At 02:40 a technologist verifies a potassium of 6.9 mmol/L on a 71-year-old inpatient with chronic kidney disease; the laboratory's critical limit is 6.0 mmol/L or higher. The hemolysis index is normal, the platelet count is normal, and yesterday's potassium was 5.7 mmol/L, so she releases the result. She phones the ward, where a unit clerk answers, writes 'K 6.9' on a note and promises to pass it on. The technologist records 'critical called to 5 North' in the laboratory system and returns to the bench. At 05:30 the patient has a cardiac arrest, and his nurse says she never received the result. Where did the testing process fail?

Postanalytical: no caregiver received it with read-back

Preanalytical: a new sample was not drawn to confirm it

Analytical: the potassium was not repeated before release

Postanalytical: only a physician may receive the result

A 66-year-old man on a continuous heparin infusion needs a stat basic metabolic panel. Blood is drawn from the arm without the infusion into a serum separator tube, which is centrifuged 6 minutes after collection to save time. The analyzer raises no flags, and QC and other patients' results are acceptable. Results: Na 97 mmol/L, K 2.9 mmol/L, Cl 71 mmol/L, glucose 77 mg/dL (4.3 mmol/L), BUN 13 mg/dL (4.6 mmol/L), calcium 6.3 mg/dL (1.57 mmol/L). The patient is alert and well. The same tube is centrifuged again and rerun: Na 139 mmol/L, K 4.1 mmol/L, Cl 102 mmol/L, glucose 110 mg/dL (6.1 mmol/L), BUN 18 mg/dL (6.4 mmol/L), calcium 9.0 mg/dL (2.25 mmol/L). The first results are best explained by:

Short sampling caused by latent fibrin in the serum

Dilution of the sample by intravenous fluid

A specimen drawn from the wrong patient

Delayed separation of serum from the cells

Compared with last month, an outpatient's total protein is 8.1 vs 7.3 g/dL (81 vs 73 g/L), albumin 4.7 vs 4.2 g/dL (47 vs 42 g/L) and calcium 10.4 vs 9.4 mg/dL (2.59 vs 2.35 mmol/L). Sodium, potassium and glucose are unchanged. Which collection problem best explains this?

Prolonged tourniquet application

Hemolysis during the venipuncture

Serum left too long on the cells

A sample from a patient with rhabdomyolysis gives CK 85,000 U/L. The next sample on the same fixed-probe analyzer gives CK 410 U/L; rerun later in the day it gives 118 U/L. QC is in range. The most likely cause of the 410 U/L result is:

Carryover from the preceding sample

Calibration drift on the CK method

Hemolysis in the second patient's sample

A healthy 13-year-old boy has a sports physical. His alkaline phosphatase is 320 U/L, flagged high against the laboratory's single interval of 40-150 U/L. GGT, bilirubin, calcium and phosphate are normal. The best interpretation is:

Normal for age; the interval used is wrong

Early rickets from vitamin D deficiency

Cholestasis needing an imaging workup

Falsely high from EDTA in the sample

Preanalytical, analytical and postanalytical errors

13 ASCP MLS practice questions on Preanalytical, analytical and postanalytical errors, part of Laboratory Operations. The first 13 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
At 02:40 a technologist verifies a potassium of 6.9 mmol/L on a 71-year-old inpatient with chronic kidney disease; the laboratory's critical limit is 6.0 mmol/L or higher. The hemolysis index is normal, the platelet count is normal, and yesterday's potassium was 5.7 mmol/L, so she releases the result. She phones the ward, where a unit clerk answers, writes 'K 6.9' on a note and promises to pass it on. The technologist records 'critical called to 5 North' in the laboratory system and returns to the bench. At 05:30 the patient has a cardiac arrest, and his nurse says she never received the result. Where did the testing process fail?
1. APreanalytical: a new sample was not drawn to confirm it
2. BAnalytical: the potassium was not repeated before release
3. CPostanalytical: only a physician may receive the result
4. DPostanalytical: no caregiver received it with read-back
Show answerHide answer
Answer: D · Postanalytical: no caregiver received it with read-back
The result itself was valid: the sample was not hemolyzed, a normal platelet count makes pseudohyperkalemia unlikely, and a rise from 5.7 mmol/L fits kidney disease. The failure came after analysis, in communication. A critical result must be given directly to a licensed caregiver responsible for the patient, such as the nurse or physician, who reads back the result and the patient identifiers, and the record must show the receiver's name, the time and the read-back. A unit clerk cannot act on the result, and 'called to 5 North' cannot be traced to anyone. If no responsible caregiver can be reached, the laboratory follows its escalation policy, for example calling the physician or the nursing supervisor.
- A: The sample was valid (no hemolysis, normal platelets, a plausible trend), so recollection was not needed.
- B: Repeating a valid critical result is not required and only delays the call; the result was correct.
- C: A responsible licensed caregiver, such as the patient's nurse, may receive and act on a critical result.
Question 2 · Hard
A 66-year-old man on a continuous heparin infusion needs a stat basic metabolic panel. Blood is drawn from the arm without the infusion into a serum separator tube, which is centrifuged 6 minutes after collection to save time. The analyzer raises no flags, and QC and other patients' results are acceptable. Results: Na 97 mmol/L, K 2.9 mmol/L, Cl 71 mmol/L, glucose 77 mg/dL (4.3 mmol/L), BUN 13 mg/dL (4.6 mmol/L), calcium 6.3 mg/dL (1.57 mmol/L). The patient is alert and well. The same tube is centrifuged again and rerun: Na 139 mmol/L, K 4.1 mmol/L, Cl 102 mmol/L, glucose 110 mg/dL (6.1 mmol/L), BUN 18 mg/dL (6.4 mmol/L), calcium 9.0 mg/dL (2.25 mmol/L). The first results are best explained by:
1. ADilution of the sample by intravenous fluid
2. BA specimen drawn from the wrong patient
3. CDelayed separation of serum from the cells
4. DShort sampling caused by latent fibrin in the serum
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Answer: D · Short sampling caused by latent fibrin in the serum
Every analyte in the first run, sodium and chloride included, is about 30% lower than in the rerun of the same tube, which points to a smaller volume of sample being aspirated rather than a change in the patient. Serum spun before clotting is complete, especially from a patient receiving heparin, goes on forming fibrin after centrifugation, and fibrin strands can partly block the sample probe without always triggering a flag. Re-centrifuging the tube cleared the problem and gave plausible results. Contamination with saline or dextrose fluid raises chloride or glucose, a wrong-patient tube would give the same results on rerun, and delayed separation raises potassium. Serum tubes need full clotting time (about 30 minutes, longer in anticoagulated patients), or a heparin plasma tube should be used for stat work.
- A: Saline or dextrose contamination raises chloride or glucose; it does not lower every analyte by the same 30%.
- B: A wrong-patient tube would give the same wrong results when the same tube was rerun.
- C: The tube was spun within minutes, and delayed separation raises potassium rather than lowering it.
Question 3 · Medium
Compared with last month, an outpatient's total protein is 8.1 vs 7.3 g/dL (81 vs 73 g/L), albumin 4.7 vs 4.2 g/dL (47 vs 42 g/L) and calcium 10.4 vs 9.4 mg/dL (2.59 vs 2.35 mmol/L). Sodium, potassium and glucose are unchanged. Which collection problem best explains this?
1. AHemolysis during the venipuncture
2. BProlonged tourniquet application
3. CContamination with K2EDTA
4. DSerum left too long on the cells
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Answer: B · Prolonged tourniquet application
A tourniquet left on for more than about 1 minute forces water and small solutes out of the vein, concentrating cells and large molecules. Proteins and protein-bound analytes such as calcium and cholesterol rise by up to about 10-15%, while small, freely diffusible analytes change little. CLSI GP41 advises releasing the tourniquet within 1 minute. Posture has a similar effect, so serial samples should be drawn in the same position.
- A: Hemolysis would raise potassium, which is unchanged.
- C: EDTA contamination raises potassium and lowers calcium.
- D: Prolonged contact with cells lowers glucose and raises potassium.
Question 4 · Hard
A sample from a patient with rhabdomyolysis gives CK 85,000 U/L. The next sample on the same fixed-probe analyzer gives CK 410 U/L; rerun later in the day it gives 118 U/L. QC is in range. The most likely cause of the 410 U/L result is:
1. ACarryover from the preceding sample
2. BCalibration drift on the CK method
3. CHemolysis in the second patient's sample
4. DMacro-CK in the second patient
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Answer: A · Carryover from the preceding sample
Carryover occurs when a trace of a very high sample stays in the probe or cuvette and contaminates the next sample. A very high result followed by a falsely raised one that is normal on rerun is the typical pattern. Laboratories measure carryover during method verification and set rules to repeat any sample run straight after a result above a set limit. Probe washing and the order of loading also matter.
- B: Calibration drift would show in QC and other samples, and QC is in range.
- C: Hemolysis would also affect the rerun of the same tube.
- D: Macro-CK gives a raised result that persists on repeat.
Question 5 · Hard
A healthy 13-year-old boy has a sports physical. His alkaline phosphatase is 320 U/L, flagged high against the laboratory's single interval of 40-150 U/L. GGT, bilirubin, calcium and phosphate are normal. The best interpretation is:
1. ANormal for age; the interval used is wrong
2. BEarly rickets from vitamin D deficiency
3. CCholestasis needing an imaging workup
4. DFalsely high from EDTA in the sample
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Answer: A · Normal for age; the interval used is wrong
Bone growth during puberty raises ALP well above adult levels, often to several times the adult upper limit. Applying a single adult interval wrongly flags a normal result, a postanalytical reporting error that can lead to needless tests. Laboratories should report age- and sex-specific intervals for analytes that change with growth, such as ALP, phosphate and creatinine. Normal GGT and bilirubin support a bone rather than liver source.
- B: Normal calcium and phosphate in a well child do not suggest rickets.
- C: Normal GGT and bilirubin argue against cholestasis.
- D: EDTA chelates zinc and magnesium and falsely lowers ALP.
Question 6 · Easy
Which of these is an analytical error?
1. ADrawing blood above an intravenous line
2. BTesting with a reagent past its expiry date
3. CCollecting a sample in the wrong tube
4. DFailing to phone a critical value
Answer and explanation in LearnFree account
Question 7 · Medium
A 70-year-old's calcium is 9.2 mg/dL (2.30 mmol/L) on the analyzer and in the laboratory system. After a new interface goes live, the hospital record shows 2.30 mg/dL, flagged critical. In which phase did the error occur?
1. AAnalytical: calibration of the method
2. BPostanalytical: transfer of the result
3. CAnalytical: failure of quality control
4. DPreanalytical: choice of collection tube
Answer and explanation in LearnFree account
Question 8 · Easy
In studies of the total testing process, most laboratory errors arise in which phase?
1. AThe preanalytical phase
2. BThe postanalytical phase
3. CAll three phases equally
4. DThe analytical phase
Answer and explanation in LearnFree account
Question 9 · Hard
Hemolyzed samples from one ward rise from 2% to 14% in the week after a new pneumatic tube station opens for that ward. Other wards, using the old route, stay at 2%, and staff and collection techniques are unchanged. What should be investigated first?
1. AThe additive in the ward's collection tubes
2. BThe hemolysis index setting on the analyzer
3. CIn vivo hemolysis among the ward's patients
4. DThe speed and landing of the new tube route
Answer and explanation in LearnFree account
Question 10 · Medium
A well, alert inpatient had Na 140 mmol/L, K 4.0 mmol/L and glucose 92 mg/dL (5.1 mmol/L) at 06:00. At 08:00: Na 126 mmol/L, K 3.6 mmol/L, glucose 581 mg/dL (32.2 mmol/L). The most likely cause is:
1. ADelayed separation of the serum
2. BHyperosmolar hyperglycemic state
3. CEDTA carryover from another tube
4. DContamination with a dextrose infusion
Answer and explanation in LearnFree account
Question 11 · Hard
A stable inpatient's results today versus yesterday: Na 71 vs 141 mmol/L, K 2.0 vs 4.1 mmol/L, Cl 52 vs 103 mmol/L, glucose 46 vs 93 mg/dL (2.6 vs 5.2 mmol/L), calcium 4.7 vs 9.3 mg/dL (1.17 vs 2.32 mmol/L). The most likely cause is:
1. AContamination with a saline infusion
2. BHemolysis during sample collection
3. CThe analyzer aspirated too little sample
4. DContamination with a dextrose infusion
Answer and explanation in LearnFree account
Question 12 · Hard
Patient A, with stage 5 chronic kidney disease and no dialysis, had creatinine 6.8 mg/dL (601 µmol/L) yesterday; today it is 0.9 mg/dL (80 µmol/L). Patient B in the next bed went from 0.9 to 6.7 mg/dL (80 to 592 µmol/L). One person drew both at 05:00. The laboratory should:
1. ARecalibrate creatinine and rerun the whole batch
2. BWithhold both results and recollect both patients
3. CRepeat both tubes and report if the repeats agree
4. DReport both results with a delta check comment
Answer and explanation in LearnFree account
Question 13 · Medium
A patient's MCV changes from 88 fL to 105 fL within a day, with no transfusion. Which check is designed to catch this?
1. AProficiency testing
2. BA Levey-Jennings chart
3. CA delta check
4. DCalibration verification
Answer and explanation in LearnFree account

Preanalytical, analytical and postanalytical errors

Compared with last month, an outpatient's total protein is 8.1 vs 7.3 g/dL (81 vs 73 g/L), albumin 4.7 vs 4.2 g/dL (47 vs 42 g/L) and calcium 10.4 vs 9.4 mg/dL (2.59 vs 2.35 mmol/L). Sodium, potassium and glucose are unchanged. Which collection problem best explains this?

A sample from a patient with rhabdomyolysis gives CK 85,000 U/L. The next sample on the same fixed-probe analyzer gives CK 410 U/L; rerun later in the day it gives 118 U/L. QC is in range. The most likely cause of the 410 U/L result is:

A healthy 13-year-old boy has a sports physical. His alkaline phosphatase is 320 U/L, flagged high against the laboratory's single interval of 40-150 U/L. GGT, bilirubin, calcium and phosphate are normal. The best interpretation is:

Which of these is an analytical error?

A 70-year-old's calcium is 9.2 mg/dL (2.30 mmol/L) on the analyzer and in the laboratory system. After a new interface goes live, the hospital record shows 2.30 mg/dL, flagged critical. In which phase did the error occur?

In studies of the total testing process, most laboratory errors arise in which phase?

Hemolyzed samples from one ward rise from 2% to 14% in the week after a new pneumatic tube station opens for that ward. Other wards, using the old route, stay at 2%, and staff and collection techniques are unchanged. What should be investigated first?

A well, alert inpatient had Na 140 mmol/L, K 4.0 mmol/L and glucose 92 mg/dL (5.1 mmol/L) at 06:00. At 08:00: Na 126 mmol/L, K 3.6 mmol/L, glucose 581 mg/dL (32.2 mmol/L). The most likely cause is:

A stable inpatient's results today versus yesterday: Na 71 vs 141 mmol/L, K 2.0 vs 4.1 mmol/L, Cl 52 vs 103 mmol/L, glucose 46 vs 93 mg/dL (2.6 vs 5.2 mmol/L), calcium 4.7 vs 9.3 mg/dL (1.17 vs 2.32 mmol/L). The most likely cause is:

Patient A, with stage 5 chronic kidney disease and no dialysis, had creatinine 6.8 mg/dL (601 µmol/L) yesterday; today it is 0.9 mg/dL (80 µmol/L). Patient B in the next bed went from 0.9 to 6.7 mg/dL (80 to 592 µmol/L). One person drew both at 05:00. The laboratory should:

A patient's MCV changes from 88 fL to 105 fL within a day, with no transfusion. Which check is designed to catch this?