Collection errors and specimen integrity

7 DOH Abu Dhabi (HAAD) lab licensing practice questions on Collection errors and specimen integrity, 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.

1. Question 1 · Hard

   A 46-year-old man has been on venoarterial ECMO in the cardiac ICU for 5 days. For three days his chemistry samples have been rejected as hemolyzed, and the ward has been asked to recollect each time. Today two samples are tested side by side: one drawn from the arterial line and one by venipuncture with a 21-gauge needle, both hand-carried and centrifuged within 15 minutes. Both plasmas are equally pink, with a hemolysis index of about 1.5 g/L (150 mg/dL) free hemoglobin. Results: potassium 4.3 mmol/L (3.5-5.1), LD 1,450 U/L (125-220), haptoglobin below 0.1 g/L (0.3-2.0), total bilirubin 48 µmol/L (2.8 mg/dL; 3-21). His urine is red, with dipstick blood 3+ and no red cells on microscopy. The most likely source of the hemolysis is:

   1. ARed cell damage during transport to the lab
   2. BTrauma from drawing through the arterial line
   3. CIn vivo hemolysis caused by the ECMO circuit
   4. DDelayed separation of plasma from the cells

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   Answer: C · In vivo hemolysis caused by the ECMO circuit

   The careful venipuncture sample is as hemolyzed as the line sample, and both were hand-carried and spun quickly, so the hemolysis did not happen in the tube. Undetectable haptoglobin, a raised bilirubin and hemoglobinuria without red cells show that red cells are breaking down in the circulation, a known effect of ECMO pumps and oxygenators. Potassium stays normal because potassium released in vivo is redistributed and excreted, whereas in vitro hemolysis adds it straight to the sample. Rejecting every sample only delays care: results are reported with a comment that the hemolysis is in vivo, tests whose hemoglobin interference limit is exceeded are still withheld, and the team is told, since rising plasma free hemoglobin can signal a failing circuit.

   • A: Both samples were hand-carried and spun within 15 minutes, and transport damage would not cause hemoglobinuria.
   • B: The venipuncture sample, drawn without the line, is just as hemolyzed.
   • D: Plasma was separated within 15 minutes, and a delay in the tube would not cause hemoglobinuria.

2. Question 2 · Medium

   A tourniquet left on for 3 minutes before the draw falsely raises total calcium, mainly because:

   1. ALocal acidosis frees calcium bound to albumin
   2. BCalcium leaks out of red cells under pressure
   3. CMuscle contraction releases calcium into plasma
   4. DWater leaves the vein, concentrating albumin

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   Answer: D · Water leaves the vein, concentrating albumin

   Under a prolonged tourniquet, venous pressure pushes water and small molecules into the tissues, while proteins and cells stay in the vein. Albumin, total protein, cholesterol and protein-bound substances such as total calcium therefore rise, a change called hemoconcentration. Local acidosis from stasis shifts calcium between bound and ionized forms but does not change the total. CLSI GP41 advises releasing the tourniquet within 1 minute.

   • A: Acidosis raises ionized calcium, but the total stays the same.
   • B: Red cells contain very little calcium.
   • C: Muscle activity releases potassium, not calcium, into the vein.

3. Question 3 · Hard

   An outpatient's potassium is 6.3 mmol/L. The hemolysis index is normal, the tube was spun 20 minutes after collection, and her platelets, WBC and calcium are normal. She pumped her fist repeatedly to fill the tubes. A repeat with a relaxed hand gives 4.2 mmol/L. The first result is best explained by:

   1. AHemolysis too slight to see in the serum
   2. BCarryover of EDTA from the order of draw
   3. CPotassium released by working forearm muscle
   4. DPotassium leaking from cells while the sample waited

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   Answer: C · Potassium released by working forearm muscle

   Repeated fist clenching under a tourniquet makes the forearm muscles release potassium into the local venous blood, which can raise the result by 1 to 2 mmol/L or more. The sample looks normal because no red cells have broken. Patients should keep the hand relaxed once the needle is in; a single fist to find the vein is acceptable. Recognizing this pseudohyperkalemia avoids needless treatment and repeat testing.

   • A: The hemolysis index was normal, so hemolysis does not explain it.
   • B: EDTA carryover would also lower the calcium, which is normal.
   • D: The tube was spun within 20 minutes, too soon for much leakage.

4. Question 4 · Medium

   Which collection practice is most likely to cause in vitro hemolysis?

   1. AUsing a 21-gauge needle with evacuated tubes
   2. BLetting the alcohol dry before the puncture
   3. CPushing blood from a syringe through the needle into a tube
   4. DInverting the EDTA tube gently 8 to 10 times

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   Answer: C · Pushing blood from a syringe through the needle into a tube

   Forcing blood through a narrow needle under plunger pressure shears red cells, so syringe blood should go into tubes through a transfer device that lets the vacuum fill each tube. Other common causes are very small needles, vigorous shaking, high suction through catheters and puncturing through wet alcohol. A 21-gauge needle is the usual adult size and does not damage cells. Hemolysis falsely raises potassium, LD, AST and magnesium and interferes with many photometric methods.

   • A: A 21-gauge needle is standard for adults and rarely causes hemolysis.
   • B: Letting alcohol dry prevents hemolysis; wet alcohol causes it.
   • D: Gentle inversion mixes the additive without damaging cells.

5. Question 5 · Medium

   A patient has an IV saline infusion in one arm, and the other arm cannot be used. Following CLSI GP41, blood should be drawn:

   1. ABelow the IV site, with the infusion still running
   2. BAbove the IV site, after the infusion is stopped for 2 minutes
   3. CFrom the IV cannula, with no discard volume
   4. DBelow the IV site, after the infusion is stopped for 2 minutes

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   Answer: D · Below the IV site, after the infusion is stopped for 2 minutes

   Veins above an infusion carry the infused fluid toward the heart, so blood drawn there is diluted and contaminated. CLSI asks that the infusion be stopped for at least 2 minutes and that a vein below (distal to) the IV site be used, with the tourniquet placed below the IV. A discard tube is drawn first, and the infusion is noted on the request. Using the other arm is always preferred when possible.

   • A: A running infusion can still mix with blood at the draw site.
   • B: Blood above the IV mixes with fluid flowing toward the heart.
   • C: Without a discard, fluid in the cannula dilutes the sample.

6. Question 6 · Hard

   A ward patient receives parenteral nutrition through a peripheral cannula. Today's sample, drawn from the same arm, versus yesterday's: glucose 41.0 vs 6.8 mmol/L (739 vs 122 mg/dL), potassium 5.8 vs 4.1 mmol/L, phosphate 1.6 vs 1.0 mmol/L, magnesium 1.1 vs 0.8 mmol/L, creatinine 68 vs 74 µmol/L; the plasma is lipemic. The most likely cause is:

   1. AHyperosmolar hyperglycemic state
   2. BDelayed separation of the sample
   3. CContamination with the nutrition infusion
   4. DRefeeding syndrome after starting the feeds

   Answer and explanation in LearnFree account

7. Question 7 · Easy

   Which sample should be rejected for a potassium test?

   1. AA mildly icteric serum sample
   2. BA serum sample separated within 1 hour of collection
   3. CA hemolyzed serum sample
   4. DA correctly labeled lithium heparin plasma sample

   Answer and explanation in LearnFree account