Category: Practice & Regulation | Estimated reading time: 8 minutes
Two Terms Often Considered the Same
Quality Control (QC) and Quality Assurance (QA) are often used interchangeably in daily laboratory conversations—as if they were two names for the same thing. In reality, they are two layers of a quality system that are fundamentally different. The confusion between the two is one of the reasons why many laboratories have QC running, yet their quality never seems to improve.
QA: A System That Works Before Measurement Occurs
Quality Assurance (QA) is a system designed to ensure optimal conditions before measurement occurs. It is not a single activity—it is the entire environment in which laboratory measurements take place. QA components include:
- SOP Design — well-designed operating procedures that refer to validated methods and are specific enough to ensure consistency across operators and time.
- Training — laboratory personnel (ATLM) competency that is built systematically, verified periodically, and documented. This includes not only initial onboarding training but also continuous training that keeps pace with developments in methods and instruments.
- Calibration — a planned and adhered-to calibration schedule, with a documented calibration history that allows for trace-back of issues if necessary.
- Reagent Management — a system for receiving, storing, and verifying reagents that ensures no problematic reagents enter the measurement process undetected.
- Audit — periodic checks on SOP compliance, equipment conditions, and documentation—not just during accreditation preparation, but as a mechanism for continuous improvement.
All of these components work before the first control material is measured in the morning. They are the foundation upon which QC stands.
QC: A System That Verifies the Foundation Remains Solid
With a QA foundation already in place, Quality Control (QC) is tasked with verifying every day—every run—that the foundation is still functioning properly. QC is the question asked every morning:
"Are the conditions established by QA still maintained today?"
If the answer is yes—the process continues, and patient results can be released. If the answer is no—something has changed. There is something within the QA foundation that is no longer as it should be, and the task of QC investigation is to find out what has changed.
In this framework, the relationship between QA and QC becomes very clear: it is a cycle of continuous improvement—not two systems that stand alone.
Why This Distinction Is Practically Important
Scenario one: QC rejects today. Investigation shows the control material has been open too long and has degraded. After replacing it with a new vial, QC passes. The problem is solved.
This is a QC problem solved at the QC level. Degraded control material is not a systemic issue—it is a conditional event that was detected and corrected.
Scenario two: QC has rejected repeatedly over the last two weeks, always with an upward shift pattern. Investigation shows the last calibration was performed three months ago—despite the manufacturer's recommendation being every month. No one was monitoring the calibration schedule.
This is a QA problem manifesting as a QC problem. The solution is not merely recalibration, but improving the calibration management system—ensuring there is someone responsible for monitoring the schedule and documenting this process.
Laboratories that do not understand this distinction will continue to solve QA problems at the QC level—and the problems will continue to recur.
Plebani (2017) emphasizes this sharply: a strict QC system cannot compensate for a weak QA system. QC can only detect the consequences—not prevent the causes. Studies on laboratory errors also show that the majority of errors actually occur in areas outside the reach of daily QC—the pre-analytical and post-analytical phases—which are far more influenced by the quality of the QA system (SOPs, training, sample management) than by the strictness of analytical QC rules alone.
QA Prevents, QC Detects
There is one sentence that summarizes it all:
"QA prevents. QC detects. The two cannot replace each other."
This is not a slogan—this is an accurate description of two different roles in one integrated system.
A laboratory that only has QC without QA will always be in reactive mode—constantly putting out fires without ever installing a fire prevention system. A laboratory that only has QA without QC will have a good foundation but will never know when that foundation begins to crack.
Both are needed. Both must run together. International quality frameworks such as ISO 15189 explicitly require laboratories to build both layers simultaneously—not just one—as part of an accredited quality management system.
Conclusion
QA and QC are not two names for the same thing. QA is a prevention system—it works before problems occur. QC is a detection system—it works to find problems that have already occurred before they reach the patient.
Both have their place. Both have their roles. Laboratories that understand this distinction—and build both seriously—are laboratories that can be truly trusted.
Good QC is the answer to a daily question. Good QA is the reason why that question rarely needs to be answered with a "no".