Each blood collection tube has a specific color-coded cap that tells you exactly what additive is inside and which lab tests it supports. The color system is standardized across most manufacturers, so a lavender top in one facility means the same thing in another. Here’s a breakdown of every major tube color, what it contains, and the tests it’s designed for.
Light Blue Top: Coagulation Studies
Light blue tubes contain 3.2% sodium citrate, an anticoagulant that works by binding calcium in the blood. This preserves the clotting factors in their natural state so the lab can accurately measure how long your blood takes to clot. Common tests drawn in this tube include prothrombin time (PT), partial thromboplastin time (PTT), INR, fibrinogen, and D-dimer.
Fill level matters more with this tube than any other. The ratio of blood to citrate must be precise, so the tube needs to be filled to at least 80% capacity. An underfilled blue top will be rejected by the lab because the excess citrate throws off the results. For patients with very high red blood cell counts (hematocrit above 55%), the lab may need to adjust the citrate volume.
Red Top: Serum Without Additives
A plain red top tube contains no additives at all, or in some versions, just a clot activator coating the inside walls. Blood drawn into this tube is allowed to clot naturally, then gets spun in a centrifuge to separate the serum from the clot. Red tops are used for serology, some chemistry panels, and blood bank antibody screening. Because they lack gel separators, the serum must be transferred off the clot relatively quickly to avoid changes in certain analytes.
Gold (SST) Top: Chemistry and Serology
Gold top tubes, often called SSTs (serum separator tubes), contain both a clot activator and a gel that sits at the bottom. When the tube is centrifuged, the gel migrates upward and forms a physical barrier between the serum on top and the clot below. This keeps the sample stable longer and makes it easier to handle in the lab.
Gold tops are the workhorse tube for most routine blood work. They cover the majority of chemistry panels (metabolic panels, liver function, kidney function), thyroid and other hormone tests, cholesterol and lipid panels, and infectious disease screening including hepatitis and HIV. If your doctor orders “standard blood work,” much of it is likely run from a gold top tube.
Lavender Top: Hematology
Lavender (purple) tubes contain EDTA, an anticoagulant that preserves blood cells by binding calcium. This keeps the blood in whole, unclotted form so individual cells can be counted and examined. The most common test run from a lavender top is the complete blood count (CBC), which measures red blood cells, white blood cells, platelets, and hemoglobin. Hemoglobin A1c for diabetes monitoring, ESR (sed rate), and reticulocyte counts also use this tube.
After drawing, gently inverting the tube several times ensures the EDTA mixes thoroughly with the blood. Clots that form in an EDTA tube because of poor mixing will make the sample unusable.
Pink Top: Blood Bank and Transfusion
Pink tops also contain EDTA, just like lavender tubes, but they serve a different purpose. These are specifically designated for blood banking, crossmatching, and transfusion compatibility testing. The key difference is labeling: pink top tubes must follow strict patient identification procedures, with the label applied at the bedside and verified against the patient’s identity band. This extra layer of verification prevents the potentially fatal error of transfusing the wrong blood type.
Green Top: Plasma Chemistry
Green top tubes contain heparin, an anticoagulant that works by inhibiting thrombin. They come in two main varieties. Dark green tops use sodium heparin and are typically reserved for special chemistry tests and certain whole blood assays. Light green tops (sometimes called mint green or PST tubes) contain lithium heparin plus a gel separator, functioning as the plasma equivalent of a gold SST. Light green tops handle many of the same routine chemistry panels as gold tops, but produce plasma instead of serum, which can be processed faster since there’s no waiting for the blood to clot.
The choice between a gold SST and a light green PST often depends on the lab’s preference and analyzer requirements. Some facilities default to plasma tubes because they can be centrifuged immediately after collection, speeding up results in urgent situations like emergency departments.
Gray Top: Glucose Testing
Gray top tubes contain two additives working together: potassium oxalate as an anticoagulant and sodium fluoride as a preservative. The sodium fluoride is the important part. It inhibits glycolysis, the process by which red blood cells continue consuming glucose after the blood is drawn. Without this preservative, glucose levels in a blood sample drop roughly 5 to 7% per hour at room temperature.
Gray tops are used for fasting glucose, glucose tolerance tests (GTT), and blood alcohol levels. Any test where you need an accurate snapshot of how much glucose (or alcohol) was in the blood at the moment of collection benefits from this tube’s ability to freeze metabolic activity in place.
Yellow Top: Cultures and Tissue Typing
Yellow top tubes come in two versions with different additives for very different purposes. One contains SPS (sodium polyanethol sulfonate), which is used for microbiology blood cultures to detect bacterial infections in the bloodstream. SPS prevents the blood from clotting while also neutralizing certain immune system components that would otherwise kill bacteria in the sample, giving organisms a better chance of growing in culture.
The other version contains ACD (acid citrate dextrose), which preserves cell viability. ACD tubes are used for HLA typing (tissue compatibility testing for organ transplants), DNA testing, and paternity testing. The dextrose feeds the cells, keeping them alive and functional for analysis.
Royal Blue Top: Trace Elements and Toxicology
Royal blue tubes are manufactured with extra precautions to minimize contamination from trace metals. Regular tubes can leach tiny amounts of metals from their stoppers or glass, which would skew results when you’re trying to measure substances present in very small quantities.
These tubes come in two varieties, identified by a colored stripe on the label. Royal blue tubes with an EDTA additive (lavender stripe) are used for whole blood tests like lead, mercury, arsenic, cadmium, manganese, and heavy metals panels. Royal blue tubes with no additive (red stripe) produce serum and are used for aluminum, chromium, cobalt, copper, zinc, selenium, and nickel testing.
Order of Draw
When multiple tubes are needed from one blood draw, collecting them in a specific sequence prevents additives from one tube contaminating the next. A tiny amount of additive can transfer via the needle between tubes, and even trace contamination can affect sensitive tests. The standard order, based on international clinical guidelines, is:
- Blood culture bottles (yellow or specific culture bottles), always first to maintain sterility
- Light blue (citrate) for coagulation
- Red or gold (serum tubes, with clot activator or SST)
- Green (heparin, including PST)
- Lavender or pink (EDTA)
- Gray (oxalate/fluoride)
The coagulation tube is drawn early because citrate contamination from other additives (especially EDTA) can falsely alter clotting times. However, if no blood culture is ordered, some labs require a “discard tube” to be drawn first before the light blue top, clearing any tissue fluid from the initial needle puncture that could activate clotting factors.
Pediatric and Small-Volume Tubes
The same color coding applies to smaller tubes used for infants and pediatric patients. Microtainer and pediatric-sized tubes follow the same additive and cap color conventions: a purple microtainer contains EDTA just like a full-size lavender top, and a blue pediatric tube contains sodium citrate. The difference is volume. A standard adult lavender top holds about 4 to 5 mL, while an infant microtainer may hold as little as 0.6 mL. The fill requirements still apply, particularly for blue top coagulation tubes, which need to reach at least 80% capacity regardless of tube size.