Every phlebotomy tube has a color-coded stopper that tells you what additive is inside, what specimen type it produces, and which tests it supports. Knowing this chart cold is non-negotiable for the ASCP PBT exam. It's also the difference between collecting a specimen that gets results and one that gets rejected at the lab door.
How to Read This Chart
Each tube section below covers: additive, mechanism, specimen type, inversion count, primary uses, order of draw position, and the exam-critical details that distinguish it from tubes it's commonly confused with. This isn't a quick list. It's everything you need to know about each tube to answer any question the exam throws at you.
Yellow Tube: Blood Culture / SPS
| Additive | Sodium polyanethol sulfonate (SPS) |
| Specimen | Whole blood |
| Inversions | 8-10 |
| Draw order | First |
| Primary use | Blood cultures (bacteremia, fungemia, sepsis workup) |
SPS does three things simultaneously: it neutralizes antibiotics already in the patient's blood, inhibits complement proteins that would destroy bacteria, and suppresses phagocytosis by white blood cells. The combined effect is that organisms survive long enough to grow in culture media. Without SPS, the patient's own immune system would kill the organisms before the lab could identify them.
Yellow SPS tubes come first in the order of draw because contaminating a blood culture with skin flora is the primary source of false-positive results. The first milliliters of blood from a venipuncture have the highest exposure to skin surface organisms, so they go directly into culture media before anything else touches the needle.
Don't confuse: Yellow ACD (acid citrate dextrose) tubes are also yellow and contain a completely different additive. ACD is used for HLA typing, paternity testing, and DNA studies. The difference is on the label. In clinical practice, yellow SPS is far more common in routine collections.
Light Blue Tube: Sodium Citrate (Coagulation)
| Additive | 3.2% sodium citrate |
| Specimen | Citrated plasma |
| Inversions | 3-4 (gentle) |
| Draw order | Second |
| Primary use | PT, INR, PTT, D-dimer, fibrinogen, mixing studies |
Sodium citrate chelates calcium reversibly. That reversibility is the entire point. When the lab runs a PT or PTT, they add calcium back to the sample to restart the clotting cascade, then measure how long it takes to clot. If the anticoagulant were irreversible (like EDTA), you couldn't restart the cascade. Citrate must be reversible for coagulation testing to work.
The 9:1 blood-to-citrate ratio is critical. Underfill the tube and excess citrate dilutes clotting factors, producing artificially prolonged results. A patient on warfarin with a falsely elevated INR gets an unnecessary dose reduction-all because the tube was underfilled. The underfill changes the citrate-to-blood ratio and artificially prolongs clotting time. Fill to the line. Always.
The butterfly discard rule: When using a winged infusion set, draw a discard tube before the light blue. The dead space in butterfly tubing fills with air, not blood, and that air volume displaces blood from the citrate tube, producing an underfill. The discard tube flushes the dead space first.
Black Tube: Sodium Citrate (ESR Only)
| Additive | 3.8% sodium citrate (higher concentration) |
| Specimen | Citrated whole blood |
| Inversions | 3-4 |
| Draw order | Second (with light blue) |
| Primary use | Erythrocyte sedimentation rate (ESR) by Westergren method only |
The black tube is one of the most commonly missed distinctions on the ASCP exam. It looks similar in function to the light blue (both contain sodium citrate) but it is not interchangeable. The black tube uses 3.8% sodium citrate, not 3.2%. The Westergren ESR method is specifically validated for the 3.8% concentration. Using a light blue tube for ESR or a black tube for coagulation will produce inaccurate results. They look similar on a requisition, but the test and the tube are matched.
Red Tube: Plain Serum
| Additive | None (glass) or polymer clot activator (plastic) |
| Specimen | Serum |
| Inversions | 0 (glass) / 5 (plastic) |
| Draw order | Third |
| Primary use | Serology, blood bank, some chemistry, drug testing |
The plain red tube (glass) contains nothing. Blood clots naturally when it contacts the glass surface. Plastic red tubes have a clot activator coating because blood doesn't clot on contact with plastic the same way it does with glass. Either way, the end product is serum: the liquid portion of blood after the clot forms and is removed by centrifugation. Serum doesn't contain fibrinogen or the other clotting factors, which makes it cleaner for certain assays.
Clot time is 30 minutes minimum at room temperature. Don't refrigerate before the clot forms, cold slows the process and you'll centrifuge an incompletely clotted specimen. The resulting fibrin strands cause analyzer clogs and test interference.
Gold / SST Tube: Serum Separator
| Additive | Silica clot activator + thixotropic gel |
| Specimen | Serum (gel-separated) |
| Inversions | 5 |
| Draw order | Third (with red) |
| Primary use | BMP, CMP, lipid panel, LFTs, hormones, most therapeutic drug monitoring |
The gold SST is the workhorse tube of most outpatient labs. The silica particles accelerate clot formation. The thixotropic gel is the clever part: at rest, the gel is viscous and sits at the bottom. During centrifugation, shear forces temporarily liquefy it, and the density difference between serum and cells carries the gel to the interface where it re-solidifies. The gel forms a stable physical barrier that separates serum from cells for up to 48 hours of refrigerated storage without the cells breaking down and altering analyte concentrations.
Still requires 30 minutes of clot time before centrifugation. Early centrifugation traps fibrin in the gel, creating a hazy specimen that clogs instruments and produces unreliable results.
Orange Tube: Rapid Serum (Thrombin)
| Additive | Thrombin (rapid clot activator) + gel |
| Specimen | Serum |
| Inversions | 5-6 |
| Draw order | Third (with serum tubes) |
| Primary use | STAT chemistry panels requiring serum |
Thrombin directly cleaves fibrinogen to fibrin, bypassing the entire coagulation cascade. Clot time is reduced to about 5 minutes, making this tube useful when serum results are needed urgently. Less common than SST in routine collections but shows up on the exam as a knowledge check.
Green Tube: Lithium Heparin
| Additive | Lithium heparin |
| Specimen | Plasma |
| Inversions | 8-10 |
| Draw order | Fourth (after serum tubes) |
| Primary use | STAT metabolic panels, ammonia, ionized calcium, some therapeutic drugs |
Heparin inhibits thrombin and factor Xa, blocking coagulation without removing the clotting factors from the sample. Plasma from a heparin tube still contains fibrinogen. That's the primary difference between plasma and serum: serum has had fibrinogen consumed in clot formation, plasma retains it.
The advantage of lithium heparin over SST is speed. No clot time required. Centrifuge immediately after collection, have plasma in roughly 10-15 minutes. STAT situations often run on heparin plasma rather than waiting 30 minutes for serum. The mint green (PST) variant adds a gel separator for the same plasma stability benefit the SST offers serum.
Never use heparin tubes for coagulation tests. Heparin itself is an anticoagulant drug. Running PT or PTT on heparin plasma would be measuring the drug's effect, not the patient's own clotting function.
Lavender / Purple Tube: EDTA (Hematology)
| Additive | K2 EDTA (ethylenediaminetetraacetic acid) |
| Specimen | Whole blood (anticoagulated) |
| Inversions | 8-10 |
| Draw order | Fifth |
| Primary use | CBC, WBC differential, reticulocyte count, HbA1c, blood smear, ESR (some methods), sickle cell screen |
EDTA permanently chelates calcium and magnesium, shutting down the coagulation cascade completely. Blood stays liquid, cells stay intact and countable, morphology is preserved. The K2 form is the current standard. K3 EDTA (an older formulation) has largely been replaced because K2 produces less platelet swelling and better red cell morphology for microscopy.
EDTA also uniquely causes EDTA-dependent pseudothrombocytopenia in a subset of patients, less than 0.2% of the population. Their platelets clump in EDTA tubes, making the automated platelet count falsely low. The fix is sodium citrate tubes, where the platelet clumping doesn't occur. If a patient repeatedly has abnormally low platelets with no clinical bleeding and looks fine, EDTA-dependent pseudothrombocytopenia should be on the differential.
Pink Tube: EDTA (Blood Bank)
| Additive | K2 EDTA (same as lavender) |
| Specimen | Whole blood |
| Inversions | 8-10 |
| Draw order | Fifth (with lavender) |
| Primary use | Blood bank: type and screen, crossmatch, antibody identification |
The additive is identical to the lavender tube. The pink color exists purely to distinguish blood bank specimens from routine hematology specimens, reducing the risk of a transcription or labeling error that could result in an incompatible transfusion. The consequences of a wrong blood type transfusion are severe: acute hemolytic transfusion reactions can be fatal. The color distinction is a safety mechanism, not a chemistry distinction. Many facilities require blood bank tubes to be collected under a specific protocol with a separate, phlebotomist-signed label to establish chain of custody.
Gray Tube: Sodium Fluoride / Potassium Oxalate
| Additive | Sodium fluoride + potassium oxalate |
| Specimen | Plasma or whole blood |
| Inversions | 8-10 |
| Draw order | Last (sixth) |
| Primary use | Fasting glucose, glucose tolerance test (GTT), lactate, blood alcohol concentration (BAC) |
Sodium fluoride inhibits enolase, the glycolytic enzyme that would otherwise allow red blood cells to keep consuming glucose after collection. Without fluoride, glucose drops roughly 5-7 mg/dL per hour at room temperature. A borderline fasting glucose of 99 mg/dL becomes 90 mg/dL after two hours in a plain tube. That's a clinically meaningful difference for diabetes screening.
Potassium oxalate is a secondary anticoagulant that prevents clotting by chelating calcium. It lets the tube function as a whole blood or plasma specimen, not just a cellular one.
Gray goes last in the order of draw because fluoride and oxalate interfere with enzyme-based chemistry assays. Any carryover into chemistry tubes would produce falsely low enzyme activity.
Royal Blue Tube: Trace Elements
| Additive | K2 EDTA (blue label), heparin (green label), or none (red label) |
| Specimen | Varies by variant |
| Draw order | After routine tubes, collected last or separately |
| Primary use | Heavy metals, trace element analysis: zinc, copper, selenium, arsenic, lead, manganese |
The royal blue tube is manufactured with ultra-low metal content rubber stoppers and metal-free materials throughout. Standard tube stoppers leach trace amounts of zinc, chromium, and other metals that would contaminate trace element specimens and falsify results. The additive variant (indicated by label color) depends on the element being tested. Know that the tube exists for trace elements and that additive varies by specimen type.
Tan Tube: Lead Testing
Additive: K2 EDTA. Use: Lead (Pb) level exclusively. Like royal blue, uses metal-free stopper materials to prevent stopper contamination. The tan tube is specifically validated for lead and is collected after routine tubes.
Inversions: The Right Technique
Inversion numbers are specific for a reason. Too few inversions leave additive clumped at the bottom, producing inconsistent anticoagulation. Too many can mechanically damage red blood cells and cause hemolysis.
Proper technique: rotate the tube end-over-end completely (180 degrees), then back to upright. That's one inversion. Slow, complete rotation. Not a wrist flick, not shaking. Shaking creates turbulence that ruptures red blood cells, releasing intracellular contents including potassium, LDH, AST, and hemoglobin. A shaken lavender tube might come back with a potassium of 7.0 and a visibly red-tinged plasma. That's hemolysis, and the specimen goes back for recollection.
Serum vs. Plasma: Why It Matters Clinically
Serum (red, gold) is what remains after blood clots. The clotting process consumes fibrinogen and other clotting factors. Serum does not contain these proteins. Plasma (green, lavender, light blue) is anticoagulated whole blood after cells are removed. Plasma retains fibrinogen, clotting factors, and everything else that would have been in the clot. Some tests can only run on one or the other. Coagulation tests must run on plasma because you need the clotting factors present to test the clotting pathway. Most chemistry tests can run on either, though reference ranges may differ slightly.
Frequently Asked Questions
What is the difference between lavender and pink tubes?
The additive is identical: K2 EDTA in both. Pink tubes are reserved exclusively for blood bank (type and screen, crossmatch). The color distinction is a patient safety mechanism to prevent labeling errors that could result in an incompatible transfusion.
What is the difference between light blue and black tubes?
Both contain sodium citrate, but different concentrations. Light blue contains 3.2% sodium citrate and is used for coagulation testing (PT, PTT, INR). Black contains 3.8% sodium citrate and is used exclusively for Westergren ESR. They are not interchangeable.
Why does the light blue tube need to be completely filled?
The sodium citrate tube requires a 9:1 blood-to-citrate ratio. Underfilling changes this ratio, causing excess citrate to dilute clotting factors and artificially prolong PT and PTT. Most laboratories reject underfilled citrate tubes.
What tubes produce serum vs. plasma?
Serum tubes: red (plain) and gold/SST. Plasma tubes: green (heparin), light green/PST (lithium heparin + gel), light blue (sodium citrate), and gray (fluoride/oxalate). Lavender and pink produce whole blood specimens (cells are analyzed, not separated).
What is the correct number of inversions for each tube?
Light blue: 3-4 gentle inversions. Gold/SST: 5 inversions. Green/PST and lavender/pink/gray: 8-10 inversions. Red glass: 0 inversions. Red plastic: 5 inversions.
What tube is used for a STAT potassium result?
Green (lithium heparin) or light green (PST), which produce plasma without clot time. A gold SST requires 30 minutes before centrifugation. If the clinical situation is truly urgent, heparin plasma is faster. Note that lithium in lithium heparin tubes interferes with lithium assays, so a red or gold tube is needed for lithium drug levels.