ASCP Phlebotomy Technician Order of Draw and Additives 2 — Questions and Answers
Question 1: What additive is found in yellow-topped blood culture tubes?
- EDTA
- Sodium polyanethol sulfonate (SPS) (Correct answer)
- Lithium heparin
- Sodium fluoride
Correct answer: Sodium polyanethol sulfonate (SPS)
Yellow-topped blood culture tubes contain SPS, an anticoagulant that also inhibits complement, phagocytes, and certain antibiotics, creating an optimal environment for bacterial growth.
Sodium polyanethol sulfonate (SPS) at 0.025-0.050% concentration is the additive in yellow-topped blood culture tubes. SPS prevents blood from clotting, inhibits complement activation, neutralizes lysozyme, inactivates aminoglycoside antibiotics, and inhibits phagocytosis. SPS tubes are first in the order of draw to prevent contamination from other tube additives.
Question 2: Why must light blue-topped coagulation tubes be filled to the exact fill line?
- To prevent hemolysis
- To maintain the correct 9:1 blood-to-anticoagulant ratio (Correct answer)
- To ensure enough serum for testing
- To prevent tube breakage during centrifugation
Correct answer: To maintain the correct 9:1 blood-to-anticoagulant ratio
Light blue coagulation tubes require a precise 9:1 ratio of blood to sodium citrate. Underfilling or overfilling alters this ratio and produces inaccurate coagulation results.
Light blue-topped tubes contain a fixed volume of 3.2% sodium citrate. When filled to the indicated line, a 9:1 ratio of blood to anticoagulant is achieved. Underfilling causes excess citrate, falsely prolonging PT and aPTT. Most laboratories reject coagulation specimens that are less than 90% filled.
Question 3: Which tube additive works by activating clot formation rather than preventing it?
- EDTA
- Clot activator (silica particles) (Correct answer)
- Sodium citrate
- Sodium heparin
Correct answer: Clot activator (silica particles)
Clot activators such as silica or glass particles are coated on the inner walls of red/gold-topped tubes to promote and accelerate clot formation for serum testing.
Clot activators are fine silica particles or micronized glass that coat the inner surface of gold-topped (SST) and some red-topped tubes. These particles activate Factor XII of the intrinsic coagulation pathway, accelerating clot formation from 30-60 minutes to approximately 15-30 minutes.
Question 4: In the order of draw, why are EDTA (lavender) tubes drawn after heparin (green) tubes?
- EDTA is more expensive than heparin
- EDTA contamination would falsely affect electrolyte and enzyme results in heparin tubes (Correct answer)
- Green tubes need to be drawn first because they expire faster
- There is no specific reason; the order is arbitrary
Correct answer: EDTA contamination would falsely affect electrolyte and enzyme results in heparin tubes
EDTA is drawn after heparin because EDTA contamination of chemistry tubes would falsely elevate potassium and chelate calcium, affecting multiple test results.
K2-EDTA or K3-EDTA contains potassium salts that would falsely elevate potassium results if they contaminated a chemistry tube. EDTA also strongly chelates calcium, magnesium, and zinc. Heparin does not significantly affect hematology results if a small amount carries over into EDTA tubes, making this order the safest.
Question 5: What type of gel is found in gold-topped SST (serum separator) tubes and what is its function?
- Silicone gel; it prevents hemolysis
- Thixotropic gel; it forms a barrier between serum and clot during centrifugation (Correct answer)
- Acrylic gel; it preserves glucose levels
- Polyethylene gel; it activates clotting
Correct answer: Thixotropic gel; it forms a barrier between serum and clot during centrifugation
SST tubes contain thixotropic gel that has a density between serum and clotted blood. During centrifugation, the gel migrates to form a stable barrier between the two layers.
Serum separator tubes contain a thixotropic gel with a specific gravity intermediate between serum (1.02-1.03) and clotted blood cells (1.08-1.09). During spinning, the gel liquefies and migrates to the interface between serum and clot, where it resolidifies into a stable physical barrier that extends specimen stability.
Question 6: A phlebotomist accidentally draws a green-topped tube before a light blue-topped tube. What should be done?
- Continue with the remaining draw and submit all tubes
- Discard the light blue tube and redraw it using a new venipuncture (Correct answer)
- Note the order error on the requisition and submit anyway
- Switch the labels between the two tubes
Correct answer: Discard the light blue tube and redraw it using a new venipuncture
If the order of draw is not followed and a coagulation tube may be contaminated with heparin carry-over, the light blue tube must be discarded and a new one drawn from a fresh venipuncture.
Heparin contamination in a light blue coagulation tube is a critical error because even trace amounts of heparin falsely prolong aPTT results. The light blue tube must be discarded and a new venipuncture performed. Simply noting the error is insufficient because the degree of contamination cannot be determined. Switching labels is a patient safety violation.
ASCP Phlebotomy Technician Order of Draw and Additives 2
What additive is found in yellow-topped blood culture tubes?