Phlebotomy Test Quality Assurance and Quality Control 1 — Questions and Answers

Question 1: A Levey-Jennings chart plots control values over time against which reference points?

The patient's normal range for each analyte
The mean and standard deviations (±1SD, ±2SD, ±3SD) of the control material (Correct answer)
The manufacturer's stated acceptable QC ranges
The previous day's QC values as a baseline

Correct answer: The mean and standard deviations (±1SD, ±2SD, ±3SD) of the control material

Levey-Jennings charts plot consecutive control values against established mean ±1, 2, and 3 standard deviation (SD) lines, allowing visual detection of trends and rule violations.

A Levey-Jennings chart is a graphical tool used in laboratory quality control to monitor the stability and accuracy of analytical processes over time. For each control level, the mean (average) and standard deviations (SD) of the control values are calculated from 20–30 baseline measurements. The chart displays horizontal lines at the mean, ±1SD, ±2SD, and ±3SD. As control values are measured each day (or run), they are plotted on the chart. Visual patterns reveal QC problems: (1) Systematic error (shift) — consecutive values above or below the mean; (2) Random error — a value outside ±3SD; (3) Trend — gradual drift in one direction over multiple runs. The chart allows visual monitoring that is faster and more intuitive than reviewing numerical tables alone, and is the foundation for applying Westgard rules.

Question 2: Westgard's 1₃ₛ rule states that a QC run is rejected when:

Two consecutive controls exceed ±2SD
One control value exceeds ±3SD (Correct answer)
The range between two controls exceeds 4SD
Ten consecutive controls fall on the same side of the mean

Correct answer: One control value exceeds ±3SD

The 1₃s (one-three-sigma) Westgard rule rejects a run when any single control value exceeds 3 standard deviations from the mean, indicating a probable random error.

Westgard rules are a set of statistical decision rules applied to QC data to determine when to accept or reject a laboratory analytical run. The 1₃s rule: reject the run when a single control measurement exceeds ±3SD. This rule identifies random errors (sudden shifts causing a single outlier) or systematic errors severe enough to move a result beyond 3SD. Statistical basis: in a normally distributed dataset, only 0.27% of values fall outside ±3SD by chance, meaning a 1₃s violation is rarely due to chance alone. Key Westgard rules include: 1₂s (warning—one value >±2SD), 2₂s (reject—two consecutive values >±2SD same side), R4s (reject—range between two controls >4SD), 4₁s (reject—four consecutive values >±1SD same side), 10x̄ (reject—10 consecutive values on same side of mean). These rules distinguish random from systematic errors.

Question 3: A 'shift' in a Levey-Jennings chart is detected when:

A single control value exceeds ±3SD
Six or more consecutive control values fall on the same side of the mean (Correct answer)
The control range (high minus low) exceeds 4SD
QC is not run for more than 24 hours

Correct answer: Six or more consecutive control values fall on the same side of the mean

A shift is identified by 6 or more consecutive control values on the same side of the mean (all above or all below), indicating a systematic change in the assay.

A shift in QC data refers to a sudden systematic change in the analytical process that moves control values consistently to one side of the mean. It is typically defined as 6 or more consecutive control values all above the mean or all below the mean (without crossing). The probability of this occurring by chance is (0.5)^6 = 1.56%, indicating the shift is likely due to a real change in the system. Common causes of shifts: (1) New lot of reagent or calibrator; (2) New calibration; (3) Replacement of a lamp or electrode; (4) New QC material lot; (5) Change in assay conditions. A shift differs from a trend (gradual directional drift) in that a shift is sudden. Both require investigation and corrective action before releasing patient results. Compare: a trend is identified by 6 or more consecutive values that are progressively increasing or decreasing.

Question 4: What is the purpose of a 'delta check' in laboratory quality assurance?

To verify the calibration of an analyzer before each run
To compare a patient's current result with their previous result to detect implausible changes (Correct answer)
To check the difference between high and low QC control values
To compare results from two different analyzers

Correct answer: To compare a patient's current result with their previous result to detect implausible changes

Delta checks compare a patient's current result to their previous result; implausible differences flag potential sample mislabeling, wrong patient, or hemolysis errors.

Delta checks are a post-analytical quality assurance tool where the laboratory information system (LIS) automatically compares a newly resulted value with the patient's most recent previous value for the same analyte. If the change exceeds a predefined acceptable delta (e.g., sodium changing by more than 10 mEq/L, or potassium changing by more than 1.0 mEq/L), an alert is triggered. The specimen is held and reviewed before releasing results. Delta checks primarily detect: (1) Specimen mislabeling (wrong patient's sample); (2) Specimen mix-up at collection or processing; (3) Transcription errors; (4) Genuine rapid clinical changes (true if clinically consistent). Delta check thresholds are analytically and clinically defined—tight enough to catch errors, loose enough to avoid excessive false alarms that slow throughput. Delta checks are part of the six sigma quality framework and are required by many laboratory accreditation standards.

Question 5: Which of the following is a PRE-ANALYTICAL error that can affect laboratory results?

Analyzer malfunction during measurement
Incorrect reference range in the laboratory information system
Hemolyzed specimen due to difficult venipuncture (Correct answer)
Transcription error when entering results

Correct answer: Hemolyzed specimen due to difficult venipuncture

Pre-analytical errors occur before the specimen reaches the analyzer; hemolysis from difficult venipuncture is a classic pre-analytical error that alters multiple analyte values.

Laboratory errors are classified by phase: Pre-analytical (before measurement): test ordering errors, patient misidentification, improper patient preparation (not fasting, wrong medication timing), wrong tube type, improper collection technique (hemolysis from squeezing, wrong order of draw), inadequate specimen volume, improper specimen transport or storage (wrong temperature, light exposure), and specimen processing errors (wrong centrifuge speed, not centrifuging promptly). Analytical (during measurement): reagent problems, calibration errors, analyzer malfunction, QC failure. Post-analytical (after measurement): transcription errors, incorrect reference ranges, delayed result reporting, critical value notification failures. Studies show that approximately 60–70% of all laboratory errors are pre-analytical, making this phase the most error-prone. Hemolysis from difficult venipuncture or excessive squeezing is one of the most common pre-analytical errors.

Question 6: When a laboratory documents QC results, which of the following is the MINIMUM information that must be recorded?

Only the final pass/fail decision
Date, time, operator ID, lot numbers of controls and reagents, control values, and pass/fail decision (Correct answer)
QC values and technologist name only
Control lot number and expiration date only

Correct answer: Date, time, operator ID, lot numbers of controls and reagents, control values, and pass/fail decision

Comprehensive QC documentation is required for regulatory compliance and includes all identifiers (date, time, operator, lot numbers), values, and decision so that patterns can be retrospectively investigated.

CLIA regulations and CAP accreditation standards require comprehensive QC documentation for each analytical run. Required elements include: (1) Date and time of QC run; (2) Operator/analyst ID or name; (3) Analyzer identification (if multiple instruments); (4) Control material lot number and expiration date; (5) Reagent lot number and calibration lot number; (6) Control level (low, normal, high); (7) Observed QC value; (8) Acceptable range (mean ±2SD); (9) Pass/fail decision; (10) Corrective action taken if failed (description, outcome); (11) Supervisor review signature. This complete record allows: retrospective investigation when a patient complaint or result question arises; identification of patterns of QC failure; demonstration of regulatory compliance during inspections; and root cause analysis for systematic errors. Electronic QC tracking systems capture most of this automatically.

Question 7: The Westgard 10x̄ rule is used to detect which type of QC error?

Random error
Systematic error (bias) (Correct answer)
Gross error
Precision error

Correct answer: Systematic error (bias)

The 10x̄ rule detects systematic bias: when 10 consecutive control values fall on the same side of the mean, it indicates the method has shifted from its calibrated target.

The 10x̄ (ten-mean) Westgard rule: reject the run when 10 consecutive QC control values (across any combination of levels and runs) all fall on the same side of the mean. This is one of the most sensitive Westgard rules for detecting persistent systematic error (bias). Systematic error means the method produces results that are consistently too high or too low compared to the true value. It does not produce single outliers (that would be random error detected by the 1₃s rule) but rather a consistent directional shift. The probability of 10 consecutive values on the same side by chance is (0.5)^10 = 0.098%, making a 10x̄ violation extremely unlikely to be random. Causes of systematic error detected by 10x̄: gradual reagent degradation, calibrator drift, temperature fluctuation, light exposure of photosensitive reagents, or developing electrochemical electrode problems.

Question 8: Turnaround time (TAT) monitoring is classified as which type of quality indicator?

Pre-analytical quality indicator
Analytical quality indicator
Post-analytical quality indicator
Process quality indicator covering all phases (Correct answer)

Correct answer: Process quality indicator covering all phases

TAT spans all phases—from order to specimen collection (pre-analytical), analysis (analytical), and result reporting (post-analytical)—making it a comprehensive process quality indicator.

Turnaround time (TAT) is a comprehensive quality indicator that measures the entire testing process from a defined start point to a defined end point. It spans all three phases: (1) Pre-analytical TAT — from order entry to specimen receipt in the laboratory (includes phlebotomy draw time, transport, registration); (2) Analytical TAT — from specimen receipt to result verification (includes specimen processing, analysis, review); (3) Post-analytical TAT — from result verification to result availability in the medical record. Total TAT (order to report) is the most clinically meaningful for providers and patients. Monitoring TAT identifies which phase is causing delays (phlebotomy draw lag, transport issues, processing backlog, slow analyzers, physician review delays). TAT targets are defined by test type: STAT tests typically target 30–60 minutes total; routine tests 4–24 hours. TAT measurement and reporting are required by CAP and TJC accreditation standards.

Phlebotomy Test Quality Assurance and Quality Control 1

A Levey-Jennings chart plots control values over time against which reference points?

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