Specimens for norovirus testing may be:
- Whole stool or stool
- Cary Blair medium
Whole stool is the preferred clinical specimen for laboratory diagnosis of norovirus illness. During outbreak investigations, specimens should be collected from at least five ill people. Ideally, stool specimens should be collected during the acute phase of illness (within 48 to 72 hours after symptoms start). The greatest amount of virus is shed during the acute phase of illness.
Norovirus can sometimes be detected in stool specimens that are collected later in the illness or after the symptoms have resolved (up to 7 to 10 days after onset).
Whole stool and Cary-Blair specimens should be refrigerated at 39°F (4°C). Clair-Blair specimens should be tested within 2 to 3 days of collection and whole stool should be tested within 2-3 weeks. Cary-Blair specimens should be frozen at -4°F (-20°C) if stored for shorter than one month, or at -94°F (-70°C) when stored for longer periods. Norovirus can be detected for at least 5 years.
If the specimens are shipped to a laboratory for testing, each sample should be
- Clearly labelled with a unique identifier (such as specimen ID),
- Placed in a leak-proof plastic container and sealed in a separate bag, and
- Kept on frozen refrigerant packs in an insulated, waterproof polystyrene container.
Vomitus can be collected in addition to stool specimens during an outbreak investigation. These specimens should be collected and shipped in the same way as stool specimens. Vomitus samples should be stored frozen at -4°F (-20°C) or -94°F (-70°C) at all times.
Serum specimens are not recommended for routine laboratory diagnosis of norovirus illness.
If feasible and warranted for special studies, acute (symptoms showing) and convalescent (recovering from symptoms)-phase serum specimens may be collected and tested for a greater than four-fold rise in IgG titer to noroviruses. Serum samples should be stored frozen at -4°F (-20°C).
- Acute-phase serum specimens should be collected during the first 5 days after the symptoms start.
- Convalescent-phase specimens should be collected during the third to fourth week after the symptoms start.
Food, Water, and Environmental Specimens
In principle, norovirus can be detected in water, food, and environmental specimens. However, the virus first needs to be concentrated and/or extracted from the water or food specimen. Validated methods for these techniques are available at CDC for water and at U.S. Food and Drug Administration (FDA)External for several food commodities including shellfish, leafy greens, and berries. Water can be tested for norovirus by processing large volumes (up to 100 L) through specially designed filters. Water samples should be stored refrigerated or chilled on ice at 39°F (4°C).
If food or water is the suspected cause of a norovirus outbreak, samples should be collected as soon as possible after people were exposed and immediately refrigerated. Contact FDA for further instructions on testing.
Swabs of objects, such as doorknobs and handrails, and hard surfaces, such as kitchen counters can also be tested for norovirus. However, the amount of virus on surfaces is often low and detection of virus from swabs is not always possible. For information about swabbing and processing samples in the laboratory, see Swab Sampling Method for the Detection of Human Norovirus on SurfacesExternal. Results should be interpreted with caution and in the context of the available epidemiologic evidence.