Quality Control Refresher: Using Positive and Negative Cultures

Culture controls help ensure the accuracy and reliability of test results by verifying the proper growth of microorganisms in media and the correct functioning of reagents, equipment, and personnel. From method validation to proficiency testing, culture controls help safeguard the reliability of water testing results and reinforce public health protection.

Let's take a look at the framework for quality in the lab and four ways positive and negative cultures help assure quality.

The Quality Framework in Three Parts

Root encouraged lab technicians and analysts to zoom out and consider the full structure that supports lab quality. At the broadest level, that begins with the quality system.

"The quality system is essentially how you manage your company—how you define policies, set objectives, articulate principles, and assign responsibilities for all quality-related activities," Root said.

This includes policies, employee training, supplier oversight, data management, and procedures for continuous improvement. All of these elements should be documented and updated in a quality manual. Importantly, the quality system also covers how a lab handles corrective and preventive action plans when things don't go as expected.

Nested within the quality system is quality assurance (QA), which focuses on planning, implementation, assessment, reporting, and improvement.

"Quality assurance is a design of the process you're using to address quality," Root explained. "You can document what you do in standard operating procedures (SOPs). Basically, QA leads to reliable, robust data every time a process is followed."

At the core is QC, where you measure test performance against predefined standards, check that systems are within prescribed limits, and use operational techniques to ensure quality. For example, if the incubator is supposed to maintain 35 ± 0.5°C but is found to be at 32°C, the incubator has failed. That's using QC to identify the issue and QA to understand why it failed and fix it.

Each layer works together to ensure that a lab not only complies with regulations but also consistently delivers trustworthy data to customers, regulators, and the public.

The Purpose of Positive and Negative Culture Controls

Positive and negative culture controls are essential for QC to validate whether your test methods and media are functioning correctly.

"At a basic level, these are organisms that help demonstrate that your media either will find the correct target or will not detect a non-target," Root said, going on to explain their specific roles as follows:

• Positive controls demonstrate that your test can detect the target organism, typically using a concentration as low as a single organism in 100 mL of water.
• Negative controls confirm that the test doesn't mistakenly detect non-target organisms, often using a very high concentration of non-targets—up to 2 million non-target organisms per 100 mL of reagent.

In short, low numbers prove detection, while high numbers prove non-detection. It's also critical to understand that blanks are not negative controls and shouldn't be used as a substitute.

4 Ways to Use Culture Controls

Root outlined four key applications where culture controls support QC:

1. Media Checks

If your lab is accredited under ISO 17025, follows The NELAC Institute (TNI) standard, or complies with a state certification process, you know that every batch made in-house or lot purchased must be validated for performance and sterility. This typically involves combining the media with sterile water and either a positive or negative culture strain, incubating according to the SOP, and reading results as you would for a typical sample.

For a positive control, you should observe expected colony growth, color, or fluorescence. For a negative control, you should observe no reaction. This process helps prove that the media continues to perform as expected over time.

Labs should document performance criteria clearly in SOPs and never use expired media. Quick tip: If you format your SOP with spaces for recording results, it can serve as both directions and a record.

2. Method QC

Before introducing a new method into the lab, you should verify that it produces accurate and reliable results. First, check whether the method is approved for compliance use and review the method itself, along with supplementary standards, accreditation guidance, and ISO 17025 requirements.

3. Demonstration of Capability (DOC)

All analysts and technicians must demonstrate they can correctly perform a test before conducting it with samples for customers—both when using a new method and periodically after that.

"At a minimum, the analyst must successfully analyze a positive sample with a known count," Root said.

Your quality manual should detail when DOCs must be performed, how results should be recorded, and what criteria define a "successful" DOC.

4. Proficiency Testing (PT)

Proficiency testing proves that your lab can perform a method as written and compares your results with those of peers. Most accreditation bodies require PT at least once per year, but TNI states may require it twice.

The provider typically analyzes PT results and sends them directly to your state or province to ensure an unbiased assessment of lab performance. To support PT success, it helps if routine culture controls closely resemble the format and procedure of your PT samples. That's why many labs choose to purchase both from the same vendor.

Labs should document all aspects of QC and PT in SOPs and quality manuals—not just to meet requirements but also to build internal confidence and external trust.

The Power of Culture Controls to Assure Quality

Culture controls do more than validate media, tests, capabilities, and proficiency—they validate your lab. When used appropriately, they confirm that media, methods, and people are all performing as expected. Integrated into a robust quality system, they help labs deliver on the promise of protecting public health through reliable, defensible data.