Phlebotomy Test Order of Draw and Additives 3 — Questions and Answers
Question 1: Why is the serum separator tube (SST) drawn AFTER the light blue coagulation tube?
- SST contains clot activators that could contaminate the citrate tube (Correct answer)
- SST must be warmed before collection
- Light blue tubes expire faster and must be used first
- CLSI recommends it for patient comfort
Correct answer: SST contains clot activators that could contaminate the citrate tube
SST tubes contain silica clot activators that would contaminate the sodium citrate coagulation tube if drawn first, falsely activating clotting.
Question 2: What is the purpose of the gel separator in an SST or PST tube?
- It activates clotting faster than silica alone
- It forms a barrier between serum/plasma and cells after centrifugation (Correct answer)
- It chelates calcium to prevent clotting
- It stabilizes glucose levels during transport
Correct answer: It forms a barrier between serum/plasma and cells after centrifugation
The thixotropic gel migrates during centrifugation to form a stable barrier between the cellular portion and the serum or plasma.
Question 3: Which tube is used for a complete blood count (CBC)?
- Light blue (sodium citrate)
- Lavender (EDTA) (Correct answer)
- Green (lithium heparin)
- Yellow (ACD)
Correct answer: Lavender (EDTA)
Lavender top tubes containing EDTA are the standard for CBCs because EDTA preserves cell morphology and prevents platelet clumping.
Question 4: A phlebotomist is drawing multiple tubes and needs a sodium citrate tube. What should be done if no discard tube is available when using a butterfly needle?
- Draw the sodium citrate tube last instead
- Use the first tube drawn as a discard tube to purge air from the tubing (Correct answer)
- Invert the sodium citrate tube 10 times before drawing the next tube
- Collect the sodium citrate tube with a syringe instead
Correct answer: Use the first tube drawn as a discard tube to purge air from the tubing
When using a butterfly needle, the tubing dead space fills with air before blood — drawing a discard tube first ensures the sodium citrate tube fills to the correct volume.
Question 5: Which anticoagulant is used in yellow-topped ACD (acid citrate dextrose) tubes and what are they primarily used for?
- EDTA — used for hormone panels
- Acid citrate dextrose — used for blood bank and paternity testing (Correct answer)
- Sodium heparin — used for chromosome analysis
- Potassium oxalate — used for lead testing
Correct answer: Acid citrate dextrose — used for blood bank and paternity testing
ACD tubes contain acid citrate dextrose, which preserves red blood cell viability, making them ideal for blood bank, DNA, and paternity studies.
Question 6: How many times should a sodium citrate (light blue) tube be inverted after collection?
- 3–4 times
- 5–6 times (Correct answer)
- 8–10 times
- 15–20 times
Correct answer: 5–6 times
Light blue tubes should be inverted 3–4 times according to most manufacturers; however, CLSI and many references cite 3–4 gentle inversions to mix the anticoagulant without activating platelets.
Question 7: Which statement about the gold or tiger-striped SST tube is correct?
- It contains EDTA and must be kept on ice
- It contains a clot activator and gel separator for serum chemistry testing (Correct answer)
- It is used exclusively for coagulation studies
- It requires immediate centrifugation without allowing clotting time
Correct answer: It contains a clot activator and gel separator for serum chemistry testing
Gold or tiger-striped SST tubes contain silica particles to activate clotting and a gel separator, and are used for serum-based chemistry tests after centrifugation.
Phlebotomy Test Order of Draw and Additives 3
Why is the serum separator tube (SST) drawn AFTER the light blue coagulation tube?