An outpatient's blood is collected at 09:00 in a gel separator tube, centrifuged at 09:40 and tested: potassium 4.2 mmol/L (reference 3.5-5.1), with a normal hemolysis index. The capped tube is stored upright at 2-8 °C. Next morning the doctor adds a magnesium request. A small aliquot is first poured off the tube for another add-on test, and its potassium reads 4.3 mmol/L. A processor then re-centrifuges the tube before loading it, and the technologist also repeats potassium on it: 5.5 mmol/L, again with a normal hemolysis index. What best explains the last potassium result?

Re-centrifuging the stored gel tube

Evaporation from the tube during storage

Refrigerating the tube before separation

In vitro hemolysis during overnight storage

A 52-year-old with hepatitis C has purpura and joint pain, and cryoglobulins are requested. How should the blood be handled before the serum is separated?

Kept at 37 °C until the serum is separated

Left at room temperature, then refrigerated

Placed in ice water and spun at once

Refrigerated unspun until the next morning

Why must a serum tube with clot activator stand for about 30 minutes before it is centrifuged?

Early spinning leaves fibrin strands in the serum

Glucose must first equilibrate between cells and serum

The activator must dissolve to stop glycolysis

Spinning fresh blood hemolyzes the red cells

An EDTA sample from a 4-year-old, collected after a difficult draw, has a small clot. The analyzer hemoglobin looks normal. What is the best action?

Reject it and request a new sample

Run it and add a comment about the clot

Remove the clot and run the sample

Report the hemoglobin but not the counts

Specimen transport, processing and storage

7 DOH Abu Dhabi (HAAD) lab licensing practice questions on Specimen transport, processing and storage, part of General Laboratory, Quality and Safety. The first 7 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
An outpatient's blood is collected at 09:00 in a gel separator tube, centrifuged at 09:40 and tested: potassium 4.2 mmol/L (reference 3.5-5.1), with a normal hemolysis index. The capped tube is stored upright at 2-8 °C. Next morning the doctor adds a magnesium request. A small aliquot is first poured off the tube for another add-on test, and its potassium reads 4.3 mmol/L. A processor then re-centrifuges the tube before loading it, and the technologist also repeats potassium on it: 5.5 mmol/L, again with a normal hemolysis index. What best explains the last potassium result?
1. ARe-centrifuging the stored gel tube
2. BEvaporation from the tube during storage
3. CRefrigerating the tube before separation
4. DIn vitro hemolysis during overnight storage
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Answer: A · Re-centrifuging the stored gel tube
The gel separated the serum from the cells within an hour, and the aliquot poured off next morning still reads 4.3 mmol/L, so overnight storage did not change potassium. The rise appeared only after the tube was spun a second time. Re-centrifuging a stored gel tube lets potassium-rich fluid that has stayed in contact with the cells below the barrier mix with the serum above, which fits a higher potassium with a normal hemolysis index. Gel tubes should not be re-spun: add-on tests use the serum already separated, or a new sample if stability does not allow it.
- B: The tube was capped, and the aliquot taken just before re-spinning shows no rise.
- C: The gel had already separated the serum, and the aliquot taken before re-spinning shows no rise.
- D: The hemolysis index is normal, and the aliquot taken next morning is unchanged.
Question 2 · Easy
Unless stability data show otherwise, CLSI advises separating serum or plasma from the cells within:
1. A2 hours of collection
2. B24 hours of collection
3. C8 hours of collection
4. D30 minutes of collection
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Answer: A · 2 hours of collection
CLSI GP44 sets 2 hours from collection as the usual maximum contact time between serum or plasma and the cells. Beyond that, cells keep using glucose and leak potassium, phosphate and LD into the fluid. Analytes with proven longer stability may have a longer limit, while unstable ones such as ammonia need much faster handling. Samples that must wait longer should be centrifuged at the collection site.
- B: After 24 hours of cell contact many results are unreliable.
- C: Glucose and potassium change well before 8 hours.
- D: 30 minutes is a typical clotting time for serum tubes, not the separation limit.
Question 3 · Medium
A 52-year-old with hepatitis C has purpura and joint pain, and cryoglobulins are requested. How should the blood be handled before the serum is separated?
1. ALeft at room temperature, then refrigerated
2. BPlaced in ice water and spun at once
3. CKept at 37 °C until the serum is separated
4. DRefrigerated unspun until the next morning
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Answer: C · Kept at 37 °C until the serum is separated
Cryoglobulins precipitate in the cold, so blood is drawn into prewarmed tubes, carried at 37 °C and allowed to clot at 37 °C. If the sample cools before separation, the cryoglobulin is trapped in the clot and the test is falsely negative. The separated serum is then kept at 4 °C for several days and checked for a precipitate that dissolves on rewarming. Ammonia and lactate need the opposite, chilling and prompt testing.
- A: Cooling at room temperature can already start precipitation.
- B: Chilling makes the cryoglobulin precipitate into the clot.
- D: Refrigerating whole blood traps the cryoglobulin in the clot.
Question 4 · Medium
Why must a serum tube with clot activator stand for about 30 minutes before it is centrifuged?
1. AEarly spinning leaves fibrin strands in the serum
2. BGlucose must first equilibrate between cells and serum
3. CThe activator must dissolve to stop glycolysis
4. DSpinning fresh blood hemolyzes the red cells
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Answer: A · Early spinning leaves fibrin strands in the serum
Clotting continues after collection, even with a silica activator, and most tube makers advise about 30 minutes (plain tubes without activator need about 60). If the tube is spun too early, fibrin goes on forming in the separated serum as strands or small clots. These can block the analyzer probe and cause short sampling and erratic results. Patients on anticoagulants may need longer, or a plasma tube instead.
- B: Glucose does not need to equilibrate; it falls the longer it waits.
- C: Clot activators speed clotting; they do not stop glycolysis.
- D: Correct centrifugation of a fresh sample does not hemolyze it.
Question 5 · Medium
An EDTA sample from a 4-year-old, collected after a difficult draw, has a small clot. The analyzer hemoglobin looks normal. What is the best action?
1. ARun it and add a comment about the clot
2. BRemove the clot and run the sample
3. CReject it and request a new sample
4. DReport the hemoglobin but not the counts
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Answer: C · Reject it and request a new sample
Clotting consumes platelets and traps red and white cells in the clot, so the counts from the remaining fluid are unreliable, usually falsely low. Removing the clot does not return the trapped cells, and clots can also block the analyzer aperture. Even the hemoglobin is uncertain, because the fluid left may not represent the whole sample. Clots are prevented by gently inverting the EDTA tube 8 to 10 times right after collection.
- A: A comment does not make falsely low counts usable.
- B: Removing the clot does not return the platelets and cells trapped in it.
- D: Hemoglobin is also unreliable once cells are trapped in a clot.
Question 6 · Hard
A frozen serum aliquot is thawed, and the analyzer samples from the top of the tube without mixing. Total protein reads 56 g/L; the original result was 70 g/L. After gentle inversion, the same aliquot reads 69 g/L. What explains the first result?
1. AFrost water got into the tube and diluted the serum
2. BThawing left solutes concentrated at the bottom
3. CSerum evaporated during frozen storage
4. DProteins were broken down by freezing
Answer and explanation in LearnFree account
Question 7 · Hard
Unspun samples from a remote clinic reach the laboratory 6 hours after collection, kept at room temperature. Which requested test can still be performed reliably?
1. APotassium on a gel serum tube
2. BGlucose on a plain serum tube
3. CHbA1c on EDTA whole blood
4. DAmmonia on a heparin tube
Answer and explanation in LearnFree account

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Specimen transport, processing and storage

Unless stability data show otherwise, CLSI advises separating serum or plasma from the cells within:

A 52-year-old with hepatitis C has purpura and joint pain, and cryoglobulins are requested. How should the blood be handled before the serum is separated?

Why must a serum tube with clot activator stand for about 30 minutes before it is centrifuged?

An EDTA sample from a 4-year-old, collected after a difficult draw, has a small clot. The analyzer hemoglobin looks normal. What is the best action?

A frozen serum aliquot is thawed, and the analyzer samples from the top of the tube without mixing. Total protein reads 56 g/L; the original result was 70 g/L. After gentle inversion, the same aliquot reads 69 g/L. What explains the first result?

Unspun samples from a remote clinic reach the laboratory 6 hours after collection, kept at room temperature. Which requested test can still be performed reliably?