Mainlined Science

Let’s Talk Blood Collection Tubes and What the Colors Mean!

Ever wonder what those different colored tops on blood collection tubes actually mean? Each color tells us something important about what’s inside the tube and how the blood will be processed. Let’s break it down, adding some cool science along the way!

Red Top – No Additive Your blood is your blood! This tube contains no additives. A key property of blood is clotting—when exposed to air (more accurately activating factors), blood naturally forms a clot as platelets and clotting factors work to seal off a wound. In the lab, we let this clotting happen. As the blood clots, it traps certain components, like cells and proteins, forming a pellet. What’s left behind is serum, which is the liquid portion of blood without cells or clotting factors. This is used for tests like liver function or kidney panels where serum chemistry is important.

Gold or Tiger Top (SST) – Serum Separator Tube Think of this as “Red with a plug!” This tube has a gel that sits between the clot and the serum after spinning the sample in a centrifuge. The gel forms a barrier, making it easier for lab techs to separate serum from the clot—no extra pipetting required. It’s a time-saver in the lab, used for tests similar to those with the red top.

Light Blue Top – Sodium Citrate This tube stops clotting by binding calcium, a key player in the clotting cascade. Without calcium, the cascade can’t complete, so the blood remains unclotted. This is crucial for coagulation studies, like PT/INR (Prothrombin Time/International Normalized Ratio) and aPTT (Activated Partial Thromboplastin Time), which measure how well your blood is clotting. These tests are essential for monitoring patients on blood thinners or those with clotting disorders. The light blue tube is all about studying the clotting process without letting it happen in the tube.

Lavender or Purple Top – EDTA (Ethylenediaminetetraacetic acid) EDTA works similarly to sodium citrate by binding calcium, but it’s used for a different purpose. While citrate prevents clotting for coagulation tests, EDTA is perfect for preserving the shape of blood cells. This makes it essential for tests like Complete Blood Counts (CBCs), where we’re looking at the size, shape, and number of blood cells under a microscope. EDTA doesn’t alter the cells, allowing for accurate analysis, whereas citrate is mainly interested in the clotting factors themselves.

Gray Top – Sodium Fluoride/Potassium Oxalate Here’s where we get a bit more technical. This tube contains sodium fluoride, which inhibits enolase—a key enzyme in glycolysis, the process by which cells break down glucose for energy. Following Le Chatelier's principle, by blocking enolase, the reaction shifts away from breaking down glucose, keeping it stable for measurement. This is why the gray top is used for glucose testing. It also contains potassium oxalate, which further prevents clotting. Additionally, it’s often used for lactate testing, important for understanding conditions like sepsis.

Light Yellow Top – ACD (Acid Citrate Dextrose) This one’s a bit more niche but fascinating. ACD is used in blood banking, especially for DNA testing or preserving blood for extended periods. The citrate prevents clotting by binding calcium, and the dextrose acts as a sugar source to keep the cells happy and viable for a longer shelf life. In donated blood, this mix helps preserve the cells for several weeks, making it critical for blood storage and transfusion medicine.

Pink Top – EDTA (Like Lavender) You might wonder why we have pink when we already have lavender. Pink tops are specifically designated for blood banking, like crossmatches and blood typing. While it uses the same EDTA as lavender, the different color coding helps labs differentiate between general hematology tests (lavender) and blood banking tests (pink), ensuring the right sample goes to the right department.

Dark Blue (Royal Blue) Top – Trace Element-Free This tube is special because it’s trace element-free. The tube itself is rigorously cleaned and prepared to avoid contamination from metals like zinc, copper, or lead. This is critical when testing for trace elements, where even a tiny contamination from the tube could alter the results. Some versions have EDTA, while others are additive-free, depending on the test being performed.

Black Top – Sodium Citrate (Like Light Blue) Similar to the light blue top, this tube also contains sodium citrate, but with a different citrate-to-blood ratio. It’s used for the ESR (Erythrocyte Sedimentation Rate) test, which measures how quickly red blood cells settle in a test tube over an hour. ESR can give clues about inflammation in the body, making it useful for diagnosing conditions like autoimmune disorders or infections. The black top tube’s ratio is optimized for the slow sedimentation process.

Orange Top – Thrombin This tube contains thrombin, a clotting agent that accelerates the clotting process. It’s used when you need serum quickly for emergency chemistry tests, like in the case of urgent cardiac enzyme tests. The thrombin speeds up the clot formation so the serum can be separated faster than in a red or gold top tube.

Green Top – Heparin This guy contains heparin, an anticoagulant that prevents clotting by inhibiting thrombin and other clotting factors. This tube is used for plasma testing in clinical chemistry labs, especially for tests like electrolytes, ammonia, and troponins. Heparinized plasma allows for quicker processing because the blood doesn’t need to clot before being tested, making it ideal for urgent tests, such as when ruling out acute myocardial infarction (MI).

Serum vs. Plasma - a reCAP

Serum is the liquid portion of blood after it has clotted. When blood clots, the cells and clotting factors form a clot, leaving behind a clearer liquid—serum. This is what you get in tubes like the red or gold top (with no additives or a serum separator). Serum contains everything plasma does except the clotting factors, which are trapped in the clot. It’s often used for chemistry panels or hormone tests.

Plasma is the liquid portion of blood before clotting. It’s what you get when you use tubes like the green top (heparin) or lavender top (EDTA) that prevent blood from clotting. Plasma contains water, proteins, electrolytes, hormones, and clotting factors (because the blood hasn’t clotted yet). It’s ideal for tests where you need to measure things like troponins or electrolytes quickly since the sample doesn’t need to clot first.

And there you have it—a breakdown of the colorful world of blood collection tubes and their specific uses. The next time you’re in a lab or having blood drawn, you’ll know exactly what those tubes are doing and why they’re so important!

Have any other lab-related questions? Let’s chat about them below!