Free Phlebotomy Practice Test · Topic Drill
Order of Draw Practice Test
25 exam-style questions, one at a time — just like exam day. Score and full explanations at the end. Free, no signup.
The CLSI order of draw is the single most-tested procedure in phlebotomy, because drawing tubes out of sequence lets additives carry over and quietly ruin results. These 25 questions drill the full order — blood cultures, light-blue citrate, serum tubes, green heparin, lavender EDTA, then gray — plus tube colors, additives, and why the sequence exists. Every explanation shows the additive-carryover reason behind the rule, so the order sticks instead of being memorized cold.
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Question 1 of 25.According to the current CLSI order of draw for a multi-tube venipuncture, which tube is collected FIRST?
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According to the current CLSI order of draw for a multi-tube venipuncture, which tube is collected FIRST?
- Blood culture bottle or SPS tube (correct answer)
- Light-blue sodium citrate (coagulation) tube
- Lavender K2EDTA tube
- Gold-top serum separator tube
Blood cultures are always drawn first so the specimen has the lowest possible risk of contamination with skin flora or additive carryover, protecting the sterility needed to detect true bloodstream infection. The light-blue citrate tube is drawn next, not first, because it follows any sterile culture collection; drawing EDTA or serum tubes before the culture would defeat the whole reason cultures lead the sequence.
In the CLSI order of draw, which tube is collected immediately after blood culture tubes?
- Green sodium heparin tube
- Gray sodium fluoride tube
- Light-blue sodium citrate (coagulation) tube (correct answer)
- Lavender EDTA tube
The light-blue sodium citrate coagulation tube is drawn right after blood cultures so that additive carryover from later tubes cannot contaminate coagulation testing, where even trace EDTA or heparin would falsely alter clotting results. The green, gray, and lavender tubes all come later in the sequence precisely because their additives interfere with coagulation assays if carried backward into the citrate tube.
Which sequence correctly reflects the CLSI order of draw?
- Blood culture, citrate, serum, heparin, EDTA, fluoride/oxalate (correct answer)
- Serum, citrate, blood culture, EDTA, heparin, fluoride/oxalate
- EDTA, blood culture, citrate, heparin, serum, fluoride/oxalate
- Blood culture, EDTA, heparin, serum, citrate, fluoride/oxalate
The correct order is blood culture, then light-blue citrate, then serum (red or gold/SST), then green heparin, then lavender EDTA, then gray fluoride/oxalate. This progression keeps the most contamination-sensitive tube (culture) first and places strongly interfering additives like EDTA and fluoride last. The other options fail on two counts: some misplace EDTA or serum earlier, allowing additive carryover into coagulation and serum tubes, and options 1 and 2 also displace the blood culture out of the first position (behind serum or EDTA), which risks contaminating the sterile culture as well as producing additive-carryover errors.
A phlebotomist must draw a serum separator tube (SST), a light-blue citrate tube, and a lavender EDTA tube. In what order should they be collected?
- EDTA, then SST, then citrate
- SST, then EDTA, then citrate
- EDTA, then citrate, then SST
- Citrate, then SST, then EDTA (correct answer)
Following CLSI order of draw, the citrate tube comes before serum tubes, and serum tubes come before EDTA tubes: citrate, then SST, then EDTA. Drawing EDTA first is the classic mistake because potassium EDTA carried into the next tubes elevates potassium and lowers calcium, and it would ruin the coagulation citrate specimen. Keeping EDTA last protects every additive-sensitive tube ahead of it.
Which additive is contained in a light-blue-top tube?
- Potassium EDTA
- Lithium heparin
- Sodium citrate (correct answer)
- Sodium fluoride
The light-blue-top tube contains sodium citrate (typically 3.2%), the anticoagulant of choice for coagulation testing such as PT/INR and PTT. Potassium EDTA belongs in the lavender tube, lithium heparin in a green tube, and sodium fluoride in the gray tube. Matching citrate to light blue is essential because using the wrong anticoagulant makes clotting results uninterpretable.
A lavender (purple)-top tube most commonly contains which additive?
- Sodium citrate
- Sodium polyanethol sulfonate
- Sodium fluoride and potassium oxalate
- K2 or K3 EDTA (correct answer)
The lavender-top tube contains K2 or K3 EDTA, which preserves cell morphology and prevents clotting for CBC and other hematology tests. Sodium citrate is in the light-blue tube, sodium polyanethol sulfonate (SPS) is in blood culture tubes, and sodium fluoride with potassium oxalate is in the gray tube. Choosing EDTA for the lavender tube reflects its role as the standard whole-blood anticoagulant for cell counts.
Which additive is found in a gray-top tube used for glucose and lactate testing?
- Sodium fluoride with potassium oxalate (correct answer)
- Lithium heparin
- Thrombin clot activator
- K2 EDTA
The gray-top tube contains sodium fluoride, a glycolytic inhibitor, usually paired with potassium oxalate, an anticoagulant, so glucose is preserved by stopping cells from consuming it. Lithium heparin is a green-tube additive and does not block glycolysis, so glucose would keep falling. Thrombin is a rapid clot activator for stat serum tubes, and EDTA is for hematology; neither preserves glucose, which is why fluoride is the additive of choice.
A green-top tube is required. Which additive should it contain?
- Heparin (sodium, lithium, or ammonium) (correct answer)
- Sodium citrate
- EDTA
- Sodium fluoride
Green-top tubes contain heparin, available as sodium, lithium, or ammonium salts, which accelerates antithrombin to inhibit thrombin and prevent clotting for many chemistry and STAT tests. Sodium citrate is in the light-blue tube, EDTA is in the lavender tube, and sodium fluoride is in the gray tube. Heparin defines the green tube because it yields plasma quickly without diluting the sample as liquid citrate would.
The primary function of EDTA as a tube additive is to prevent clotting by:
- activating antithrombin to neutralize thrombin
- binding (chelating) calcium so the coagulation cascade cannot proceed (correct answer)
- inhibiting glycolysis within red blood cells
- activating the clotting cascade to form a firm clot rapidly
EDTA is a chelating anticoagulant: it binds calcium, and because calcium (factor IV) is required at several steps of the coagulation cascade, removing it stops clot formation and preserves whole blood for cell counts. Activating antithrombin describes heparin's mechanism, not EDTA's. Inhibiting glycolysis describes sodium fluoride, and activating the cascade describes clot activators in serum tubes, which do the opposite of what an anticoagulant does.
How does sodium citrate in a light-blue tube prevent the specimen from clotting?
- By potentiating antithrombin III
- By providing a silica clot activator
- By reversibly binding calcium ions needed for coagulation (correct answer)
- By poisoning the glycolytic pathway
Sodium citrate anticoagulates by reversibly binding (chelating) calcium ions, and because calcium is essential to the coagulation cascade, clotting is halted; the binding is reversible, which is what allows the lab to add calcium back and measure clotting times like PT and PTT. Potentiating antithrombin is heparin's action, a silica clot activator promotes clotting rather than preventing it, and poisoning glycolysis is what fluoride does in a gray tube.
What is the main function of heparin as an anticoagulant in green-top tubes?
- It chelates calcium to remove it from the reaction
- It inhibits glycolysis to preserve glucose
- It separates serum from cells with an inert gel
- It accelerates antithrombin, which inhibits thrombin and factor Xa (correct answer)
Heparin works by binding and accelerating antithrombin, which then neutralizes thrombin and factor Xa, blocking clot formation without removing calcium from the sample. Chelating calcium is the mechanism of EDTA and citrate, inhibiting glycolysis is the role of sodium fluoride, and gel separation is a physical feature of serum/plasma separator tubes rather than an anticoagulant action. Because heparin does not bind calcium, it is preferred for many electrolyte and STAT chemistry assays.
A serum separator tube (SST, gold or red/gray tiger top) contains which two components?
- Sodium citrate and a gel barrier
- Lithium heparin and a gel barrier
- A clot activator and a thixotropic separator gel (correct answer)
- K2 EDTA and a clot activator
An SST contains a clot activator (such as silica) to speed clotting plus a thixotropic gel that moves to form a barrier between serum and cells during centrifugation, yielding clean serum. It contains no anticoagulant, so citrate, heparin, and EDTA are all incorrect for an SST; a heparin-plus-gel tube is a plasma separator tube (PST), which is a different product used for plasma rather than serum.
Why must the light-blue sodium citrate tube be filled completely to the indicated fill line?
- An underfilled tube changes the blood-to-additive ratio and falsely prolongs coagulation results (correct answer)
- Citrate evaporates if the tube is not full
- A short draw causes the gel barrier to fail
- Underfilling activates glycolysis and lowers glucose
The citrate tube relies on a fixed 9:1 blood-to-anticoagulant ratio; a short draw leaves excess liquid citrate relative to plasma, which binds more calcium during testing and falsely prolongs PT and PTT results, so short-draw citrate tubes are rejected. Citrate does not simply evaporate, coagulation tubes have no gel barrier, and glycolysis and glucose relate to the gray tube, not the citrate tube.
A phlebotomist draws a lavender EDTA tube before the light-blue citrate tube. What error is most likely to result?
- Falsely low glucose in the citrate tube
- EDTA carryover contaminating the citrate tube and altering coagulation results (correct answer)
- Hemolysis of the EDTA specimen
- Clotting of the serum specimen
Drawing EDTA before citrate can carry EDTA over into the coagulation tube, and EDTA chelates additional calcium and interferes with clotting factor measurement, invalidating PT/PTT results. Correct order of draw places citrate before EDTA specifically to prevent this carryover. Glucose belongs to the gray tube, hemolysis is not caused by tube sequence itself, and clotting of a serum tube is unrelated to placing EDTA before citrate.
Which of the following is the classic example of additive carryover error described by the order of draw?
- Serum activator carried into a blood culture tube
- K2 EDTA carried into a later tube, raising potassium and lowering calcium results (correct answer)
- Citrate carried into a gray tube, raising glucose
- Gel carried into an EDTA tube, lowering the cell count
The best-known carryover error is potassium EDTA transferred into a subsequent tube: it artificially elevates potassium (from the K salt) and depresses calcium (which EDTA chelates), producing dangerous false chemistry results. This is exactly why EDTA is placed near the end of the sequence. Serum activator does not create this specific electrolyte pattern, citrate does not raise glucose, and gel is a physical barrier that is not liquid enough to carry over between tubes.
Which tube color is typically used for a complete blood count (CBC)?
- Light blue
- Gold (SST)
- Lavender (correct answer)
- Gray
The lavender EDTA tube is standard for a CBC because EDTA preserves the size and shape of blood cells and prevents clumping so cells can be counted accurately. The light-blue citrate tube is for coagulation, the gold SST yields serum for chemistry, and the gray tube preserves glucose; none of these keep cell morphology intact for hematology the way EDTA does.
Which tube is the correct choice for prothrombin time (PT/INR) and activated partial thromboplastin time (aPTT)?
- Light-blue sodium citrate tube (correct answer)
- Green lithium heparin tube
- Lavender EDTA tube
- Gray fluoride/oxalate tube
Coagulation studies such as PT/INR and aPTT require the light-blue sodium citrate tube because citrate binds calcium reversibly, letting the lab recalcify the plasma and measure clotting time accurately. Heparin would inhibit thrombin and invalidate the assay, EDTA irreversibly disrupts clotting factors, and fluoride is designed for glucose preservation, so none of these can substitute for citrate in coagulation testing.
A blood culture set is drawn along with several tubes. Why are blood cultures positioned first in the order of draw?
- Because the SPS additive interferes with all other tubes
- To minimize the risk of microbial and additive contamination of the sterile culture (correct answer)
- Because culture bottles fill faster than other tubes
- To reduce hemolysis in the culture bottle
Blood cultures go first to minimize contamination of the sterile specimen with skin flora or additive carryover from other tubes, which is essential because a contaminated culture can trigger unnecessary antibiotics and false diagnoses. It is not that SPS interferes with later tubes, nor a matter of fill speed, and hemolysis is not the driving concern; preserving sterility for accurate microbiology is the reason cultures lead the sequence.
When a winged (butterfly) collection set is used to draw a light-blue citrate tube first, CLSI recommends:
- discarding the citrate tube because it will always clot
- drawing a discard tube first to prime the tubing and prevent a short draw (correct answer)
- filling the citrate tube only halfway to save the additive
- switching to an EDTA tube instead
With a butterfly set, the air in the tubing (dead space) would prevent the first citrate tube from filling completely, so a discard tube is drawn first to prime the line and preserve the critical 9:1 blood-to-citrate ratio. The citrate tube does not automatically clot, deliberately underfilling would falsely prolong results, and substituting EDTA would make coagulation testing impossible; priming the tubing is the correct fix.
Which of the following is a common cause of specimen hemolysis during blood collection?
- Drawing blood through a resolving hematoma at the site
- Prolonged, overly tight tourniquet application before the draw
- Using an oversized needle on a small, low-flow vein
- Vigorously shaking the tube or forcing blood through a small-gauge needle (correct answer)
Hemolysis, the rupture of red cells, is commonly caused by mechanical trauma such as vigorously shaking a tube or forcing blood through a needle that is too small, which shears the cells and releases potassium and hemoglobin. The other options are also genuine contributors to hemolysis in practice: drawing through a hematoma pulls in already-damaged cells, a prolonged tight tourniquet promotes cell rupture and hemoconcentration, and a needle too large for a small vein creates turbulent, high-shear flow. The keyed choice is the most direct and classic mechanical cause. Hemolyzed samples falsely elevate potassium and are often rejected.
After collection, additive tubes should be mixed by:
- vigorously shaking them 5-10 times
- gently inverting them the manufacturer's recommended number of times (correct answer)
- vortexing them on a mixer
- leaving them completely still to avoid hemolysis
Additive tubes are mixed by gently inverting them the manufacturer's specified number of times so the anticoagulant or clot activator blends thoroughly with the blood without traumatizing the cells. Vigorous shaking or vortexing causes hemolysis, while leaving an anticoagulant tube completely still allows microclots to form, which ruins hematology and coagulation results; gentle inversion balances adequate mixing against cell damage.
A gel-barrier serum tube (SST) is drawn but only half fills before the vein collapses. What effect on the potassium result is expected from this short-draw SST?
- Falsely decreased potassium from additive dilution of the sample
- Falsely increased potassium from concentration of the additive
- An unusable result because the blood-to-additive ratio is disrupted
- No significant effect, because SSTs contain no liquid additive to alter a ratio (correct answer)
Unlike liquid-anticoagulant tubes, an SST contains only a clot activator and gel with no liquid additive, so a partial fill does not change a blood-to-additive ratio and a short draw alone will not skew the potassium result the way it would in a citrate tube. The dilution, concentration, and disrupted-ratio effects in the distractors all depend on a liquid additive whose proportion changes with fill volume, which the SST does not have; that is precisely why a short-draw SST potassium is not compromised by fill volume alone.
Why is the gray-top fluoride/oxalate tube placed last in the CLSI order of draw?
- Because fluoride and oxalate carried into earlier tubes can interfere with other assays (correct answer)
- Because it must be drawn while the tourniquet is still tight
- Because glucose degrades unless it is drawn first
- Because it contains no additive and cannot contaminate anything
The gray tube is drawn last because its sodium fluoride and potassium oxalate additives would interfere with other tests if carried backward; for example, oxalate can distort cell morphology and potassium salts can falsely raise potassium, so placing it at the end protects earlier tubes. Tourniquet tightness is a separate technique issue, drawing it first would risk contaminating other tubes, and it clearly does contain additives, which is exactly why it goes last.
A phlebotomist notices a small clot in a lavender EDTA tube submitted for a CBC. What is the most appropriate action?
- Run the sample anyway because small clots do not affect counts
- Manually break up the clot and then run the sample
- Reject and redraw the specimen because clots invalidate cell counts (correct answer)
- Add more EDTA to the tube to dissolve the clot
A clot in an EDTA tube means cells and platelets were consumed into the clot before mixing was adequate, so counts (especially platelets) will be falsely low and the specimen must be rejected and redrawn. Running or manually disturbing a clotted sample yields unreliable results, and adding EDTA after the fact cannot reverse a clot that has already formed. Prompt, gentle inversion at collection is what prevents this error.
A pink-top tube used for blood bank (type and screen) testing contains which additive?
- Sodium citrate
- Lithium heparin
- Sodium fluoride
- K2 EDTA (spray-dried) (correct answer)
The pink-top tube contains spray-dried K2 EDTA and shares the EDTA position in the order of draw with the lavender tube; it is used for blood bank and immunohematology testing and has special labeling requirements for patient identification. Sodium citrate is for coagulation, lithium heparin is a green-tube additive, and sodium fluoride is for glucose preservation, so EDTA is the correct additive that links the pink tube to the lavender tube's placement.
FAQ: Order of Draw
What is the CLSI order of draw, in order?
Blood culture bottles (sterile) first, then light-blue sodium citrate (coagulation), then serum tubes (red or gold/SST, with or without clot activator), then green sodium/lithium heparin, then lavender K2EDTA, and gray sodium fluoride/potassium oxalate last. The logic is additive carryover: culture tubes lead so the specimen stays sterile, citrate follows so clot activators and other additives never contaminate a coagulation sample, and EDTA sits near the end because its potassium and chelating action skew chemistry and coagulation tests if it carries backward.
Why does drawing tubes out of order ruin results?
Because a trace of additive from the previous tube clings to the needle and carries into the next one. EDTA drawn before a coagulation tube falsely prolongs PT/PTT and drops calcium; a clot activator carried into a citrate tube can trigger clotting; heparin in a lavender tube distorts a CBC. The CPT exam leans hard on this because it is invisible — the tubes fill and look fine, but the numbers are wrong, so the technician has to know the sequence cold rather than catch the error by eye.
Do I still follow the order of draw with a butterfly (winged) set?
Yes — the sequence is identical, with one addition: if a light-blue citrate tube is the first or only tube, draw a plain (non-additive) discard tube first to prime the tubing and remove the air in the line, so the citrate tube fills to its exact fill line and keeps the 9:1 blood-to-anticoagulant ratio. A short-fill citrate tube is one of the most common rejected specimens, and this drill drills that scenario specifically.
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