Whole stool is the preferred clinical specimen for laboratory diagnosis of norovirus. Ideally, specimens should be collected during the acute phase of illness (within 48 to 72 hours after symptoms start) while stools are still liquid or semisolid. Virus is excreted in the greatest amount during this time.
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 specimens should be kept refrigerated at 39°F (4°C) if testing is done within 2 to 3 weeks.
If the specimens are shipped to a laboratory for testing, each sample should be—
- sealed in a separate bag, and
- kept on frozen refrigerant packs in an insulated, waterproof polystyrene container.
If testing will be done more than 3 weeks after the specimens are collected, they should be frozen at -4°F (-20°C) or -94°F (-70°C). When the specimens are stored in this way, norovirus can be detected after at least 5 years.
Vomitus can be collected to supplement stool specimens during an investigation. These specimens should be collected, stored, and shipped in the same way as stool specimens.
Serum specimens are not recommended for routine laboratory diagnosis of norovirus.
If feasible and warranted for special studies, acute- and convalescent-phase serum specimens may be collected and tested for a greater than fourfold rise in IgG titer to noroviruses.
- 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 and Water Specimens
In principle, norovirus can be detected in water, food, and environmental specimens. However, the virus first needs to be concentrated or extracted or both from the specimen. Validated methods for these techniques are available only for water (at CDC) and shellfish [at the Gulf Coast Seafood Laboratory, Food and Drug Administration (FDA)].
If food or water is the suspected cause of a norovirus outbreak, samples should be collected as soon as possible after people were exposed.
Food specimens should be stored frozen at -4°F (-20°C).
Water can be tested for norovirus by processing large volumes (up to 100L) through specially designed filters. Water samples should be stored refrigerated or chilled on ice at 39°F (4°C).
Norovirus RNA has been detected in swabs of environmental surfaces collected in specific outbreak settings. However, obtaining virus from swabs is highly variable. Results should be interpreted with caution and in the context of the available epidemiologic evidence.
Contact CDC for additional guidance on testing water and environmental samples, and FDA for guidance on testing shellfish.
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