QA vs QC: quality assurance, quality control and standard operating procedures

QA vs QC: the difference between quality assurance and quality control in a lab, what a standard operating procedure is, ALCOA, and where a COA fits.

By Zachery Casey · Published 5 November 2026 · Reviewed 9 November 2026 · Editorial policy

"Quality" in a laboratory or factory is two different jobs that are easy to mix up. Quality control (QC) checks the product: it tests samples against a specification. Quality assurance (QA) checks the system: it makes sure the work is planned, documented and done the same way every time, so that good results are expected rather than lucky. Standard operating procedures (SOPs) are one of the main tools that tie the two together. This guide explains each, and where a certificate of analysis (COA) fits.

Terms are defined in the glossary.

What is quality control?

Quality control is the testing side of quality. It asks: does this product, or this batch, meet its specification?

Typical QC work in a chemistry lab includes:

Sampling a batch in a defined way, so the sample represents the whole.
Testing the sample against a specification: for a peptide, typically purity by HPLC and identity by mass spectrometry.
Checking the instruments before testing, with calibration and system suitability checks. See reading a chromatogram.
Recording the results and comparing them with the limits.
Investigating any result that falls outside its limits, rather than simply retesting until a pass appears.

QC is detective. It finds problems in a product that has already been made.

What is quality assurance?

Quality assurance is the system side of quality. It asks: are we set up so that products will meet their specification every time, and can we show it?

Typical QA work includes:

Writing and controlling procedures, including SOPs (below), so everyone works the same way.
Training staff on those procedures and keeping records of who is trained in what.
Controlling documents and records, so the current version of each procedure is the one in use and records can't be quietly changed.
Change control: assessing and approving changes to a method, material or piece of equipment before they are made.
Handling deviations, when something didn't go to plan, and corrective and preventive actions (CAPA), so the same problem doesn't happen again.
Audits of the lab's own work and of suppliers.
Reviewing and approving results before a batch is released.

QA is preventive. It aims to stop problems happening in the first place.

QA vs QC

Diagram of quality assurance as a system around the quality control cycle of sample, test, record, review and release — QC is the test cycle each batch goes through. QA is the system around it, and signs off the review before a batch is released.

	Quality assurance (QA)	Quality control (QC)
Focus	The process and system	The product and its results
Approach	Preventive	Detective
Main question	Is the work set up to get it right every time?	Does this batch meet its specification?
Typical activities	Procedures, training, document control, audits, change control, CAPA	Sampling, testing, calibration checks, recording results
Typical output	An approved procedure, an audit report, a release decision	A test result, a COA
When	Before, during and after production	During and after production

The two depend on each other. QC testing is only meaningful if QA has made sure the method is validated, the instrument calibrated and the analyst trained. And QA relies on QC results to show the system is working.

In a small lab, the same person may do both jobs. What matters is that the checks happen, and that someone other than the analyst reviews important results.

Standard operating procedures

A standard operating procedure (SOP) is a written, approved set of instructions for doing one routine task the same way every time. In good manufacturing practice, written procedures and the records made when following them are a core requirement (EudraLex Volume 4).

SOPs exist so that:

different people get the same result from the same task,
new staff can be trained against a fixed standard,
the method can be checked, audited and improved, and
a result can be traced back to exactly how it was produced.

What goes in an SOP

Section	What it covers
Title and number	A unique reference, such as "SOP-012 Calibrating the pH meter"
Version and dates	Version number, effective date and review date
Purpose	What the procedure is for, in a sentence or two
Scope	Where and to what it applies, and what it doesn't cover
Responsibilities	Who does each part, and who approves
Safety	Hazards and precautions, with a link to the risk assessment
Equipment and materials	What is needed, including grades of reagents
Procedure	The numbered steps
Acceptance criteria	What counts as a pass, where relevant
Records	What must be written down, and where
References	Related SOPs, standards and manuals
Approval	Names and signatures of the author and approver

Writing a good SOP

One task per SOP. Short procedures are easier to follow and update.
Number the steps, and start each with a verb: "Rinse", "Record", "Check".
Be specific. "Allow to equilibrate for 30 minutes" is better than "allow to equilibrate".
Write for the person doing the work, and test the draft by having someone follow it.
Say what to do when something goes wrong, such as a calibration that fails.
Control versions. Remove superseded copies so only the current one is in use.
Review regularly, often every one to three years, and whenever the method or equipment changes.

Common lab SOPs cover tasks such as calibrating a pH meter, checking a balance, receiving and labelling reagents, and storing samples.

Records and data integrity

Both QA and QC depend on good records. UK regulators judge records against principles known as ALCOA (MHRA). Records should be:

Letter	Principle	Meaning
A	Attributable	You can tell who did the work and when
L	Legible	It can be read for as long as it is kept
C	Contemporaneous	It was recorded at the time, not written up later
O	Original	It is the first record, or a true copy of it
A	Accurate	It is correct and complete, with no undocumented changes

An extended version, often called ALCOA+, adds complete, consistent, enduring and available. The MHRA's guidance on data integrity sets out what these mean for laboratories working under good practice rules (MHRA). In practice that means writing results down at the time, in ink or in a system with an audit trail, and correcting mistakes in a way that leaves the original visible. These principles are also why some certificates carry more weight than others; see certificate of conformity vs certificate of analysis.

Good documentation practice

Good documentation practice (GDP) is the everyday habit that keeps records meeting those principles. Record results at the time, in permanent ink or an audited system. Correct an error with a single line that leaves the original readable, then add the correction, a date and initials. Never use correction fluid, and never copy results onto a fresh sheet to tidy them up.

Where a COA fits

A certificate of analysis is a QC document: it reports the tests run on one lot and the results. Signs of QA behind it include:

a stated method for each test,
a report or document number,
a review or approval by someone other than the analyst, with a role or signature, and
a lot number that traces back to the batch records.

None of these proves a result is right, but together they show it passed through a system rather than being typed up on its own. See how to check a certificate of analysis, and lot numbers and traceability.

For what a COA reports, see what a certificate of analysis tells you. For how labs show their test results are credible, see peptide purity testing in the UK. For the numbers on a COA, see accuracy, precision and significant figures. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.

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