Blood Collection Tubes

Blood collection tubes are sterile, single-use vacuum tubes designed to safely collect venous blood for laboratory tests. Each tube contains a predefined vacuum and, if needed, a specific additive (anticoagulant, clot activator, preservative, or separator gel) tailored to the type of test. The vacuum passively draws blood into the tube once the patient’s vein is accessed, making collection safer and more convenient. Tubes are color-coded by cap shank (per ISO 6710) to indicate the additive inside. For example, light-blue tops hold sodium citrate (an anticoagulant for coagulation studies), lavender (purple) tops hold EDTA (a calcium-chelator for hematology), green tops hold heparin, and red or gold tops have no additive (or a clot activator plus gel) for serum chemistry tests. In short, the cap color “tells you exactly what additive is inside, and the additive determines what happens to the blood after collection”.

Blood collection tubes (often called vacutainer tubes) are sterile glass or plastic tubes pre-filled with a vacuum. When venipuncture is performed, the vacuum draws a precise volume of blood into the tube. Each tube has a color-coded cap indicating which additive (if any) is inside. These additives (anticoagulants, clot activators, separating gels, or preservatives) stabilize the blood for specific laboratory tests. For example, anticoagulants like EDTA, citrate, or heparin prevent clotting so that plasma or whole blood can be tested; clot activators (silica particles) induce clotting to yield serum; and gels (in “separator” tubes) form a barrier between cells and serum after centrifugation Some tubes also contain preservatives (e.g. sodium fluoride) that inhibit metabolic reactions (such as glycolysis) to stabilize analytes like glucose.

Types of Tubes (Color and Additive)

Common blood collection tubes (with their typical additives and uses) include:

• Light Blue Top (Citrate): Contains 3.2% sodium citrate anticoagulant. Used for coagulation studies (e.g. prothrombin time PT and activated partial thromboplastin time aPTT). Citrate binds calcium to prevent clotting, producing plasma upon centrifugation. Because accurate testing requires a strictly 9:1 blood-to-citrate ratio, the tube must be filled to the marked line. After draw, immediately invert gently to mix.

• Red Top (Plain/Serum): No anticoagulant (glass tube) or contains clot activator (plastic tube). Blood in a red-top tube clots naturally; after 10–60 minutes at room temperature the clot is removed by centrifugation to yield serum. Red tops are used for general chemistry and serology tests that require serum. (Note: For many chemistries, a “tiger-top” or Gold SST tube is preferred.)

• Gold or “Tiger-Top” (SST – Serum Separator Tube): These have a clot activator plus an inert gel. The gel forms a barrier between clot and serum when centrifuged. They are colloquially called gold, marble-top, or tiger-top tubes. Use gold SST tubes for chemistry panels, immunoassays, and other tests on serum. After collection, invert 5–10 times to mix clot activator, allow clotting (≈30–60 min), then centrifuge. (CDC recommends waiting at least 30 min up to 60 min for complete clot formation.)

• Green Top (Heparin): Coated with heparin (either sodium heparin or lithium heparin). Heparin inhibits thrombin, preventing clotting so that plasma is obtained. Green-top tubes are used for many STAT plasma chemistries (e.g. electrolytes, chemistry panels, blood gases). There is also a light-green/Lithium Heparin tube with a gel separator (PST) for plasma—used similarly in chemistry.

• Lavender (or Purple) Top (EDTA): Contains K₂EDTA anticoagulant. EDTA chelates calcium and prevents clotting. Lavender top tubes yield whole blood or plasma and are the standard for hematology (e.g. CBC) and blood bank compatibility testing. Pink-top tubes are similar (K₂EDTA) and are typically used for transfusion crossmatch and other immunohematology tests.

• Gray Top (Oxalate/Fluoride): Contains potassium oxalate (anticoagulant) and sodium fluoride (glycolysis inhibitor). Used for glucose and lactate measurements (fluoride stabilizes blood sugar by inhibiting enzymes) and for alcohol levels. After draw, invert to mix additives.

• Yellow Top (ACD or SPS):

  • ACD (Acid-Citrate-Dextrose): Some yellow-top tubes contain ACD solution, used for specialized tests (e.g. HLA phenotyping, DNA/PBMC collection, autosomal genetic tests) because it preserves cells.
  • SPS (Sodium Polyanethol Sulfonate): Blood culture bottles (for microbiology) often have yellow or pink stoppers. These contain SPS, which prevents clotting and inhibits complement. (Don’t confuse these culture bottles with the serum/plasma tubes above.)

• Other Colors: Specialty tubes include royal blue (trace element tubes, either EDTA or clot-activated, for lead/analyses), tan (lead EDTA), black (citrate for erythrocyte sedimentation rate), orange (rapid serum tubes with thrombin), and others. These are used only for specific tests. The exact coloring and additives are standardized (CLSI) but can vary slightly by manufacturer

Venipuncture Technique and Order of Draw

Patient Preparation: Always use two patient identifiers (e.g. name and DOB) and confirm them against the lab requisition before drawing blood. Obtain consent and explain the procedure. Address any latex sensitivities (use non-latex tourniquets and gloves if needed).

Equipment: Use a sterile needle (typically 21–23 gauge for adults; 23–25 G butterfly for children or difficult veins) with a holder (vacutainer adapter) and the required tubes. Check each tube for an intact seal and suitable expiration date; a tube with a compromised vacuum will not fill properly.

Vein Selection: Apply the tourniquet 7–10 cm above the intended puncture site to engorge veins, but no longer than 1 minute. Palpate veins (usually median cubital in the antecubital fossa). Clean the site with 70% isopropyl alcohol and allow it to air-dry (~30 seconds). Do not touch the site after disinfection.

Order of Draw: If multiple tubes are needed, follow the CLSI-recommended sequence to prevent additive carry-over:

1. Blood cultures (sterile bottles) – first, to avoid contamination.
2. Coagulation tube (citrate, light blue) – next, to avoid contamination by EDTA or clot activators. (Always fill citrate tubes to the fill line for a 9:1 ratio.)
3. Serum tubes (red or gold SST) – for chemistry/serology.
4. Heparin tubes (green) – for stat chemistries.
5. EDTA tubes (lavender or pink) – for hematology/blood bank.
6. Glycolytic inhibitor tubes (gray) – last, as additives here can alter other tests.

Follow any institutional variations, but the principle is “sterile first, then citrate, then serum, then heparin, then EDTA, then gray”. Always gently invert each tube immediately after collection to mix blood with additive (see below) and label each tube at the bedside with patient ID, date/time, and initials.

Drawing the Blood: Insert the needle with the bevel up at a shallow angle (~15°) through the skin into the vein. Stabilize the needle and holder arm with your hand. Once blood flow begins, release the tourniquet as soon as enough blood is flowing (to avoid hemoconcentration. Lower the patient’s arm (to ensure flow) and allow each tube to fill under vacuum. Do not remove the needle or the tube during filling; the vacuum will automatically draw the correct volume if the tube seal and vacuum are intact.

After the Draw: Remove the last tube, then the needle. Release the tourniquet before removing the needle to prevent hematoma. Immediately cover the puncture site with clean gauze and ask the patient to apply pressure until bleeding stops (especially important if on anticoagulants). Dispose of the needle in a sharps container (no recapping) and place used tubes in a biohazard rack or container for transport. Label all tubes securely before leaving the patient.

Mixing and Handling of Samples

Right after collection, mix anticoagulated tubes by gently inverting them end-over-end 5–10 times (as per manufacturer). A full inversion is 180° to 0° and back. This ensures the blood thoroughly contacts the anticoagulant or clot activator. Do not shake the tubes vigorously, as that can cause hemolysis or microclots. For example, CLSI guidelines state that inadequate mixing leads to clotting or hemolysis, producing inaccurate results. Once mixed, keep the tubes upright.

Clotting (for serum tubes): After drawing blood into a clot-activator tube (red/gold), allow it to clot at room temperature (typically 15–30 minutes) before centrifugation. Do not centrifuge wet clots – fibrin strands can contaminate serum and clog analyzers. After clotting, centrifuge as soon as possible (ideally within 2 hours). Serum/plasma separated from cells promptly minimizes analyte changes (cells metabolize glucose, etc.).

Transportation: Carry tubes to the lab immediately in a tube rack or sealed transport box to avoid breakage or spillage. If tests are delayed, follow handling instructions (some require refrigeration, light protection, etc.). For example, some specimens (like ammonia) require chilling on ice. Label each sample clearly and ensure documentation matches.

Collection and Handling

Blood is drawn using a double-ended Vacutainer needle and holder system. After venipuncture, the first tube is pushed onto the holder’s inner needle; the tube cap is punctured and its vacuum pulls blood into the tube. When the tube fills, it is removed and the next tube is inserted. Each tube’s vacuum stops drawing once its preset volume is reached.

Key handling steps:

• Mixing: Immediately after filling, tubes containing anticoagulant must be gently inverted (usually 5–10 times) to mix blood with the additive. Overly vigorous shaking can cause hemolysis; under-mixing can allow small clots to form.
• Clot formation: Serum tubes (red, gold) require time to clot before centrifugation (~30–60 min).
• Labeling: Tubes should be labeled at the bedside with patient ID and collection time before leaving the patient to avoid misidentification.
• Storage: After collection, tubes are transported to the lab (some tests require prompt processing or specific temperature).

All tubes are single-use and sterile. They bear manufacturer expiration dates: an expired tube may have lost its vacuum and will not fill properly. Always check expiry before use.

Indications and Tests

Doctors and nurses choose tubes based on the ordered tests. Key associations are: a lavender-top EDTA tube is used for CBC/Hematology; a light-blue citrate tube for coagulation profiles (e.g. PT/INR); red or gold-top serum tubes for general chemistry and immunology panels); a green-top heparin tube for stat chemistries and some hematology DNA tests; and a gray-top tube for glucose and alcohol assays. For example, drawing a potassium level in an EDTA tube will artifactually raise potassium (EDTA releases intracellular potassium), and drawing a coagulation test in a plain tube will allow clotting before analysis. Thus, using the correct tube is essential for valid results.

Tips, Precautions, and Best Practices

• Patient Identification: Always re-check patient ID (name/DOB) with the armband and lab request to prevent mix-ups. Mislabeling is a serious error.
• Sterile Technique: Never touch the tip of the needle, and keep the puncture site from recontamination after cleaning. Use gloves and change gloves if they become contaminated.
• Tourniquet Use: Apply the tourniquet just long enough to find a vein, then release as soon as blood flows. Prolonged tourniquet time (>1 minute) can cause hemoconcentration (spuriously high protein, calcium, potassium). It can also increase patient discomfort.
• Vein Selection: Avoid veins that have hematomas, sclerosis, or are adjacent to an infusion site. If the patient has an IV in one arm, draw from the other (or below the IV site, stopping infusion first). Avoid fist clenching or pumping (can falsely raise potassium). Have the patient make a loose fist only if needed to fill, and then relax.
• Needle Gauge: Use an appropriate gauge – smaller needles (e.g. 25G butterfly) for children, elderly, or fragile veins. A too-small needle or too-rapid suction can cause hemolysis. In general practice, a 21–22G needle is standard for adults.
• Avoid Syringe Transfers: Whenever possible use a vacuum tube system (vacutainer). Transferring blood from a syringe into tubes increases hemolysis risk and needle-stick risk. CLSI guidelines explicitly recommend against using syringes for routine draws. If you must use a syringe, transfer slowly and carefully to avoid hemolyzing cells.
• Mixing: Mix anticoagulated tubes immediately by gentle inversion. Failure to mix promptly can lead to clot microfibrils or cell settling. Mix each tube the recommended number of times (generally 5–10 flips). After mixing, store tubes upright.
• Preventing Hemolysis: Avoid trauma to the vein. Do not use the same tourniquet or harsh stripping of the arm. Remove the needle smoothly. Hemolysis (red tint in serum/plasma) falsely elevates potassium, LDH, AST and can invalidate tests. The Korean guidelines note that improper collection techniques causing hemolysis or inadequate volume account for “60%–70% of laboratory errors”. Gentle technique, correct order of draw, and prompt mixing all reduce these errors.
• Tube Fill Volume: Fill tubes to the indicated level (marked line) whenever possible. Some tests (especially coagulation and blood gas) require exact volumes for accurate results. An underfilled citrate tube will have excess anticoagulant, prolonging clot times. Overfilling can also skew ratios.
• Labelling: Label each tube immediately after collection (bedside or in front of patient) with at least two identifiers. Never label in advance or allow tubes to be unlabeled longer than necessary.
• Disposal: Never reuse tubes. Dispose of used collection tubes and needles in appropriate biohazard sharps containers. Used tubes should not be opened or manipulated outside the lab.
• Patient Comfort and Safety: Watch for signs of syncope (ask patients to lie down if they feel faint). If a hematoma begins forming, stop the draw and apply pressure. If patients are on anticoagulant therapy (warfarin, DOACs), hold pressure longer (3–5 minutes or until bleeding stops). Advise patients not to bend their arm at the puncture site for a few hours after.

In summary, using blood collection tubes correctly means choosing the right tube for each test, following proper venipuncture technique, and handling the sample carefully. Each tube’s additive and volume are carefully calibrated for specific assays. Respecting the order of draw and mixing requirements prevents additive carryover and clots. Gentle, efficient collection minimizes hemolysis and pre-analytical error (which, per quality studies, accounts for most lab errors). When done properly, these tubes provide high-quality samples for accurate lab results, supporting optimal patient diagnosis and care.