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Exam Prep

Quality Control & Lab Statistics practice questions.

Lab statistics, QC principles, Westgard rules, method evaluation, lab calculations, lab water and errors.

All subjects

Independent practice questions written by Labologics. Not affiliated with DHA, DOH (formerly HAAD), SCFHS, Prometric or any exam body. Not recalled exam questions.

Lab statistics

Q1 Lab statistics · Easy

The sum of all control values divided by the number of values is the:

  1. A: Mean
  2. B: Mode
  3. C: Range
  4. D: Median
Show answer

Answer: A — Mean

The median is the middle value; the mode is the most frequent value; the range is highest minus lowest.

Reference: Tietz Fundamentals of Clinical Chemistry

Q2 Lab statistics · Easy

Which statistic measures how widely values are spread around the mean?

  1. A: Mean
  2. B: Standard deviation
  3. C: Median
  4. D: Sensitivity
Show answer

Answer: B — Standard deviation

SD is the square root of the variance. A small SD means good precision.

Reference: Tietz Fundamentals of Clinical Chemistry

Q3 Lab statistics · Easy

The variance of a control is 144. The standard deviation is:

  1. A: 12
  2. B: 14.4
  3. C: 72
  4. D: 144
Show answer

Answer: A — 12

SD = square root of variance = square root of 144 = 12.

Reference: Tietz Fundamentals of Clinical Chemistry

Q4 Lab statistics · Easy

The coefficient of variation (CV%) is calculated as:

  1. A: SD x mean
  2. B: Variance / mean
  3. C: Mean / SD x 100
  4. D: SD / mean x 100
Show answer

Answer: D — SD / mean x 100

CV expresses precision as a percentage, so methods at different concentrations can be compared.

Reference: Tietz Fundamentals of Clinical Chemistry

Q5 Lab statistics · Medium

A control has a mean of 200 mg/dL and an SD of 6 mg/dL. The CV is:

  1. A: 1.2%
  2. B: 3%
  3. C: 6%
  4. D: 33%
Show answer

Answer: B — 3%

6 / 200 x 100 = 3%.

Reference: Tietz Fundamentals of Clinical Chemistry

Q6 Lab statistics · Medium

A control has a mean of 38.5 and an SD of 4.2. Its 2SD acceptable range is:

  1. A: 34.3 - 42.7
  2. B: 30.1 - 46.9
  3. C: 25.9 - 51.1
  4. D: 36.4 - 40.6
Show answer

Answer: B — 30.1 - 46.9

Mean +/- 2SD = 38.5 +/- 8.4. Mean +/- 1SD (34.3 - 42.7) is too narrow to use as the acceptable range.

Reference: Westgard: Basic QC Practices

Q7 Lab statistics · Medium

CLSI recommends that a laboratory establish a new reference interval by testing at least:

  1. A: 20 healthy people
  2. B: 120 healthy people per group
  3. C: 1,000 patients
  4. D: 10 hospital patients
Show answer

Answer: B — 120 healthy people per group

A lab can verify an existing (transferred) interval with about 20 healthy people. The reference interval usually covers the central 95% of results.

Reference: CLSI EP28 (reference intervals) / C24 (statistical QC)

Q8 Lab statistics · Easy

Getting the same result on repeated analysis of one sample describes:

  1. A: Sensitivity
  2. B: Specificity
  3. C: Accuracy
  4. D: Precision (reproducibility)
Show answer

Answer: D — Precision (reproducibility)

Accuracy is closeness to the true value. A method can be precise but inaccurate.

Reference: Tietz Fundamentals of Clinical Chemistry

Q9 Lab statistics · Medium

The ability of a test to correctly identify people who have the disease is:

  1. A: Sensitivity
  2. B: Positive predictive value
  3. C: Precision
  4. D: Specificity
Show answer

Answer: A — Sensitivity

Sensitivity = TP / (TP + FN). Specificity = TN / (TN + FP) and correctly identifies people without the disease.

Reference: Tietz Fundamentals of Clinical Chemistry

Q10 Lab statistics · Hard

The positive predictive value of a test increases when:

  1. A: Disease prevalence decreases
  2. B: Disease prevalence increases
  3. C: Sensitivity falls
  4. D: The test is repeated on the same sample
Show answer

Answer: B — Disease prevalence increases

PPV depends on prevalence; screening low-prevalence populations gives more false positives.

Reference: Tietz Fundamentals of Clinical Chemistry

QC principles

Q11 QC principles · Easy

Material with a precisely known concentration used to calibrate an analyzer is a:

  1. A: Patient pool
  2. B: Control
  3. C: Standard (calibrator)
  4. D: Blank
Show answer

Answer: C — Standard (calibrator)

Controls monitor performance; calibrators (standards) set the relationship between signal and concentration.

Reference: Tietz Fundamentals of Clinical Chemistry

Q12 QC principles · Easy

A Levey-Jennings chart plots:

  1. A: Absorbance against wavelength
  2. B: Sensitivity against specificity
  3. C: Patient results against age
  4. D: Control results over time against the mean and SD limits
Show answer

Answer: D — Control results over time against the mean and SD limits

Lines are drawn at the mean and +/-1, 2 and 3 SD.

Reference: Westgard: Basic QC Practices

Q13 QC principles · Medium

Six or more consecutive control values on one side of the mean, after an abrupt change, is called a:

  1. A: Shift
  2. B: Random error
  3. C: Outlier
  4. D: Trend
Show answer

Answer: A — Shift

Common causes: new reagent or calibrator lot, recalibration, or a change in instrument settings.

Reference: Westgard: Basic QC Practices

Q14 QC principles · Medium

Control values moving steadily in one direction over several runs is a:

  1. A: Shift
  2. B: Trend
  3. C: Random error
  4. D: Delta failure
Show answer

Answer: B — Trend

Common causes: gradual reagent or calibrator deterioration, aging lamp, slow temperature change or dirty optics. Find and fix the cause; don't just change reagents by habit.

Reference: Westgard: Basic QC Practices

Q15 QC principles · Medium

Errors that are predictable and push all results in one direction (bias) are:

  1. A: Pre-analytical errors only
  2. B: Random errors
  3. C: Systematic errors
  4. D: Clerical errors
Show answer

Answer: C — Systematic errors

Systematic error affects accuracy and is usually fixed by recalibration or reagent correction. Random error affects precision.

Reference: Westgard: Basic QC Practices

Q16 QC principles · Medium

Random error is typically caused by:

  1. A: Reagent lot change
  2. B: Wrong standard concentration
  3. C: Incorrect calibrator value
  4. D: Bubbles, pipetting variation, electrical fluctuation
Show answer

Answer: D — Bubbles, pipetting variation, electrical fluctuation

Random error is unpredictable and shows as wider scatter.

Reference: Westgard: Basic QC Practices

Q17 QC principles · Medium

For most quantitative chemistry tests, the minimum QC is:

  1. A: Two levels of control every 24 hours of testing (or as required by the method)
  2. B: Only when results look abnormal
  3. C: Once per reagent lot only
  4. D: One level per week
Show answer

Answer: A — Two levels of control every 24 hours of testing (or as required by the method)

Many labs run controls each shift and after calibration, maintenance or a new reagent lot.

Reference: Westgard: Basic QC Practices

Q18 QC principles · Easy

External quality assessment (proficiency testing) means:

  1. A: Running internal controls
  2. B: Testing unknown samples from an outside provider and comparing results with other labs
  3. C: Calibrating the analyzer
  4. D: Inspecting the lab's paperwork
Show answer

Answer: B — Testing unknown samples from an outside provider and comparing results with other labs

PT samples must be tested like patient samples by the staff who routinely do the test.

Reference: Westgard: Basic QC Practices

Q19 QC principles · Medium

Comparing a patient's result with their previous result to detect sample mix-ups is a:

  1. A: Linearity check
  2. B: Westgard rule
  3. C: Delta check
  4. D: Proficiency test
Show answer

Answer: C — Delta check

A large unexplained change can point to a mislabeled sample or contamination.

Reference: Tietz Fundamentals of Clinical Chemistry

Westgard rules

Q20 Westgard rules · Medium

Under Westgard multirules, one control outside +/-2SD (1-2s) is:

  1. A: Ignored
  2. B: A systematic error
  3. C: Always a rejection
  4. D: A warning that triggers checking with the other rules
Show answer

Answer: D — A warning that triggers checking with the other rules

1-2s alone gives too many false rejections.

Reference: Westgard: Basic QC Practices

Q21 Westgard rules · Medium

One control value outside +/-3SD (1-3s) most often indicates:

  1. A: Random error - reject the run
  2. B: Acceptable run
  3. C: A trend
  4. D: Systematic error
Show answer

Answer: A — Random error - reject the run

1-3s is sensitive to random error and can also show large systematic error.

Reference: Westgard: Basic QC Practices

Q22 Westgard rules · Medium

Two consecutive control values exceeding the same 2SD limit (both +2SD or both -2SD) is the:

  1. A: R-4s rule
  2. B: 2-2s rule (systematic error)
  3. C: 1-3s rule
  4. D: 10x rule
Show answer

Answer: B — 2-2s rule (systematic error)

It can be two levels in one run or the same level in two consecutive runs.

Reference: Westgard: Basic QC Practices

Q23 Westgard rules · Hard

Within one run, one control exceeds +2SD and the other exceeds -2SD. This violates:

  1. A: 10x
  2. B: 2-2s
  3. C: R-4s (random error)
  4. D: 4-1s
Show answer

Answer: C — R-4s (random error)

R-4s applies within a run: the range between the two controls exceeds 4SD.

Reference: Westgard: Basic QC Practices

Q24 Westgard rules · Hard

Four consecutive control values exceeding the same 1SD limit is the 4-1s rule, which detects:

  1. A: No error
  2. B: Clerical error
  3. C: Random error
  4. D: Systematic error
Show answer

Answer: D — Systematic error

4-1s and 10x detect smaller systematic shifts.

Reference: Westgard: Basic QC Practices

Q25 Westgard rules · Medium

Ten consecutive control values on the same side of the mean (10x) indicate:

  1. A: Systematic error (shift)
  2. B: Precise and accurate method
  3. C: Contaminated sample
  4. D: Random error
Show answer

Answer: A — Systematic error (shift)

Check calibration, reagent lots and instrument settings.

Reference: Westgard: Basic QC Practices

Q26 Westgard rules · Medium

QC is rejected. Emergency samples are waiting. The correct action is:

  1. A: Report the emergency results first, then fix QC
  2. B: Stop reporting, find and correct the problem, rerun QC, then run patient samples (emergencies first)
  3. C: Repeat the same control until it passes
  4. D: Report all results with a comment
Show answer

Answer: B — Stop reporting, find and correct the problem, rerun QC, then run patient samples (emergencies first)

Patient results from a rejected run must not be released. Repeating a control without investigating hides real problems.

Reference: Westgard: Basic QC Practices

Q27 Westgard rules · Medium

Only one control level is out. The control vial was opened 5 days ago and its open-vial stability is 6 days. The best first step is:

  1. A: Change the analyzer
  2. B: Recalibrate immediately
  3. C: Check the control material: open a fresh vial and rerun, then investigate further if it still fails
  4. D: Ignore it because two levels passed
Show answer

Answer: C — Check the control material: open a fresh vial and rerun, then investigate further if it still fails

Deteriorated or mishandled control material is a common cause of an isolated failure.

Reference: Westgard: Basic QC Practices

Method evaluation

Q28 Method evaluation · Hard

Before using an unmodified, approved commercial method for patient testing, the lab must perform:

  1. A: Full validation like a new in-house test
  2. B: Only proficiency testing
  3. C: No checks
  4. D: Verification (confirm the manufacturer's precision, accuracy and reportable range in the lab)
Show answer

Answer: D — Verification (confirm the manufacturer's precision, accuracy and reportable range in the lab)

Full validation is needed for lab-developed or modified methods.

Reference: CLSI EP28 (reference intervals) / C24 (statistical QC)

Q29 Method evaluation · Medium

A result is above the analyzer's measuring range. The correct action is:

  1. A: Dilute the sample, re-run, and multiply by the dilution factor
  2. B: Report as 'high' with no value always
  3. C: Recalibrate
  4. D: Report as is
Show answer

Answer: A — Dilute the sample, re-run, and multiply by the dilution factor

Check the dilution is validated for that analyte.

Reference: Tietz Fundamentals of Clinical Chemistry

Calculations

Q30 Calculations · Easy

In a twofold serial dilution starting at 1:2, the sixth tube is:

  1. A: 1:12
  2. B: 1:32
  3. C: 1:64
  4. D: 1:128
Show answer

Answer: C — 1:64

1:2, 1:4, 1:8, 1:16, 1:32, 1:64.

Reference: Tietz Fundamentals of Clinical Chemistry

Q31 Calculations · Easy

To prepare 100 mL of a 1% (w/v) albumin solution you need:

  1. A: 0.1 g
  2. B: 1 g
  3. C: 10 g
  4. D: 100 g
Show answer

Answer: B — 1 g

1% w/v = 1 g per 100 mL.

Reference: Tietz Fundamentals of Clinical Chemistry

Q32 Calculations · Easy

Moles of solute per liter of solution is:

  1. A: Normality
  2. B: Osmolality
  3. C: Molality
  4. D: Molarity
Show answer

Answer: D — Molarity

Molality is moles per kilogram of solvent.

Reference: Tietz Fundamentals of Clinical Chemistry

Q33 Calculations · Hard

Total calcium is 2.10 mmol/L and albumin 31 g/L. Using corrected Ca = Ca + 0.02 x (40 - albumin), the corrected calcium is:

  1. A: 2.12 mmol/L
  2. B: 2.19 mmol/L
  3. C: 2.28 mmol/L
  4. D: 2.50 mmol/L
Show answer

Answer: C — 2.28 mmol/L

2.10 + 0.02 x 9 = 2.10 + 0.18 = 2.28 mmol/L. Ionized calcium is better when accuracy matters.

Reference: Tietz Fundamentals of Clinical Chemistry

Q34 Calculations · Medium

To convert BUN (mg/dL) to urea (mg/dL), multiply by:

  1. A: 0.467
  2. B: 1.0
  3. C: 2.14
  4. D: 6.0
Show answer

Answer: C — 2.14

Urea (60 g/mol) contains 28 g of nitrogen: 60/28 = 2.14.

Reference: Tietz Fundamentals of Clinical Chemistry

Q35 Calculations · Medium

Using a standard in a photometric assay, the sample concentration is:

  1. A: Absorbance x wavelength
  2. B: Sample absorbance x standard absorbance
  3. C: (Sample absorbance / standard absorbance) x standard concentration
  4. D: Standard concentration / sample absorbance
Show answer

Answer: C — (Sample absorbance / standard absorbance) x standard concentration

Based on Beer-Lambert law: absorbance is proportional to concentration.

Reference: Tietz Fundamentals of Clinical Chemistry

Lab water & errors

Q36 Lab water & errors · Medium

Ion-exchange resins are used to produce:

  1. A: Tap water
  2. B: Saline
  3. C: Distilled water
  4. D: Deionized water
Show answer

Answer: D — Deionized water

Distillation uses boiling and condensation. Most analyzers need clinical laboratory reagent water (CLRW, formerly Type I), as specified by the manufacturer.

Reference: Tietz Fundamentals of Clinical Chemistry

Q37 Lab water & errors · Easy

Most laboratory errors occur in which phase?

  1. A: Pre-analytical
  2. B: Post-analytical
  3. C: Equally in all phases
  4. D: Analytical
Show answer

Answer: A — Pre-analytical

Examples: wrong patient ID, wrong tube, hemolysis, delays. Post-analytical errors include transcription and reporting errors.

Reference: Tietz Fundamentals of Clinical Chemistry

Q38 Lab water & errors · Medium

High total protein with low sodium measured by indirect ISE suggests:

  1. A: True hyponatremia
  2. B: Pseudohyponatremia
  3. C: Hemolysis
  4. D: EDTA contamination
Show answer

Answer: B — Pseudohyponatremia

Very high lipids or proteins reduce the water fraction of plasma. Direct ISE (e.g., blood gas analyzer) gives the true value.

Reference: Tietz Fundamentals of Clinical Chemistry