NCCT Equipment, Tubes, and Additives 2 — Questions and Answers
Question 1: What is the function of sodium fluoride in a gray-top tube?
- Prevents platelet aggregation
- Inhibits glycolysis to preserve glucose levels (Correct answer)
- Activates clotting factors
- Chelates calcium ions
Correct answer: Inhibits glycolysis to preserve glucose levels
Sodium fluoride is a glycolytic inhibitor that prevents blood cells from consuming glucose, preserving accurate glucose levels in the specimen.
After blood is drawn, red blood cells and white blood cells continue to metabolize glucose through glycolysis at a rate of approximately 5-7% per hour at room temperature. Sodium fluoride inhibits the enzyme enolase in the glycolytic pathway, effectively stopping glucose consumption. Gray-top tubes also contain potassium oxalate (an anticoagulant) since sodium fluoride alone is a weak anticoagulant. These tubes are specifically used for glucose and lactate testing where accurate preservation is critical.
Question 2: Which tube color contains the anticoagulant EDTA?
- Light blue
- Green
- Lavender/purple (Correct answer)
- Red
Correct answer: Lavender/purple
Lavender (purple) top tubes contain EDTA (ethylenediaminetetraacetic acid), which is the standard anticoagulant for hematology testing.
EDTA (ethylenediaminetetraacetic acid) in lavender/purple-top tubes prevents coagulation by chelating (binding) calcium ions, which are essential for the clotting cascade. EDTA is the preferred anticoagulant for hematology tests (CBC, differential, platelet count, reticulocyte count) because it preserves cell morphology better than other anticoagulants. It is available as K2EDTA (dipotassium) or K3EDTA (tripotassium), with K2EDTA being the CLSI-recommended form.
Question 3: What is the purpose of a butterfly (winged infusion) needle set?
- To collect large volumes of blood quickly
- To access small or difficult veins with greater control (Correct answer)
- To prevent needle stick injuries only
- To draw arterial blood samples
Correct answer: To access small or difficult veins with greater control
Butterfly needles have flexible tubing and a smaller needle, allowing better control and access to small, fragile, or rolling veins.
Winged infusion (butterfly) sets consist of a small-gauge needle (typically 23-gauge) with flexible plastic wings for grip and a length of tubing connecting to a hub for tube or syringe attachment. They provide superior control for accessing small, rolling, or fragile veins in the hand, wrist, or antecubital area. They are commonly used for elderly patients, pediatric patients, and patients with difficult venous access. The tubing creates dead space that must be accounted for in short-draw situations.
Question 4: What does the thixotropic gel in a serum separator tube (SST) do during centrifugation?
- Activates clotting to speed up coagulation
- Forms a barrier between serum and the cell clot (Correct answer)
- Dissolves fibrin to prevent clotting
- Binds to proteins for analysis
Correct answer: Forms a barrier between serum and the cell clot
During centrifugation, the thixotropic gel migrates to the interface between the serum and the clot, forming a stable physical barrier that prevents mixing.
Thixotropic gel is a synthetic material with a density between that of serum (lighter) and the cellular clot (heavier). During centrifugation, the gel liquefies under centrifugal force, migrates to the interface between the serum above and the packed cells below, and then re-solidifies into a stable barrier. This prevents glycolysis and the exchange of substances between cells and serum, extending specimen stability.
Question 5: When using a syringe for blood collection, what is the correct method for transferring blood to evacuated tubes?
- Push the plunger forcefully to fill tubes quickly
- Remove the needle and pour blood into each tube
- Attach a transfer device and allow the vacuum to pull blood in (Correct answer)
- Squirt blood through the needle directly into tubes
Correct answer: Attach a transfer device and allow the vacuum to pull blood in
A syringe transfer device is attached to replace the needle, and the tube's vacuum draws blood in, reducing hemolysis risk and preventing needle stick injuries.
After collection with a syringe, the needle is removed and a blood transfer device (blunt cannula that punctures tube stoppers) is attached. The evacuated tube is inserted into the device, and its internal vacuum draws blood from the syringe without the phlebotomist pushing the plunger. Pushing the plunger forces blood through a narrow opening under pressure, causing turbulence and hemolysis. The transfer device method is both safer (no exposed sharps) and gentler on the specimen.
Question 6: What gauge needle is most commonly used for routine adult venipuncture?
- 18-gauge
- 21-gauge (Correct answer)
- 25-gauge
- 27-gauge
Correct answer: 21-gauge
A 21-gauge needle is the standard for routine adult venipuncture, providing adequate blood flow while minimizing patient discomfort and hemolysis.
The 21-gauge needle is considered the standard for routine venipuncture in adults. It accommodates adequate blood flow to fill evacuated tubes without excessive hemolysis. Larger needles (18-20 gauge) are used for blood bank collections or when large volumes are needed but cause more discomfort. Smaller needles (23-25 gauge, typically butterfly sets) are used for difficult veins but increase hemolysis risk and slow collection.
NCCT Equipment, Tubes, and Additives 2
What is the function of sodium fluoride in a gray-top tube?