These serum tubes contain an inert gel barrier that separates the serum from the blood clot during centrifugation, preventing contamination of the serum. For example, in serum certain analytes such as potassium, phosphorus and glucose should be separated from the cells within a few hours - otherwise
the results will be significantly impacted. 

The main advantages of gel tubes versus non-gel tubes are:
• Stable barrier between serum and clotted blood during transportation and storage, leading to better analyte stability
• Better sample quality
• Optimization of the work flow: short centrifugation time, sample processing and archiving in the primary tube
• No requirement for secondary tubes, reducing the possibility of mis-identification

The minimum recommended clotting time for is 30 minutes.