Every phlebotomy tube has a color-coded stopper that tells you exactly what additive is inside, what kind of specimen it produces, and what tests it’s used for. Memorizing this chart is non-negotiable for the ASCP PBT exam and for your first week on the job. Here’s every tube with the information you actually need.
Quick Reference: Phlebotomy Tube Color Chart
Yellow — Blood Culture SPS Tube
Additive: Sodium polyanethole sulfonate (SPS)
Specimen type: Whole blood
Inversions: 8-10
Use: Blood cultures (bacteremia/sepsis workup). SPS is an antimicrobial neutralizer that prevents the patient’s own antibiotics and natural bactericidal factors from killing bacteria in the specimen before the lab can culture it.
Order of draw position: First (sterile collection, no additive carryover concern)
Note: Aerobic and anaerobic culture bottles (yellow, sometimes different color caps) are filled with blood inoculated at bedside. The SPS yellow tube is a different product, know both.
Yellow — ACD Tube (Acid Citrate Dextrose)
Additive: Acid citrate dextrose
Specimen type: Whole blood
Inversions: 8-10
Use: HLA typing, paternity testing, DNA studies, blood banking (special circumstances)
Note: Same color as SPS tubes. Distinguish by label. ACD tubes are rarely encountered in routine draws.
Light Blue — Sodium Citrate Tube
Additive: 3.2% sodium citrate
Specimen type: Plasma (citrated plasma)
Inversions: 3-4 (gentle)
Use: Coagulation studies, PT (prothrombin time), PTT (partial thromboplastin time), INR, D-dimer, fibrinogen
Order of draw position: Second (after blood cultures, before all other tubes)
Critical detail: The blood-to-citrate ratio must be exactly 9:1. Underfilling this tube falsely prolongs coagulation results because excess citrate chelates too much calcium. Overfilling is also problematic. Fill to exactly the marked line. If this is the first tube drawn (no blood cultures), use a 1-2 mL discard tube first to clear tissue thromboplastin from the needle.
Red — Serum Tube (No Additive)
Additive: None
Specimen type: Serum (after clotting)
Inversions: 0 (no additive to mix)
Use: Serology, blood bank (type and screen, crossmatch), some chemistry. Allows blood to clot naturally without any additive interference.
Order of draw position: Third (serum tubes, after sodium citrate)
Note: Must clot completely (30 minutes at room temperature) before centrifugation. Do not refrigerate before clotting.
Gold / Tiger Top (SST) — Serum Separator Tube
Additive: Clot activator (silica particles) + polymer gel separator
Specimen type: Serum (after centrifugation, gel creates barrier between serum and clot)
Inversions: 5
Use: Most chemistry panels, BMP, CMP, LFTs, hormones, most therapeutic drug levels. The most commonly used tube in most outpatient labs.
Order of draw position: Third (with other serum tubes)
Critical detail: The clot activator speeds clotting but the specimen still must clot completely (30 minutes) before centrifugation. Centrifuging early traps fibrin strands in the gel and produces a fibrinous haze that clogs analyzers and invalidates results.
Green — Lithium Heparin Tube
Additive: Lithium heparin
Specimen type: Plasma
Inversions: 8-10
Use: STAT chemistry (plasma results available faster than SST because no clotting time required), ammonia, certain point-of-care tests, some toxicology
Order of draw position: Fourth (after serum tubes, before EDTA)
Note: Lithium heparin cannot be used for potassium testing by some methods (lithium/sodium interference). Know your lab’s protocols. Never use heparin tubes for coagulation testing.
Light Green / Mint Green — PST (Plasma Separator Tube)
Additive: Lithium heparin + gel separator
Specimen type: Plasma (gel creates barrier between plasma and cells after centrifugation)
Inversions: 8-10
Use: Same as lithium heparin tube, with the convenience of a gel barrier for plasma stability
Order of draw position: Fourth (with heparin tubes)
Lavender / Purple — EDTA Tube
Additive: EDTA (K2EDTA or K3EDTA), ethylenediaminetetraacetic acid
Specimen type: Whole blood (anticoagulated)
Inversions: 8-10
Use: Complete blood count (CBC), differential, reticulocyte count, HbA1c, blood smears, ESR
Order of draw position: Fifth
How it works: EDTA chelates (binds) calcium ions, which are essential for the coagulation cascade. No calcium = no clotting = whole blood is preserved for cell counting. EDTA also preserves red cell morphology, which is why it’s used for blood smear preparation.
Critical detail: EDTA carryover into sodium citrate tubes will falsely prolong PT/PTT. EDTA carryover into chemistry tubes can affect calcium, magnesium, and iron results. This is why EDTA comes after serum and heparin tubes in the order of draw.
Pink — EDTA Tube (Blood Bank)
Additive: EDTA
Specimen type: Whole blood
Inversions: 8-10
Use: Blood bank (type and screen, crossmatch). The pink color distinguishes blood bank specimens from routine CBC specimens collected in lavender. Blood bank tubes often require a separate phlebotomist-signed label for chain-of-custody purposes.
Gray — Sodium Fluoride / Potassium Oxalate Tube
Additive: Sodium fluoride (NaF) + potassium oxalate
Specimen type: Whole blood or plasma
Inversions: 8-10
Use: Glucose (fasting or random), lactate, blood alcohol concentration (BAC)
Order of draw position: Last (sixth)
How it works: Sodium fluoride inhibits enolase, an enzyme in the glycolytic pathway. Without this inhibitor, red blood cells continue metabolizing glucose even after collection, dropping glucose levels approximately 5-7 mg/dL per hour at room temperature. The gray top preserves glucose for accurate measurement.
Note: Potassium oxalate is a secondary anticoagulant that prevents clotting by chelating calcium (similar mechanism to citrate, but not used for coagulation testing).
Royal Blue — Trace Element Tube
Additive: May be EDTA (with K), heparin, or no additive, depends on the stopper color variant (look at the label)
Specimen type: Varies by variant
Use: Heavy metal testing, trace element analysis (zinc, copper, selenium, arsenic, lead). Made with ultra-low metal content rubber to prevent contaminating the specimen with metals from the stopper itself.
Tan — Lead Testing Tube
Additive: K2EDTA
Specimen type: Whole blood
Use: Lead (Pb) testing. Like royal blue, uses low-metal materials to prevent stopper contamination of the specimen.
Inversions: Why They Matter and How to Do Them
Inversions mix the additive with blood. The number varies because over-mixing can damage cells (hemolysis) and under-mixing leaves additive unincorporated.
Proper inversion: rotate the tube end-over-end completely, 180 degrees, then back. One complete rotation = one inversion. Gentle and complete. Never shake, shaking creates turbulence that ruptures red blood cells, causing hemolysis that falsely elevates potassium, LDH, AST, and other intracellular analytes.
Common counts: 3-4 for sodium citrate (gentle, fill ratio critical), 5 for SST, 8-10 for EDTA and heparin, 8-10 for gray top.
Order of Draw Summary
Using the tube color chart above:
1. Blood cultures (yellow SPS)
2. Sodium citrate (light blue)
3. Serum tubes (red, then gold/SST)
4. Heparin tubes (green, light green)
5. EDTA tubes (lavender, pink)
6. Glycolytic inhibitor (gray)
Royal blue and tan tubes go last, or are collected separately from routine draws.
ASCP PBT Exam Tips for Tube Knowledge
Tube color questions on the exam are almost never simple recall (“what color is the EDTA tube?”). They’re scenario-based: “A phlebotomist draws an EDTA tube before a sodium citrate tube. The coagulation result is critically prolonged. What is the most likely cause?” You need to know not just the tube colors but the additive mechanisms and the consequences of carryover between tubes.
Our ASCP PBT study guide has a dedicated Equipment chapter covering every tube with full carryover scenarios and practice questions. The free quiz includes Equipment-domain questions so you can test your current tube knowledge right now.