NCCT Specimen Handling and Processing 2 — Questions and Answers
Question 1: What is the primary reason for centrifuging a blood specimen after collection?
- To destroy bacteria in the specimen
- To separate cellular components from serum or plasma (Correct answer)
- To warm the specimen to body temperature
- To dissolve any clots that may have formed
Correct answer: To separate cellular components from serum or plasma
Centrifugation uses centrifugal force to separate the heavier cellular components from the lighter serum or plasma, allowing testing on the liquid portion.
Centrifugation spins blood specimens at high speed (typically 1,000-2,000 g for 10-15 minutes), using centrifugal force to separate blood into layers based on density. Red blood cells settle to the bottom, white blood cells and platelets form a buffy coat in the middle, and serum or plasma rises to the top.
Question 2: Which specimens must be protected from light during transport to the laboratory?
- CBC specimens
- Blood cultures
- Bilirubin and vitamin B12 specimens (Correct answer)
- Type and screen specimens
Correct answer: Bilirubin and vitamin B12 specimens
Bilirubin and vitamin B12 are light-sensitive analytes that degrade when exposed to light, requiring amber tubes or foil wrapping during transport.
Several laboratory analytes are photosensitive and degrade when exposed to light. Bilirubin undergoes photoisomerization and photo-oxidation, reducing measured levels. Vitamin B12 (cobalamin), vitamin B6, folate, porphyrins, and carotene are also light-sensitive. Specimens for these tests should be collected in amber-colored tubes or wrapped in aluminum foil immediately after collection.
Question 3: What happens to potassium levels in a specimen that is hemolyzed?
- Potassium decreases significantly
- Potassium increases significantly (Correct answer)
- Potassium is not affected by hemolysis
- Potassium becomes undetectable
Correct answer: Potassium increases significantly
Hemolysis releases intracellular potassium from ruptured red blood cells into the serum, falsely elevating measured potassium levels.
Intracellular potassium concentration in red blood cells (approximately 150 mEq/L) is roughly 35 times higher than normal serum potassium (3.5-5.0 mEq/L). When hemolysis ruptures red blood cells, this large intracellular store of potassium floods into the serum, causing falsely elevated results.
Question 4: What is the maximum time a serum specimen should remain on the clot before centrifugation?
- 10 minutes
- 30 minutes
- 2 hours (Correct answer)
- 24 hours
Correct answer: 2 hours
Serum specimens should be centrifuged within 2 hours of collection to prevent glycolysis and other cellular metabolic changes from altering analyte concentrations.
After collection, blood cells continue metabolic activity: glucose is consumed by glycolysis, potassium leaks from cells, and enzymes are released. CLSI recommends that serum specimens be centrifuged within 2 hours of collection.
Question 5: Why must a blood gas specimen be placed on ice immediately after collection?
- To prevent the blood from clotting
- To slow cellular metabolism that consumes oxygen and produces carbon dioxide (Correct answer)
- To preserve the anticoagulant in the syringe
- To prevent bacterial contamination
Correct answer: To slow cellular metabolism that consumes oxygen and produces carbon dioxide
Ice slows the metabolic activity of blood cells, which would otherwise consume oxygen and produce carbon dioxide, changing the pO2 and pCO2 values.
Blood cells in an arterial blood gas specimen continue aerobic and anaerobic metabolism after collection. At room temperature, pO2 decreases by approximately 3-10 mmHg per 10 minutes, and pCO2 increases as cells produce carbon dioxide. Placing the specimen in crushed ice slows metabolism to negligible levels, preserving the gas values for up to 30 minutes.
Question 6: What is the correct procedure for aliquoting serum from a centrifuged tube?
- Pour the serum directly from the tube into a transfer tube
- Use a clean disposable pipette to transfer serum without disturbing the cell layer (Correct answer)
- Use the same pipette for multiple patient specimens to save time
- Shake the tube vigorously before transferring
Correct answer: Use a clean disposable pipette to transfer serum without disturbing the cell layer
A clean disposable pipette should be used to carefully transfer serum from above the gel or cell layer without disturbing it, preventing cellular contamination.
Aliquoting (transferring) serum requires careful technique to maintain specimen integrity. A clean, disposable transfer pipette is inserted into the serum layer above the gel barrier or buffy coat, and serum is drawn up without touching the cellular layer. Each patient requires a new pipette to prevent cross-contamination.
NCCT Specimen Handling and Processing 2
What is the primary reason for centrifuging a blood specimen after collection?