CVA Laboratory Sample Handling 2 — Questions and Answers
Question 1: What anticoagulant does a purple/lavender top tube contain and what is it primarily used for?
- Sodium citrate for coagulation profiles
- EDTA for complete blood counts (CBC) (Correct answer)
- Lithium heparin for chemistry panels
- Sodium fluoride for glucose testing
Correct answer: EDTA for complete blood counts (CBC)
Purple/lavender top tubes contain EDTA which preserves cell morphology and prevents clotting, making them ideal for CBCs.
EDTA chelates calcium to prevent clotting and preserves cell morphology better than other anticoagulants. Blue tops contain sodium citrate, green tops contain lithium heparin, and gray tops contain sodium fluoride/potassium oxalate.
Question 2: What is the maximum recommended time between blood collection and serum separation?
- 15 minutes
- 30 minutes (Correct answer)
- 60 minutes
- 4 hours
Correct answer: 30 minutes
Serum should be separated from the clot within 30 minutes to prevent falsely elevated potassium, phosphorus, and other analytes from cellular leakage.
After collection in a red top tube, blood should clot for 15-20 minutes, then be centrifuged and separated within 30 minutes. Prolonged contact causes pseudo-hyperkalemia and elevated phosphorus and LDH.
Question 3: Why must a urinalysis sample be analyzed within 30 minutes of collection?
- Urine color changes rapidly
- Crystals may form artificially, bacteria multiply, and cells degrade (Correct answer)
- Specific gravity increases over time
- The pH always becomes more acidic
Correct answer: Crystals may form artificially, bacteria multiply, and cells degrade
Delayed analysis leads to bacteria proliferation, artificial crystal formation or dissolution, and cell lysis, all producing inaccurate results.
At room temperature, bacteria double every 20-30 minutes causing alkalinization. The pH shift causes calcium phosphate crystals to form while dissolving calcium oxalate. Red and white blood cells lyse and casts disintegrate.
Question 4: Which fixative is standard for preserving tissue biopsy samples for histopathology?
- 70% ethanol
- 10% neutral buffered formalin (Correct answer)
- Normal saline
- Bouin's solution
Correct answer: 10% neutral buffered formalin
10% neutral buffered formalin is the universal standard for histopathology, preserving tissue architecture and being compatible with most staining protocols.
10% NBF cross-links proteins to preserve tissue architecture. The tissue-to-formalin ratio should be at least 1:10. Ethanol causes tissue shrinkage, saline does not fix tissue, and Bouin's is only for specific applications.
Question 5: What is the recommended storage method for fecal samples if analysis will be delayed more than 2 hours?
- Leave at room temperature
- Refrigerate at 4 degrees C or mix with formalin preservative (Correct answer)
- Freeze at -20 degrees C immediately
- Place in direct sunlight
Correct answer: Refrigerate at 4 degrees C or mix with formalin preservative
Refrigeration slows parasite egg development and larval hatching. Formalin preservative permanently stabilizes eggs and larvae.
Refrigeration at 4 degrees C preserves egg morphology for 24-48 hours. For longer delays, mixing with 10% formalin permanently preserves parasite stages. Freezing destroys the morphology of many eggs and larvae.
Question 6: What does hemolysis in a blood sample indicate and how does it affect results?
- Bacterial infection with no effect on results
- Red blood cells have ruptured, causing false elevations in potassium, AST, and LDH (Correct answer)
- Correct collection with more accurate results
- Elevated white blood cells requiring sample discard
Correct answer: Red blood cells have ruptured, causing false elevations in potassium, AST, and LDH
Hemolysis occurs when RBCs rupture, releasing intracellular contents that falsely elevate potassium, AST, LDH, and phosphorus.
Hemolysis is the most common pre-analytical error. It occurs from difficult venipuncture, small needles, forceful aspiration, or vigorous mixing. Released contents interfere with colorimetric assays. Hemolyzed samples should be noted and recollection recommended.
