Why each tube sits where it does
| # | Tube | Reason |
|---|---|---|
| 1 | Blood cultures | Drawn first so the sterile sample is not contaminated by anything on the needle. |
| 2 | Sodium citrate | Coagulation tests are thrown off by clot activator or other anticoagulants carried over, and the tube must be filled to the line. |
| 3 | Serum tubes | Clot activator carried into a citrate or heparin tube would start clotting there, so serum comes after citrate. |
| 4 | Heparin | Heparin carried into a serum tube would stop it clotting properly. |
| 5 | EDTA | EDTA is potassium salt that binds calcium: carried over, it falsely raises potassium and lowers calcium in chemistry tubes. |
| 6 | Glycolytic inhibitor | Fluoride and oxalate damage cells and would affect sodium, potassium and enzyme results in other tubes, so gray goes last. |
Mnemonic
Stop Light Red, Stay Put, Green Light Go: Sterile cultures, Light blue, Red, SST, PST, Green, Lavender, Gray.
For tube colors, additives and exam traps in more depth, read the order of draw and tube colors guide.
Try 5 phlebotomy (CPT) practice questions
Which order of draw is correct for a routine venipuncture using evacuated tubes?
Show answer
B. Blood culture, coagulation tube, SST, heparin, EDTA, glycolytic inhibitor The correct answer is blood culture, coagulation tube, SST, heparin, EDTA, glycolytic inhibitor. The order of draw prevents cross-contamination of additives between tubes. Blood cultures are drawn first to minimize contamination risk. Coagulation tubes must follow blood cultures because tissue thromboplastin can affect clotting times. SST comes before heparin and EDTA. EDTA is drawn before glycolytic inhibitors because potassium EDTA can interfere with certain tests. Incorrect order can cause erroneous results due to additive carryover between tubes.
What color top does a tube containing EDTA have?
Show answer
C. Lavender or purple The correct answer is lavender or purple. EDTA (ethylenediaminetetraacetic acid) tubes have a lavender or purple stopper and are used for hematology tests such as complete blood counts and hemoglobin A1c. EDTA binds calcium to prevent clotting while preserving cell morphology. Red-top tubes contain no additive or a clot activator. Blue-top tubes contain sodium citrate for coagulation studies. Green-top tubes contain heparin for chemistry tests. Knowing tube colors is essential for correct test ordering and specimen collection.
When collecting a blood culture, approximately how much blood should be drawn into each culture bottle?
Show answer
D. 20-30 mL for adults, divided between aerobic and anaerobic bottles The correct answer is 20-30 mL for adults, divided between aerobic and anaerobic bottles. Adequate blood volume is critical for blood culture sensitivity. The recommended volume is 20-30 mL for adults, typically split 10-15 mL per bottle (aerobic and anaerobic). Insufficient volume reduces the likelihood of detecting bacteremia. Drawing only 5-10 mL total halves the sensitivity. One to two milliliters is far too low for reliable detection. Overfilling bottles can cause false positives or processing errors. Pediatric volumes differ based on the child's weight.
What is the primary purpose of applying a tourniquet before venipuncture?
Show answer
D. To distend the veins by restricting venous blood flow The correct answer is to distend the veins by restricting venous blood flow. A tourniquet is applied 3-4 inches above the venipuncture site to impede venous return while allowing arterial flow to continue. This causes veins to fill and become more visible and palpable for needle insertion. Numbing the skin is not a function of the tourniquet. Increasing arterial flow is the opposite of what occurs. Cleaning the skin is performed with an antiseptic swab, not a tourniquet. The tourniquet should not remain in place longer than one minute.
Before performing a venipuncture, a phlebotomist must verify the patient's identity using how many patient identifiers?
Show answer
A. Two identifiers, such as name and date of birth The correct answer is two identifiers, such as name and date of birth. The Joint Commission's National Patient Safety Goal NPSG.01.01.01 requires the use of at least two patient identifiers when collecting specimens. This prevents mislabeling and ensures the specimen matches the correct patient. Relying only on name is insufficient since patients may share names. Medical record number alone is one identifier and needs pairing. Social Security numbers are not recommended identifiers and are rarely used at bedside for specimen collection.
All free phlebotomy (CPT) questions with answers Take a free phlebotomy (CPT) practice test
Common questions
Blood cultures first, then sodium citrate (light blue), serum tubes with or without clot activator or gel (red, gold, tiger top), heparin (green), EDTA (lavender, pink, tan), and glycolytic inhibitor (gray) last. This is the order in CLSI standard GP41.
A trace of additive can ride on the needle into the next tube. EDTA carried into a chemistry tube, for example, falsely raises potassium and lowers calcium. The order keeps each tube's additive from changing the next tube's results.
"Stop Light Red, Stay Put, Green Light Go": Sterile (cultures), Light blue, Red, SST (gold), PST/green, Green, Lavender, Gray. Gold and light green are the gel versions of red and green.
Draw a discard tube first (a plain red or another light blue) to fill the tubing's air space, so the citrate tube gets the full volume it needs for the 9 to 1 ratio.
No. For skin punctures the order is different: EDTA tubes first, then other additive tubes, then serum tubes, because platelets start clumping at the puncture site. That order comes from CLSI GP42.
Follow the manufacturer's label. BD's guide lists 3 to 4 inversions for citrate, 5 to 6 for serum and gel serum tubes, and 8 to 10 for heparin, EDTA and fluoride tubes. Glass red tubes with no additive need none.
Practice the phlebotomy (CPT) free
Sources, checked 2026-10-29: CLSI GP41, Collection of Diagnostic Venous Blood Specimens, 7th ed. (2017); BD Vacutainer tube guide and recommended order of draw. Your lab’s procedure manual and the tube maker’s insert come first. QuizCram is not affiliated with the agencies or standards bodies named here.
Free to use