The color of a blood collection tube is a standardized signal that dictates how a blood sample must be processed and analyzed. This color-coding system ensures the correct tests are performed and that results are reliable. Each cap color signifies a specific chemical additive necessary to maintain the integrity of the blood sample for its intended diagnostic purpose. Using the wrong tube color is a common laboratory error that can invalidate a result, potentially leading to misdiagnosis or treatment delays.
The Science Behind Blood Preservation
The tube’s contents control whether the blood clots after it is drawn. When blood is collected, it naturally begins a coagulation cascade, forming a clot and separating into a liquid component called serum. Serum is blood plasma with the clotting factors removed.
Tubes requiring serum contain clot activators to speed up this process. Conversely, if a test requires whole blood or plasma (the liquid portion containing clotting factors), an anticoagulant additive is necessary. Anticoagulants prevent clotting by interfering with the coagulation cascade, often by binding to calcium ions or inhibiting thrombin. The choice between serum and plasma, dictated by the additive, is necessary for accurate analysis.
Decoding the Most Common Tube Colors
The majority of blood tests rely on four main tube types, each identified by a distinct color and additive. Choosing the correct tube is necessary because an incompatible additive will interfere with the chemical reaction of the test.
Light Blue Top (Coagulation Studies)
The light blue-top tube is used for coagulation studies, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT), which measure the blood’s clotting ability. The additive is sodium citrate, an anticoagulant that works by binding calcium ions, which are essential for clotting. This additive requires a precise 1:9 ratio of anticoagulant to blood for accurate results.
Gold/Red-Speckled Top (Serum Separator Tubes – SSTs)
These tubes are the standard for general chemistry, hormone, and serology tests. SSTs contain a clot activator, often silica particles, to accelerate clotting, and a polymer gel barrier. During centrifugation, the gel forms a stable barrier between the serum and the clotted red blood cells, ensuring a clean serum sample.
Lavender/Purple Top (Hematology)
The lavender or purple-top tube is designated for hematology tests like the Complete Blood Count (CBC) and blood typing. This tube contains Ethylenediaminetetraacetic acid (EDTA), a strong anticoagulant that prevents clotting by irreversibly binding to calcium ions. EDTA is preferred for cellular analysis because it preserves the morphology and integrity of red and white blood cells.
Green Top (Plasma Chemistry)
Green-top tubes contain heparin (lithium or sodium heparin), an anticoagulant that works by enhancing the activity of antithrombin III, inhibiting thrombin. These tubes are primarily used for chemistry tests that require plasma rather than serum, such as emergency (“stat”) chemistry panels and ammonia levels. Because heparin stops the clotting process almost immediately, it allows for a faster turnaround time when rapid results are needed.
The Importance of the Order of Draw
When multiple blood tubes are collected, the sequence in which they are filled is known as the “Order of Draw.” This sequence is standardized by clinical guidelines to prevent the cross-contamination of additives between tubes, which could cause erroneous test results. For example, the EDTA from a lavender tube, if carried over into a light blue coagulation tube, would interfere with the sodium citrate, leading to falsely prolonged clotting times. The standard sequence begins with blood culture bottles, which must be collected first to ensure a sterile sample.
The standard order of draw is:
- Blood culture bottles
- Light blue (coagulation) tube
- Serum tubes (red or gold tops)
- Heparin tubes (green tops)
- EDTA tubes (lavender tops)
Specialized Tubes for Specific Diagnostics
Beyond the common tubes, specialized colors exist for tests with unique sensitivity requirements.
Gray Top (Glucose Testing)
The gray-top tube contains two additives: potassium oxalate as an anticoagulant and sodium fluoride as an antiglycolytic agent. Sodium fluoride inhibits glycolysis (the breakdown of glucose by blood cells). This inhibition is necessary to ensure an accurate measurement of blood glucose levels, such as for the Glucose Tolerance Test, preventing the sugar level from artificially decreasing over time.
Royal Blue Top (Trace Elements)
Royal blue-top tubes are reserved for trace element and toxicology testing (e.g., measuring lead, zinc, or copper). These tubes are manufactured with extremely low levels of trace elements in their stoppers and glass to avoid contaminating the sample, which could interfere with highly sensitive analysis.