Pre-Analytical Variables & Quality Control in Clinical Chemistry

Question-and-answer flashcards covering specimen handling, timing, critical values, and quality-control principles including Levey-Jennings and Westgard rules.

Which three major phases of laboratory testing can introduce variables affecting patient care?

Preanalytical, Analytical, and Post-analytical phases.

During which phases do studies show the largest percentage of laboratory errors occur?

Preanalytical and Post-analytical phases.

What specimen type is used most often in clinical chemistry?

Serum.

State one advantage of using serum for testing.

No anticoagulant is present, so nothing is added that might interfere with analysis.

What is the main disadvantage of serum compared with plasma?

Extra time is required for complete clot formation before centrifugation.

If serum or plasma is not promptly separated from cells, how are glucose and pH affected?

Glucose decreases and pH falls because RBCs continue glycolysis, producing lactate.

Failure to separate cells quickly causes which two analytes to rise because of intracellular release?

Phosphorus and potassium (especially potassium, which is ~150× higher inside RBCs).

How does prolonged contact of blood cells with serum/plasma affect oxygen and carbon dioxide?

Cells consume oxygen and generate carbon dioxide, altering gas values.

Why can refrigerating a tube that still contains cells increase measured potassium?

Cold enhances RBC membrane permeability, allowing intracellular K⁺ to leak into serum/plasma.

What laboratory issue occurs when red cells hemolyze?

Cellular contents are released, causing spectral interference and enzyme inhibition (e.g., lipase).

Leaving a tourniquet on >1–2 minutes can lead to what problem?

Hemoconcentration of blood below the venipuncture site, falsely elevating many analytes.

How does plasma differ from serum in specimen preparation?

An anticoagulant is added before collection; the tube is centrifuged immediately to obtain plasma.

Why are “short draws” in anticoagulant tubes undesirable?

Incorrect blood-to-anticoagulant ratio may interfere with chemical analysis.

Why must whole-blood specimens be mixed gently but thoroughly after collection?

To prevent micro-clot formation and settling of cellular and fluid components.

List the three numbered tubes collected during a lumbar puncture and their primary tests.

Tube 1: Chemistry, Tube 2: Microbiology, Tube 3: Hematology.

Which two analytes are most commonly measured in cerebrospinal fluid?

Protein and glucose.

Name three changes that occur when CSF or urine is allowed to stand too long.

Cell lysis, bacterial contamination/proliferation, and continued glycolysis leading to lactate buildup and ↓pH.

How long is a typical urine collection for quantitative chemistry tests?

24 hours.

What analytes are evaluated in synovial fluid to help diagnose gout and rheumatoid arthritis?

Uric acid crystals (gout) and inflammatory markers (rheumatoid arthritis).

An increase in serous (pleural, pericardial, peritoneal) fluid volume usually indicates what?

Inflammation or a decrease in serum protein (oncotic pressure).

Define a fasting specimen.

Collected after no food or drink (except water) is consumed; differs from NPO, which forbids even water.

What is a post-prandial (pp) specimen, and which test is most common?

Collected after a meal; the 2-hour post-prandial glucose is most common.

Give an example of a timed specimen and explain why timing matters.

6 a.m. cortisol; some analytes show diurnal variation, so collection time affects interpretation.

What does an ASAP order mean?

Analyze the specimen as soon as possible because the patient is waiting for results.

When is a STAT order appropriate?

In emergencies when the specimen must be tested immediately and results reported at once.

Why can drawing a cortisol level at 9 p.m. cause misinterpretation?

Cortisol has diurnal variation; normal levels are low at night, so a late sample may appear abnormal.

What are critical values, and how must they be handled?

Life-threatening results that must be phoned to nursing/physician staff immediately with read-back and documentation.

Differentiate between a standard and a control in quality control.

Standard: known concentration used to calibrate; Control: material run with patients to verify assay performance.

How many control levels should be used at minimum, and why?

At least two (often three) at medical decision points to ensure accuracy across the measurement range.

When QC lot numbers change, how long is typically required to re-establish target ranges?

About 30 days of data collection.

Define standard deviation (SD).

A statistic that describes the dispersion of values around the mean in a Gaussian distribution.

Approximately what percentage of values lie within ±1 SD, ±2 SD, and ±3 SD of the mean?

±1 SD: 68.2 %, ±2 SD: 95.5 %, ±3 SD: 99.7 %.

Within what SD limits do most laboratories accept QC results?

Within ±2 SD.

Provide the formula for coefficient of variation (CV).

CV (%) = (SD ÷ mean) × 100.

What does a lower CV indicate?

Greater precision (values are closer to the mean).

What CV value is generally considered acceptable for repeated lab measurements?

Less than 5 %.

What is the primary purpose of a Levey-Jennings plot?

To graphically monitor QC values over time against the mean and SD limits.

Explain the difference between a trend and a shift on a Levey-Jennings chart.

Trend: gradual change in one direction (6 consecutive rising or falling points). Shift: abrupt sustained change (≥6 points on one side of mean).

Define random error versus systematic error.

Random: isolated unpredictable deviation; Systematic: continuous error affecting all results equally (trend or shift).

What is the Westgard 12s rule and how is it used?

One control exceeds ±2 SD; serves as a warning to apply other rules but does not automatically reject runs.

Describe the Westgard 13s rule.

One control exceeds ±3 SD; indicates random error—reject patient results.

What does the Westgard 22s rule detect?

Two consecutive controls exceed the same ±2 SD limit; signals systematic error—reject results.

Explain the Westgard R4s rule.

One control is +2 SD and the other –2 SD (4 SD total range) in the same run; detects random error—reject results.

What is indicated by the Westgard 41s rule?

Four consecutive controls exceed ±1 SD on the same side of the mean; suggests systematic error—reject results.

State the meaning of the Westgard 10x rule.

Ten consecutive control results fall on one side of the mean; indicates systematic error—reject results.