What is Quality Control in Clinical Laboratories?
Understanding Quality Control (QC) and its requirements under ISO 15189:2022 for reliable laboratory results.
Introduction
Quality Control (QC) is a fundamental component of laboratory medicine. It refers to the operational techniques and activities used to monitor and verify that the analytical testing process is performing within predefined limits of accuracy and precision.
In simple terms, QC answers a critical question every laboratory must ask every day:
“Is the test system working correctly right now, so that patient results can be safely released?”
Without effective QC, even the most advanced analyzers and well-trained staff cannot guarantee the reliability of patient results.
Why is Quality Control Important?
Clinical laboratory results influence approximately 70% of medical decisions. Errors in laboratory testing can lead to:
- Incorrect diagnosis
- Delayed or inappropriate treatment
- Unnecessary additional testing
- Patient harm
Quality Control acts as an early warning system. It detects analytical problems (instrument malfunction, reagent deterioration, calibration drift, operator error, environmental issues) before incorrect patient results are reported.
Types of Quality Control
1. Internal Quality Control (IQC)
Internal Quality Control (also called Internal QC or Daily QC) is performed by the laboratory itself using control materials with known target values. These controls are analyzed at regular intervals (usually at the beginning of each analytical run or shift) alongside patient samples.
Key features of IQC:
- Performed frequently (daily or per run)
- Uses stable control materials
- Results are plotted on Levey-Jennings charts
- Interpreted using statistical rules (e.g., Westgard Rules)
- Detects both random and systematic errors in real time
2. External Quality Assessment (EQA) / Proficiency Testing
External Quality Assessment (EQA), also known as Proficiency Testing (PT), involves the laboratory analyzing samples provided by an external organization. The laboratory’s results are compared with peer group results or reference values.
Key features of EQA:
- Performed periodically (monthly, quarterly, or as scheduled)
- Provides independent evaluation of laboratory performance
- Helps identify long-term bias and inter-laboratory variation
- Required for accreditation under ISO 15189
Quality Control under ISO 15189:2022
ISO 15189:2022 is the international standard for medical laboratories – Requirements for quality and competence. The 2022 edition places strong emphasis on risk-based thinking and the effectiveness of the quality management system.
Key requirements related to Quality Control include:
Clause 7.3.7 – Ensuring the validity of examination results
The laboratory shall have a procedure for monitoring the validity of results. This includes:
- Use of quality control materials
- Review of results using statistical methods
- Participation in interlaboratory comparison programmes (EQA/PT)
- Use of trend analysis and statistical techniques
Important expectations of ISO 15189:2022 regarding QC:
- QC materials should, as far as possible, behave like patient samples
- The frequency of QC testing shall be based on the stability of the examination system and risk to the patient
- Acceptance criteria (control limits) must be defined and documented
- When QC results are unacceptable, the laboratory must take corrective action and evaluate the impact on previously released patient results
- EQA participation is mandatory for examinations where it is available
- The laboratory must review and respond to EQA results in a timely manner
Practical Implementation of Quality Control
A robust QC programme typically includes the following steps:
- Selection of control materials – Preferable materials that closely mimic patient samples (matrix-matched, multi-level).
- Establishment of mean and SD – Calculated from data generated under stable conditions (usually 20–30 data points).
- Setting of control limits – Commonly ±2 SD or ±3 SD, or based on Westgard multi-rule criteria.
- Regular analysis of controls – At the frequency defined by laboratory policy and risk assessment.
- Plotting and interpretation – Using Levey-Jennings charts and statistical rules (Westgard Rules).
- Immediate action on failures – Investigation, corrective action, and documentation.
- Trend analysis – Regular review of QC data to detect gradual shifts or increases in imprecision.
- Participation in EQA – And timely review of external performance reports.
Benefits of an Effective QC Programme
- Early detection of analytical errors
- Improved reliability and consistency of patient results
- Support for laboratory accreditation (ISO 15189, CAP, NABL, etc.)
- Reduced risk of incorrect clinical decisions
- Increased confidence of clinicians and patients in laboratory reports
- Continuous improvement of laboratory processes
Summary
Quality Control is not just a regulatory requirement — it is an essential daily practice that protects patients. Under ISO 15189:2022, laboratories are expected to implement a risk-based, statistically sound QC system that includes both internal quality control and external quality assessment.
When QC is properly designed, monitored, and acted upon, it becomes one of the strongest tools a laboratory has to ensure the accuracy and reliability of every result released.
- Westgard Rules in Clinical Laboratory Quality Control
- Levey-Jennings Charts – How to Plot and Interpret
- Internal Quality Control (IQC) Best Practices
- Understanding ISO 15189:2022 Requirements
Educational Disclaimer: This article is intended for educational and professional reference purposes. Laboratories should implement QC procedures in accordance with ISO 15189:2022, applicable national regulations, and their own quality management system.
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