AACC AACC Specimen Collection & Processing 1 — Questions and Answers
Question 1: Which tube additive is found in a lavender/purple-top tube used for hematology testing, and what is its mechanism of action?
- Sodium citrate — chelates calcium to prevent clotting
- EDTA — chelates calcium and magnesium to inhibit coagulation (Correct answer)
- Sodium fluoride — inhibits glycolysis
- Lithium heparin — neutralizes thrombin activity
Correct answer: EDTA — chelates calcium and magnesium to inhibit coagulation
EDTA (ethylenediaminetetraacetic acid) in lavender-top tubes chelates divalent cations required for the coagulation cascade, preserving cell morphology for CBC analysis.
Question 2: What is the recommended order of draw when collecting multiple tubes by venipuncture, according to CLSI guidelines?
- Lavender → light blue → red → green
- Blood cultures → light blue → red/SST → green → lavender → gray (Correct answer)
- Red → green → lavender → light blue → gray
- Gray → lavender → green → red → light blue
Correct answer: Blood cultures → light blue → red/SST → green → lavender → gray
The CLSI-recommended order of draw minimizes additive carryover: blood cultures first (sterility), then light blue (citrate), red/SST, green (heparin), lavender (EDTA), and gray (fluoride/oxalate).
Question 3: What is the maximum acceptable time for a serum specimen to remain as a clot before centrifugation under standard pre-analytical conditions?
- 5 minutes
- 15–30 minutes at room temperature (Correct answer)
- 2 hours refrigerated
- 4 hours at any temperature
Correct answer: 15–30 minutes at room temperature
Serum tubes should be allowed to clot for 15–30 minutes at room temperature before centrifugation to ensure complete fibrin formation and prevent fibrin strands in the serum.
Question 4: Which pre-analytical variable is most likely to cause falsely elevated serum potassium levels?
- Excessive tourniquet time causing hemoconcentration
- Hemolysis releasing intracellular potassium into the sample (Correct answer)
- Refrigerating the sample before centrifugation
- Using a light blue-top tube instead of a red-top tube
Correct answer: Hemolysis releasing intracellular potassium into the sample
Hemolysis releases potassium from erythrocytes into the serum, causing falsely elevated results because intracellular potassium concentration is approximately 20–30 times higher than plasma levels.
Question 5: For glucose testing, why is a gray-top tube (sodium fluoride/potassium oxalate) preferred over a serum tube?
- It prevents lipemia from interfering with the glucose assay
- Sodium fluoride inhibits glycolysis, preserving glucose concentration (Correct answer)
- It contains a gel barrier that separates glucose from cells faster
- It prevents hemolysis that would falsely lower glucose
Correct answer: Sodium fluoride inhibits glycolysis, preserving glucose concentration
Sodium fluoride inhibits enolase, a glycolytic enzyme, halting glucose metabolism by red blood cells and stabilizing glucose levels for up to 24 hours.
Question 6: When collecting a urine specimen for culture and sensitivity, which collection method is preferred to minimize contamination?
- First-void morning specimen in a regular container
- Midstream clean-catch in a sterile container (Correct answer)
- 24-hour urine collected in a preservative jug
- Catheterized specimen from the urinary bag port
Correct answer: Midstream clean-catch in a sterile container
A midstream clean-catch specimen, collected after cleaning the urethral area and discarding the initial stream, minimizes contamination from periurethral flora.
AACC AACC Specimen Collection & Processing 1
Which tube additive is found in a lavender/purple-top tube used for hematology testing, and what is its mechanism of action?