Quality Control (QC) in Medical Laboratory
Introduction
Quality Control (QC) is a fundamental concept in medical laboratory science that ensures laboratory test results are accurate, consistent, and reliable. Every laboratory result influences clinical decisions, patient diagnosis, and treatment plans. Therefore, QC acts as a protective system that detects errors before incorrect results are reported.
For students, Quality Control is not only an important final examination topic, but also a core professional responsibility in real laboratory practice.
What Is Quality Control (QC)?
Quality Control refers to the continuous monitoring of laboratory testing procedures using specific techniques and control materials to confirm that test results meet acceptable standards.
Simple Exam Definition:
Quality Control is a process used in laboratories to check the accuracy and precision of test results before reporting them.
Why Quality Control Is Important in Laboratories ?
Quality Control plays a critical role in healthcare because it:
- Ensures correct diagnosis and treatment
- Protects patient safety
- Maintains consistency in test performance
- Detects instrument, reagent, and procedural errors
- Builds trust in laboratory reports
- Supports laboratory accreditation and quality standards
Without QC, laboratory results can become unreliable and harmful.
Main Objectives of Quality Control
The major objectives of QC include:
- Monitoring accuracy and precision of tests
- Identifying errors at an early stage
- Maintaining standard laboratory performance
- Improving technical competence
- Ensuring compliance with regulatory guidelines
Types of Quality Control in Medical Laboratory
Quality Control systems are broadly classified into two main types:
1. Internal Quality Control (IQC)
Internal Quality Control refers to routine checks performed inside the laboratory during daily testing.
Key Characteristics of IQC:
- Conducted daily or with each test run
- Uses control samples with known values
- Detects random and systematic errors
- Evaluates instrument and reagent performance
Examples of IQC:
- Running normal and abnormal control samples
- Monitoring calibration status
- Checking pipette accuracy and temperature control
2. External Quality Control (EQC) / Proficiency Testing
External Quality Control is an independent assessment of laboratory performance using unknown samples provided by an external agency.
Key Characteristics of EQC:
- Conducted at regular intervals
- Compares results with reference laboratories
- Measures overall test reliability
- Identifies long-term performance issues
Control Materials Used in Quality Control
Control materials are stable samples with known concentrations of analytes used to validate testing accuracy.
Types of Control Materials:
- Normal level controls
- High and low (abnormal) controls
- Commercially prepared controls
- Laboratory-prepared controls
Accuracy and Precision Explained
Understanding accuracy and precision is essential for exams and laboratory work.
Accuracy
Accuracy indicates how close a test result is to the true value.
Example: If the correct potassium value is 4.0 mmol/L and the result is 4.0 mmol/L, the test is accurate.
Precision
Precision describes how consistent repeated test results are.
Example: Results of 4.0, 4.1, and 4.0 mmol/L show good precision.
Types of Errors Detected by Quality Control
Quality Control helps identify different types of errors:
Random Errors
- Occur unpredictably
- Affect precision
- Caused by improper pipetting, electrical noise, or bubbles
Systematic Errors
- Occur consistently in one direction
- Affect accuracy
- Caused by incorrect calibration or deteriorated reagents
Levey–Jennings Chart in Quality Control
A Levey–Jenning’s chart is a graphical method used to track control values over time.
Purpose:
- Visualize daily QC results
- Detect trends, shifts, and out-of-control events
- Support decision-making before reporting patient results
Westgard Rules
Westgard rules are statistical decision criteria applied to QC data to determine acceptability.
Commonly used rules include:
- 1-2s (warning rule)
- 1-3s (rejection rule)
- 2-2s
- R-4s
- 4-1s
- 10x
These rules help laboratories prevent reporting erroneous results.
Quality Control in Different Laboratory Sections
Clinical Chemistry
- Routine use of control sera
- Daily calibration and maintenance
Hematology
- Analyzer control materials
- Peripheral smear quality checks
Microbiology
- Culture media quality testing
- Use of control organisms
Immunology & Serology
- Positive and negative controls
- Kit validation and lot verification
Corrective Actions When QC Fails
If QC results fall outside acceptable limits, laboratories must:
- Stop patient testing
- Investigate the cause
- Check reagents and instruments
- Repeat QC testing
- Document corrective actions
Documentation and Record Keeping
Proper QC documentation includes:
- Daily QC logs
- Calibration records
- Equipment maintenance reports
- Corrective action documentation
Documentation is essential for audits, inspections, and accreditation.
Quality Control – Important Exam Points
- QC ensures accuracy and precision
- IQC is daily, EQC is periodic
- Accuracy = closeness to true value
- Precision = reproducibility
- QC prevents diagnostic errors
Conclusion
Quality Control is the foundation of reliable laboratory testing. It ensures that laboratory results are accurate, precise, and clinically meaningful. For students, mastering QC concepts is essential not only for final examinations, but also for developing safe and professional laboratory practices.