The standard order of draw for venipuncture follows a specific sequence: blood culture bottles first, then light blue, red or gold, green, lavender (purple), and gray. This sequence exists to prevent additives in one tube from contaminating the next and skewing test results. Even tiny amounts of carryover, as little as 5 microliters, can cause clinically significant errors.
The Complete Order of Draw
The Clinical and Laboratory Standards Institute (CLSI) sets the standard that labs and hospitals follow. Here is the full sequence:
- Blood culture bottles (yellow or yellow-black caps): Always drawn first to minimize contamination risk from skin bacteria or additives in other tubes.
- Light blue top: Contains sodium citrate for coagulation tests like PT and PTT.
- Red, gold, or red-speckled top: Serum tubes, some with clot activator or gel separator. Used for chemistry panels, thyroid tests, and many other common blood tests.
- Green top (dark green, light green, or speckled green): Contains heparin. Used for chemistry tests that require plasma rather than serum.
- Lavender (purple), pink, or pearl top: Contains EDTA. Used for complete blood counts and blood bank testing.
- Gray top: Contains sodium fluoride and potassium oxalate. Used specifically for glucose and lactate levels.
A simple memory aid many phlebotomy students use: “Boys Love Red Green Lights Going” for blood culture, light blue, red/gold, green, lavender, gray.
Why the Sequence Matters
Each tube contains a different additive (or no additive at all), and when one tube’s additive transfers into the next tube, it can interfere with test results. This happens because a small amount of the previous tube’s additive can cling to the inside of the needle or tube holder.
The consequences are real and measurable. Research published in Clinical Chemistry and Laboratory Medicine found that contamination of heparin tubes with citrate caused clinically significant changes in ionized calcium results at volumes as low as 5 to 50 microliters. That’s a barely visible droplet. Heparin carryover into coagulation tubes caused significant interference at similarly small volumes, potentially making a normal clotting result look abnormal or vice versa.
EDTA is particularly problematic if it contaminates other tubes. It aggressively binds calcium, which would falsely lower calcium levels in a chemistry panel and can interfere with coagulation testing. That’s why lavender tops are drawn near the end of the sequence, well after the light blue coagulation tube and the chemistry tubes.
What Each Tube Color Does
Light blue tubes contain sodium citrate, which works by binding calcium in the blood. Since calcium is essential for clotting, removing it keeps the blood in a liquid state so the lab can measure how long clotting takes under controlled conditions.
Red top tubes contain no anticoagulant at all. The blood is meant to clot naturally so the lab can separate out the serum, the liquid portion left after clotting. Gold and red-speckled tubes work the same way but include a gel that physically separates the serum from the clot during centrifugation, plus a clot activator to speed things along.
Green tubes use heparin, which prevents clotting by blocking thrombin, one of the key proteins in the clotting process. These are used when a test needs plasma (blood’s liquid component with clotting factors still present) rather than serum.
Lavender tubes contain EDTA, which also prevents clotting by binding calcium. EDTA is the preferred anticoagulant for blood counts because it preserves the size and shape of blood cells better than other options.
Gray tubes serve a specialized purpose. The potassium oxalate prevents clotting, while sodium fluoride acts as a preservative that stops red blood cells from continuing to consume glucose after the blood is drawn. Without this preservative, glucose levels in the tube would drop over time, giving a falsely low reading.
Special Rules for Light Blue Tubes
The light blue tube has a unique requirement. It must be filled to the exact line marked on the tube because coagulation tests are sensitive to the ratio of blood to sodium citrate. An underfilled tube will have too much citrate relative to blood, which can produce inaccurate clotting times.
When using a butterfly needle (winged infusion set), the tubing contains air that displaces some of the first tube’s volume. If the light blue tube is drawn first in this situation, it won’t fill completely. The standard practice is to draw a small “discard” tube first, typically 1 to 2 milliliters into another light blue citrate tube, then discard it and collect the actual specimen. This ensures the real tube fills to the correct level. If blood cultures are ordered alongside the light blue, the blood culture bottles serve as that first draw and no discard tube is needed.
Mixing Tubes After Collection
After filling each tube, you need to gently invert it (turn it upside down and back) a specific number of times to mix the blood with the additive inside. This isn’t optional: insufficient mixing leads to small clots forming in tubes that are supposed to stay liquid, which can ruin the sample.
The inversion counts vary by tube type:
- Light blue (citrate): 4 to 5 inversions
- Red/gold (serum separator): 5 inversions
- Green (heparin): 8 to 10 inversions
- Lavender (EDTA): 8 to 10 inversions
- Gray (fluoride/oxalate): 8 to 10 inversions
The tubes with more aggressive anticoagulants need more mixing. Gentle inversions are key. Shaking tubes vigorously can damage blood cells (a problem called hemolysis), which ruins many test results.
Capillary Draws Use a Different Order
When blood is collected from a fingerstick or heelstick rather than a vein, the order of draw essentially reverses. The EDTA tube (lavender) is collected first, followed by tubes for chemistry and blood bank testing. This is because capillary collection produces much smaller volumes of blood, and platelets begin clumping almost immediately at the puncture site. Collecting the hematology specimen first minimizes the effect of platelet clumping on the complete blood count, which is typically the most important test ordered from a capillary draw.
This reversed order catches many students off guard, so it’s worth remembering that the venipuncture sequence and the capillary sequence have different priorities. Venipuncture focuses on preventing additive contamination between tubes. Capillary collection focuses on getting the most accurate cell counts before the tiny sample starts clotting at the skin surface.