22 Equipment & Order of Draw Practice Questions & Answers
Every Equipment & Order of Draw practice question from the Phlebotomy Technician Practice Test, with the correct answer and a short explanation.
Start practice test →1. According to the CLSI recommended order of draw, which tube is always collected first?
- A.Light-blue sodium citrate (coagulation) tube
- B.Blood culture / sterile tube✓ Answer
- C.Lavender/purple EDTA tube
- D.Gray sodium fluoride tube
Blood cultures are drawn first because sterility is critical; drawing them before any additive tube prevents additive carryover and skin-flora contamination that would produce false-positive or invalid culture results.
Source: CLSI GP41, recommended order of draw (step 1: blood culture/sterile)Report a problem with this question
2. What anticoagulant/additive is contained in a light-blue-top evacuated tube?
- A.3.2% sodium citrate✓ Answer
- B.K2 EDTA
- C.Lithium heparin
- D.Sodium fluoride / potassium oxalate
The light-blue tube contains 3.2% sodium citrate, a reversible anticoagulant used for coagulation testing because its exact 9:1 blood-to-additive ratio can be reversed in the lab to run PT/aPTT accurately.
Source: CLSI GP41 color-code additive mapping (light blue = sodium citrate 3.2%)Report a problem with this question
3. A lavender/purple-top tube contains which additive?
- A.Sodium citrate
- B.EDTA✓ Answer
- C.Heparin
- D.No additive
Lavender/purple tubes contain EDTA, which binds calcium to prevent clotting and preserves cell morphology, making it the standard tube for hematology tests such as the CBC.
Source: CLSI GP41 color-code additive mapping (lavender = EDTA)Report a problem with this question
4. A green-top evacuated tube contains which anticoagulant?
- A.EDTA
- B.Sodium citrate
- C.Heparin✓ Answer
- D.Sodium fluoride
Green-top tubes contain heparin (lithium or sodium), which works by activating antithrombin to inhibit thrombin, keeping the plasma unclotted for many chemistry tests.
Source: CLSI GP41 color-code additive mapping (green = heparin)Report a problem with this question
5. Which additive combination is found in a gray-top tube?
- A.Clot activator + gel
- B.EDTA + gel
- C.Sodium fluoride / potassium oxalate✓ Answer
- D.Sodium citrate
Gray-top tubes contain sodium fluoride as an antiglycolytic agent plus potassium oxalate as the anticoagulant; the fluoride preserves glucose by inhibiting cell metabolism, making it ideal for glucose and lactate testing.
Source: CLSI GP41 color-code additive mapping (gray = sodium fluoride/potassium oxalate)Report a problem with this question
6. What does a gold-top (tiger-top) SST tube contain?
- A.EDTA and a gel separator
- B.A clot activator and a gel separator✓ Answer
- C.Sodium citrate only
- D.Heparin and a gel separator
The gold/SST tube contains a clot activator to speed clotting plus a thixotropic gel that forms a barrier between serum and cells after centrifugation, yielding serum for chemistry testing.
Source: CLSI GP41 color-code additive mapping (gold/SST = clot activator + separator gel)Report a problem with this question
7. In the order of draw, the lavender/EDTA tube is collected in which relative position?
- A.Before the citrate tube
- B.Before the serum tubes
- C.After heparin but before the gray fluoride tube✓ Answer
- D.First, before blood cultures
EDTA is drawn late (after serum and heparin, before the gray tube) because EDTA carryover is especially damaging to earlier tubes; placing it near the end minimizes contamination of chemistry and coagulation specimens.
Source: CLSI GP41 recommended order of draw (step 5: EDTA)Report a problem with this question
8. Why is EDTA carryover into a subsequent chemistry tube a problem?
- A.It falsely lowers potassium and raises calcium
- B.It falsely elevates potassium and falsely lowers calcium✓ Answer
- C.It has no effect on electrolytes
- D.It falsely raises glucose
Potassium-EDTA salt carryover falsely elevates the potassium result, while EDTA's calcium-binding (chelating) action falsely lowers the measured calcium; this is a key reason EDTA is placed after chemistry tubes in the order of draw.
Source: CLSI GP41 rationale for order of draw (EDTA carryover: high K, low Ca)Report a problem with this question
9. What is the required blood-to-anticoagulant ratio for a sodium citrate (light-blue) coagulation tube?
- A.1:1
- B.4:1
- C.9:1✓ Answer
- D.1:9
Coagulation tubes require a 9:1 ratio of blood to citrate; this precise ratio is essential because the liquid citrate volume is fixed, and the correct fill neutralizes exactly the right amount of calcium for valid clotting times.
Source: CLSI GP41 / GP42 citrate tube fill (9:1 blood-to-anticoagulant)Report a problem with this question
10. Why is a short-draw (underfilled) sodium citrate tube rejected for coagulation testing?
- A.Excess citrate falsely prolongs clotting times (PT/aPTT)✓ Answer
- B.Too little citrate causes the sample to clot instantly
- C.It has no effect if capped properly
- D.It falsely shortens clotting times
An underfilled tube has too much citrate relative to plasma, so extra citrate binds calcium added during testing and falsely prolongs PT/aPTT; that is why citrate tubes must be filled to at least ~90% of their draw volume.
Source: CLSI GP41 / H21 short-draw citrate rejection criterionReport a problem with this question
11. Regarding needle gauge, which statement is correct?
- A.A larger gauge number means a larger bore
- B.A larger gauge number means a smaller bore✓ Answer
- C.Gauge number is unrelated to bore size
- D.A larger gauge number means a longer needle
Gauge is inversely related to bore diameter: the larger the gauge number, the smaller the internal diameter of the needle, so a 23-gauge needle has a narrower bore than a 21-gauge needle.
Source: NHA CPT Test Plan / CLSI GP41 needle selection (gauge inverse to bore)Report a problem with this question
12. Which needle gauge is typically used with a winged (butterfly) collection set?
- A.16 gauge
- B.18 gauge
- C.23 gauge✓ Answer
- D.14 gauge
A 23-gauge needle is typical for butterfly sets because its smaller bore suits small, fragile, or difficult veins (such as hand or pediatric veins), whereas routine straight venipuncture usually uses 21–22 gauge.
Source: NHA CPT Test Plan / CLSI GP41 (butterfly typically 23 g; routine 21–22 g)Report a problem with this question
13. When a light-blue citrate tube must be the first tube collected using a butterfly (winged) set, what should be done first?
- A.Draw the citrate tube immediately with no other step
- B.Draw a discard tube first to prime the tubing✓ Answer
- C.Draw an EDTA tube first
- D.Remove the air from the tube with a syringe
A butterfly set's tubing holds air that would underfill the first tube; drawing a discard tube first fills (primes) that dead space so the citrate tube collects the full volume and maintains the required 9:1 ratio.
Source: CLSI GP41 winged-set discard tube for citrate as first tubeReport a problem with this question
14. By what mechanism does heparin act as an anticoagulant?
- A.It chelates (binds) calcium
- B.It inhibits thrombin✓ Answer
- C.It precipitates fibrinogen
- D.It inhibits glycolysis
Heparin works by potentiating antithrombin, which inhibits thrombin (and factor Xa), blocking the conversion of fibrinogen to fibrin; unlike EDTA or citrate, it does not act by binding calcium.
Source: CLSI GP41 additive mechanisms (heparin inhibits thrombin)Report a problem with this question
15. How does sodium fluoride in a gray-top tube preserve the specimen?
- A.It acts as an antiglycolytic agent, preserving glucose✓ Answer
- B.It activates clotting to form serum
- C.It binds calcium to prevent clotting only
- D.It lyses red blood cells
Sodium fluoride is an antiglycolytic agent that inhibits the enzyme enolase, stopping blood cells from consuming glucose; this preserves the true glucose level for accurate testing, especially when analysis is delayed.
Source: CLSI GP41 additive mechanisms (fluoride = antiglycolytic glucose preservative)Report a problem with this question
16. EDTA and sodium citrate both prevent clotting by which shared mechanism?
- A.Inhibiting thrombin directly
- B.Chelating (binding) calcium✓ Answer
- C.Inhibiting glycolysis
- D.Activating antithrombin
Both EDTA and citrate chelate calcium, and because calcium (factor IV) is required at several steps of the coagulation cascade, removing it prevents clot formation; heparin, by contrast, works on thrombin.
Source: CLSI GP41 additive mechanisms (EDTA/citrate chelate calcium)Report a problem with this question
17. How many inversions does a light-blue sodium citrate tube typically require for adequate mixing?
- A.0 inversions
- B.3–4 inversions✓ Answer
- C.8–10 inversions
- D.20 inversions
Citrate tubes are gently inverted about 3–4 times to mix the liquid anticoagulant with blood; too few inversions risks micro-clots, while EDTA, heparin, and gray tubes require more (about 8–10) inversions.
Source: CLSI GP41 tube mixing recommendations (citrate 3–4 inversions)Report a problem with this question
18. A red-top glass (non-additive) serum tube requires how many inversions?
- A.0 inversions (no mixing needed)✓ Answer
- B.3–4 inversions
- C.5 inversions
- D.8–10 inversions
A true non-additive red glass tube requires no inversions because there is no anticoagulant or clot activator to distribute; tubes with a clot activator (like plastic red or SST) do require about 5 inversions.
Source: CLSI GP41 tube mixing (non-additive glass red = 0 inversions)Report a problem with this question
19. Why does heparin carryover into a coagulation (citrate) tube cause errors?
- A.It falsely raises glucose
- B.It interferes with coagulation testing by inhibiting thrombin✓ Answer
- C.It has no effect on coagulation tests
- D.It causes hemolysis only
Heparin inhibits thrombin, so even a small carryover into a coagulation specimen falsely prolongs clotting results; this is one reason the order of draw and separate coagulation collection are strictly followed.
Source: CLSI GP41 rationale for order of draw (heparin carryover affects coagulation)Report a problem with this question
20. Which sequence correctly represents the CLSI recommended order of draw?
- A.Light blue → blood culture → red/SST → green → lavender → gray
- B.Blood culture → light blue → red/SST → green → lavender → gray✓ Answer
- C.Blood culture → lavender → green → red → light blue → gray
- D.Red → light blue → blood culture → green → lavender → gray
The CLSI order is blood culture (sterile) → light-blue citrate → red/SST serum → green heparin → lavender EDTA → gray fluoride; this sequence minimizes additive carryover between tubes with incompatible additives.
Source: CLSI GP41 recommended order of draw (full sequence)Report a problem with this question
21. A red-top (plastic, non-gel) serum tube contains which of the following?
- A.EDTA
- B.A clot activator, no anticoagulant✓ Answer
- C.Heparin
- D.Sodium citrate
Plastic red-top tubes contain a clot activator but no anticoagulant, so the blood is allowed to clot fully and yields serum; because they lack an anticoagulant, they are serum tubes rather than plasma tubes.
Source: CLSI GP41 color-code additive mapping (red = clot activator / no anticoagulant, serum)Report a problem with this question
22. Why must the coagulation (light-blue citrate) tube be drawn before EDTA, heparin, and serum-gel tubes?
- A.Because citrate is the most expensive additive
- B.To prevent additives or clot activators from contaminating the sensitive coagulation sample✓ Answer
- C.Because the light-blue tube is always largest
- D.There is no reason; order is arbitrary
The citrate tube is placed right after blood cultures (first among additive tubes) so that clot activators or other anticoagulants cannot carry over and skew the highly sensitive PT/aPTT coagulation results.
Source: CLSI GP41 rationale for order of draw (citrate before other additive tubes)Report a problem with this question
Concept-focused practice questions based on CLSI GP41 (venous blood collection), the CLSI order of draw, and the OSHA Bloodborne Pathogens Standard (29 CFR 1910.1030). Not affiliated with the NHA, ASCP, or CLSI, and not medical advice. Facility protocols and current CLSI standards take precedence — always follow your employer's procedures. About the CPT exam →