contains additives like potassium oxalate or sodium fluoride, primarily used for glucose testing or other specific tests, but not for collecting serum. Thus, for serum requirements, the red tube is the appropriate choice.