Overview
This lecture covers high-yield concepts in laboratory operations, focusing on quality management, diagnostic efficacy, key instrumentation techniques, and essential laboratory mathematics relevant for the ASCP exam.
Quality Management Systems (QMS)
- QMS is an organized, process-oriented system aiming for consistent quality and customer satisfaction in the lab.
- QMS covers pre-examination, examination, and post-examination phases.
- The 12 essential QMS elements are: organization, personnel, equipment/instrument management, process control, information management, documents/records, occurrence management, assessment, process improvement, customer service, facilities, and safety.
- Quality improvement is a continuous, cyclical process to enhance lab services.
- Quality assurance (QA) is proactive and systematic to ensure standards are met and defects are prevented.
- Quality control (QC) detects analytical errors before releasing test results, maintaining accuracy and precision.
Quality Control & Proficiency Testing
- Internal QC (intra-lab) is daily, detects short-term errors.
- External QC (inter-lab) uses proficiency testing (PT) to compare results across labs and assure long-term accuracy.
- PT by CLIA: 3 surveys/year, 5 samples/survey; 80% of results (4/5) must be acceptable.
- Commutable samples reference true values; non-commutable use peer group means.
- Acceptable: within 2 SD; needs improvement: 2–3 SD; unacceptable: outside 3 SD.
Statistical Quality Control
- Mean: average; Median: middle value; Mode: most frequent value; Mid-range: (highest+lowest)/2.
- Range: highest minus lowest.
- Standard deviation (SD): √[Σ(x-mean)²/(n-1)].
- Coefficient of variation (CV): (SD/mean) × 100; measures precision.
- Variance: SD².
- Descriptive statistics: describe one data set; Comparative/inferential statistics: compare two sets (T-test for means, F-test for SDs).
- Gaussian curve (normal distribution): 68.2% within 1SD, 95% within 2SD (confidence range), 99% within 3SD.
- Schuhart-Levy-Jennings chart & Westgard rules help interpret QC data (1-2S, 1-3S, R4S: random error; 2-2S, 4-1S, 10x: systematic error).
- Steps when control is out of range: hold results, rerun control, change control/sample/reagents, recalibrate, seek assistance, document actions, repeat patient tests if needed.
Diagnostic Efficacy/Efficiency
- Sensitivity: true positive/(true positive + false negative) × 100.
- Specificity: true negative/(true negative + false positive) × 100.
- Positive predictive value (PPV): true positive/(true positive + false positive) × 100.
- Negative predictive value (NPV): true negative/(true negative + false negative) × 100.
- Sensitive tests: for screening (minimize false negatives).
- Specific tests: for confirmation (minimize false positives).
Laboratory Mathematics
- Proportional equation (no change in concentration): w1/v1 = w2/v2.
- Dilution equation (change in concentration): C1V1 = C2V2.
- Ratio: part:whole (e.g. serum:total = 1:5).
- Dilution: solute/total volume (e.g. 1/5).
- Dilution factor is the reciprocal of dilution.
- Compound dilution: multiply sequential dilutions for final dilution.
- Actual (reported) value = diluted value × dilution factor.
Instrumentation & Analytical Techniques
- Spectrophotometer configuration: light source, entrance slit, monochromator (wavelength selector), exit slit, cuvette, photodetector, read-out device.
- Wavelength accuracy (uses didymium or holmium oxide filters) and wavelength linearity are key QC parameters.
- Analytical techniques:
- Flame emission photometry: measures light from excited atoms (now obsolete).
- Atomic absorption spectrophotometry: measures light absorbed by unexcited atoms (uses hollow cathode lamp).
- Fluorometry: sensitive/specific; absorbs at one wavelength, emits at longer.
- Turbidimetry: measures reduction of transmitted light.
- Nephelometry: measures scattered light at an angle.
- Potentiometry: measures pH and PCO₂ (pH electrode, Severinghaus electrode).
- Amperometry: measures PO₂ (Clark electrode), via current from oxidation.
- Coulometry: quantifies electricity for analyte oxidation (e.g. chloride ions).
- Voltammetry: measures current from potential changes (e.g. heavy metals like lead).
Key Terms & Definitions
- QMS (Quality Management System) — Coordinated activities ensuring laboratory quality and customer satisfaction.
- Quality Assurance (QA) — Systematic activities to assure quality requirements are met.
- Quality Control (QC) — Procedures to detect analytical errors before releasing results.
- Sensitivity — Ability of a test to correctly identify true positives.
- Specificity — Ability of a test to correctly identify true negatives.
- PPV/NPV — Likelihood that positive/negative results are true, respectively.
- Coefficient of Variation (CV) — Index measuring precision (=%RSD).
- Proficiency Testing (PT) — External QC using standardized samples to assess lab accuracy.
Action Items / Next Steps
- Review key formulas for mean, SD, variance, CV, diagnostic statistics, and dilution calculations.
- Memorize the 12 QMS elements and Westgard rules.
- Practice solving proportion, dilution, and compound dilution problems.
- Study the principles and QC of analytical techniques (especially spectrophotometry, potentiometry, fluorometry).
- Prepare for recall and application-type questions on these topics for the ASCP exam.