Choosing a contract lab can be like vetting a supplier: Put together a list of requirements that will be used to compare labs, and this will likely consider technical as well as business elements.
USDA-ARS Stephen R Ausmus

Whenever food microbiologists talk about pathogen testing, one of the topics that invariably arises is whether the results they receive from laboratories are accurate and trustworthy. In addition, the newly minted Preventive Controls for Human Foods Rule (PC rule) contains certain requirements for testing, including that methods and procedures be scientifically valid.

In response to the new requirements for supplier verification and the potential role of testing for verification in these programs, the GMA Microbiological Safety Committee published Guidance on Laboratory Selection and Evaluation. This guidance covers lab selection as well as good laboratory practice, and can be used by contract and internal labs and auditors evaluating food-testing labs. The full, detailed guidance is available free from the GMA website under Resources >Technical Guidance and Tools ( Following is a brief overview of the guidance.


In some ways, choosing a contract lab can be somewhat like vetting a supplier. A company should put together a list of requirements that will be used to compare labs, and this will likely consider technical as well as business elements, for example:

  • What are the business hours?
  • What is the turn time on key assays?
  • Is the location convenient so that samples can be delivered within the required timeframe?

A survey with such questions can be used to collect preliminary information. Then once it has been narrowed to several possibilities, audits may be used to verify that the lab complies with the requirements. Some companies choose to use a single lab while others have large networks of approved labs that they use for testing.

The building of a food safety culture in the food facility has become increasingly recognized as an important component of producing safe food. A contract lab should also share this commitment and recognize the impact that the results can have on food safety and quality. Further, ethics, confidentiality, communication, and transparency are vital to a successful relationship. Some laboratories also are able to offer a consultative relationship with subject-matter experts and guidance on certain elements of testing and/or crisis management.

Accreditation, such as ISO 17025, has become increasingly widespread, and many labs—whether internal or contact—have adopted this system. The standard addresses management and technical requirements and has led to an increased emphasis on, and a standardization of, quality systems. While a laboratory can still produce credible results without this accreditation, it has become a common criterion for lab selection. Keep in mind that the scope of accreditation usually does not reflect all of the testing that the lab conducts.


Obtaining fast and reliable test results is a key part of the lab-selection decision. Labs will offer different methods depending on their current client base and that which is currently in demand. The PC rule requires that test methods are scientifically valid. There are a number of ways that this could be achieved, but one way is to select methods that have been vetted by an organization such as AOAC, or to use FDA reference methods (e.g., from the Bacteriological Analytical [BAM]) or Food Safety Inspection Service’s Microbiology Laboratory Guidebook (MLG). A decision about the specific assay to use will depend on the type of sample (e.g., food vs. environmental) and turn-around time. Some methods may be better for different matrices. If there are issues with the method, it’s important that the lab work quickly with the test-kit manufacturer to resolve the problem.

Sample and result traceability are critical, and the laboratory should have an effective program to manage the various elements of this process. This includes an adequate system for traceability and the ability of the laboratory to tie together all the various sources of information.

The elements of this system include chain of custody, sample login information, testing records, records of results, media, and other quality-related information. These records should be linked so as to be readily accessed (e.g., in the case of an investigation). The system also should prevent potential laboratory errors (e.g., mix up of samples, login or reporting errors, etc.).


A written quality assurance program should be available to all personnel, who should implement it as written. This program should include sections on:

  • Policies and procedures.
  • Biosafety/chemical safety program.
  • Preventive and corrective actions.
  • Training.
  • Proficiency programs.
  • Equipment maintenance and calibration programs.
  • Methods.
  • Laboratory control programs.
  • Stock culture maintenance.
  • Sample receipt and handling.
  • Documentation.
  • Records.

These programs should be properly maintained and verified by the laboratory on an ongoing basis. External and internal audits can be used as part of the verification process, and review of these audits may be part of the selection process.


Adequate facilities also are necessary to obtain reliable test results. The lab and workflow should follow a “clean” to “dirty” path. Different activities such as sample receipt, setting the samples for testing, analysis of enriched samples, DNA amplification, etc. should be conducted in separate areas. Laboratory results are dependent on safe, proper operation and the maintenance of equipment used in generation of those results. Equipment should be regularly inspected, calibrated, and cleaned, and receive preventive maintenance as recommended by the manufacturer.

There is a potential for samples to become contaminated through handling or contact in the laboratory, and subsequently test positive for a pathogen (when, in fact, they were not contaminated with a pathogen when they were received by the laboratory). Potential sources for the laboratory contamination event could result from exposure to the positive control cultures, other samples, or amplified DNA products.

All efforts should be made to minimize laboratory error and to prevent contamination of samples within the laboratory. This includes adequate training, proper aseptic technique, segregation of incompatible activities (either spatially or by time), appropriate workflow, careful disposal of contaminated materials, and use of appropriate test controls. It is essential to handle positive control strains, positive samples, and amplified DNA carefully. Best practice is to separate pre- and post-PCR activities. Any positive test result should be defendable, and there should be a process in place to make sure that the laboratory is monitoring and verifying positive results and linking them to their other programs, such as environmental monitoring. The lab should develop a checklist for quick and effective root-cause analysis, including checks on environmental controls and proficiencies.


While all these steps are critical, this article provides only a brief overview of the full guidance on selecting and evaluating laboratories. Keep in mind that a microbial testing program also needs to be correctly designed and implemented to obtain the most meaningful results. How this should be approached, and how it fits with the new FSMA requirements, will be addressed in a future GMA Update article.

The authors: Hayman is GMA director of microbiology; McMahon is general manager, Mérieux NutriSciences.