Understanding the Cream-Colored Liquid in Blood Test Tubes

Have you ever noticed the cream-colored liquid at the bottom of a blood test tube before blood is drawn? This phenomenon is not uncommon and can be explained by different factors, such as the presence of a gel separator or a clot activator.

Gel Separator

Many blood collection tubes contain a gel that separates the serum or plasma from the cellular components of the blood after centrifugation. This gel is often a cream or pale yellow color and serves to facilitate the separation of blood components when the sample is processed.

The presence of this gel in an unused tube is typically normal and part of the tube's design. This gel helps with the separation of blood components during processing, making it easier to obtain the required samples for further testing. After blood collection and centrifugation, the serum or plasma will be on top, while the gel and cellular components will remain at the bottom.

Certain blood tests require blood serum only and not whole blood. When blood is drawn using a vacutainer, it is left to sit for about 30 minutes before being placed in a centrifuge. The cream-colored substance serves as a separator, allowing the serum to rise to the top and be separated from the rest of the blood and clotting factors. After the sample is spun down, the serum is poured off for testing, while the remaining materials are discarded.

Clot Activator

Some blood collection tubes contain additives to promote clotting, such as silica or certain polymers. These additives can also appear as a cream-colored liquid or substance at the bottom of the tube.

It's important to note that if the tube is unused and you see the cream-colored liquid, it is normal and part of the tube's design. However, if there is any doubt about the integrity of the tube or if it appears contaminated, it's best to consult with a healthcare professional or laboratory technician before proceeding with the blood draw.

Distinguishing Between Different Tube Types

Just as there are different colored tube tops for different tests, there are also different reagents in these tubes. Each tube is designed to accommodate specific test requirements, and it's crucial to use the correct tube for the appropriate test. For instance, blood tests that require serum only benefit from the presence of a separator gel, while tests that need whole blood may have anticoagulants added to prevent clotting.

For example, chemistry profiles, which measure blood sugar, kidney and liver function, and electrolytes, are performed using serum-only vacutainers. To obtain a complete blood count (CBC), a different vacutainer is used, one that does not have a serum separator and does not need to be spun down. These vacutainers contain small amounts of anticoagulants to prevent clotting and help in measuring red and white blood cell counts as well as other clotting factors.

Understanding the role of the cream-colored liquid in blood test tubes is essential for ensuring accurate and reliable test results. By knowing the appropriate tube to use and the purpose of the reagents, healthcare professionals can perform comprehensive and efficient diagnostic evaluations.