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Veterinary diagnostics is the set of laboratory and clinical tests used to identify disease in animals, and the laboratories that run them. In the United States those laboratories sit in layers: clinic and university labs, labs accredited by the American Association of Veterinary Laboratory Diagnosticians (AAVLD), laboratories in USDA’s National Animal Health Laboratory Network (NAHLN), and the National Veterinary Services Laboratories (NVSL) at the top as reference labs. Knowing which layer a result came from tells you how far it can be relied on.
This page is general information for researchers, laboratory and clinic staff. It is not veterinary advice and does not describe sampling or treatment procedures.
Definition and scope
A veterinary diagnostic laboratory examines samples from animals, such as tissue, blood and swabs, to find the cause of illness or death, or to screen populations for specific diseases. Work spans companion animals, livestock, poultry, aquatic species and wildlife. It overlaps with several disciplines covered elsewhere on CASRAI: histopathology, bacteriology and veterinary cytology. For the research field behind it, see What Is Veterinary Science?
How diagnostic work is organised in practice
A university example shows the usual service lines. The University of Missouri Veterinary Medical Diagnostic Laboratory describes a necropsy service that includes tests chosen at the pathologist’s discretion and a histopathology report, and lists laboratory sections including Histology/IHC, Molecular Diagnostics, and Bacteriology and Mycology (university page read 2026-11-08). In other words, one case can pass through several disciplines, and the final report combines them.
Outbreak-scale testing is organised differently. USDA APHIS describes NAHLN as “a nationally coordinated network of animal disease diagnostic laboratories” that detects biological threats to U.S. animal agriculture. The network began with 12 laboratories and now includes 66 State, Federal and University laboratories across 43 States, and can run up to 450,000 PCR tests per week for avian influenza (APHIS flyer APHIS-25-035, October 2025, read 2026-11-08).
Who runs the tests: a map of the U.S. laboratory layers
The table below is the information-gain core of this page: it separates quality and role labels that are often used loosely. Every row comes from the AAVLD or APHIS sources listed at the end.
| Layer | What the source says | Source |
|---|---|---|
| AAVLD-accredited laboratory | Voluntary program giving “formal recognition of laboratory competence and quality”; requirements follow ISO/IEC 17025; covers all laboratory disciplines; peer inspection; certificates up to five years; open to publicly funded, full-service laboratories (necropsy, histopathology, bacteriology, virology). | AAVLD |
| NAHLN Level 1 | Meets the highest quality accreditation requirements; surveillance and response testing, diagnostic development, training and Biosafety Level 3 facility support. | APHIS |
| NAHLN Level 2 | Third-party accredited quality surveillance and response testing, surge capacity, diagnostic development and training, as resources allow. | APHIS |
| NAHLN Level 3 | Surveillance testing and surge capacity support. | APHIS |
| NAHLN Affiliate | Federal laboratories that can support surge capacity. | APHIS |
| NVSL | The U.S. national reference and confirmatory laboratories, and the Nation’s reference laboratories for NAHLN and USDA. | APHIS |
APHIS also reports that over 80 percent of Level 1 NAHLN laboratories have in-house Biosafety Level 3 facilities.
Types of diagnostic work
The sources above show the main families of work: anatomic pathology (necropsy and histopathology), molecular diagnostics such as PCR, bacteriology and mycology, and disease-specific surveillance testing. NAHLN laboratories may be approved to test for named diseases, including African swine fever, bovine spongiform encephalopathy, Chronic Wasting Disease, classical swine fever, Newcastle disease, foot-and-mouth disease, infectious salmon anemia, influenza A virus in avian and in swine, pseudorabies, scrapie, spring viremia of carp, vesicular stomatitis and viral hemorrhagic septicemia, as well as antimicrobial resistance. Approval is test-specific and laboratory-specific, which is why APHIS publishes a list of NAHLN laboratories searchable by State, designation or disease.
The World Organisation for Animal Health (WOAH) Terrestrial Manual supports this work: its manuals “provide a standardised approach to the diagnosis of the diseases listed in each Code” (WOAH page read 2026-11-08).
Where it matters for labs, clinics and supply decisions
Checklist: questions to ask about any diagnostic result or laboratory
- Is the laboratory AAVLD-accredited, and for which disciplines? Accreditation attests documented processes and quality controls, not a specific product or service.
- If the disease is a regulated or foreign animal disease, is the laboratory NAHLN-approved for that test? Approval is listed by disease.
- Does a reference laboratory need to confirm the result? APHIS identifies NVSL as the national confirmatory laboratories.
- Which standard covers the laboratory’s competence? AAVLD requirements follow ISO/IEC 17025 (the AAVLD page cites ISO/IEC 17025:2021).
For clinic-side stock and consumables that support in-house testing, see Veterinary Inventory Management, What Is a Urine Analyzer? and the veterinary hematology analyzer comparison.
Common misunderstandings
- “Accredited” and “NAHLN” are the same thing. They are not. AAVLD accreditation is a voluntary quality recognition; NAHLN is a USDA network with its own levels, though APHIS ties Level 1 and Level 2 to accreditation.
- Every veterinary lab can run every test. NAHLN approval is disease-specific, and AAVLD accreditation covers defined disciplines.
- A clinic’s in-house result is the same as a reference result. NVSL, not a clinic, is the national confirmatory laboratory for NAHLN.
FAQ
What is a veterinary diagnostic laboratory?
A laboratory that tests animal samples to identify disease or its cause. Examples include university diagnostic laboratories, state laboratories and members of NAHLN.
What is NAHLN?
The National Animal Health Laboratory Network: 66 State, Federal and University laboratories across 43 States, coordinated by USDA APHIS for early detection of animal health threats (APHIS, October 2025).
What does AAVLD accreditation mean?
It is voluntary peer-inspected recognition of laboratory competence, based on ISO/IEC 17025 requirements, with certificates lasting up to five years.
Who confirms a presumptive positive result?
For NAHLN and USDA, the National Veterinary Services Laboratories are the national reference and confirmatory laboratories.
Sources
- USDA APHIS, National Animal Health Laboratory Network overview, APHIS-25-035, October 2025 (read 2026-11-08).
- AAVLD, Accreditation program (read 2026-11-08).
- WOAH, Terrestrial Manual online access (read 2026-11-08).
- University of Missouri VMDL, Necropsy with histopathology and lab tests (read 2026-11-08).
