Diagnostic methods for norovirus focus on detecting viral antigen or viral RNA (genetic material). Diagnostic tests are available at all public health laboratories and many clinical laboratories, and most use real-time reverse transcription-polymerase chain reaction (RT-qPCR) assays to detect norovirus.
TaqMan-based RT-qPCR assays detect the RNA of the virus. They can be used to test stool, vomitus, food, water, and environmental specimens. RT-qPCR assays are the preferred method to detect norovirus because they are very sensitive and specific. They can detect as few as 10 to 100 norovirus copies per gram of sample. They use different oligonucleotide primer sets to differentiate genogroup I and genogroup II norovirus. RT-qPCR assays can also be quantitative and can provide estimates of viral load.
Multiplex Gastrointestinal Platforms
Recently, several commercial platforms for detection of multiple gastrointestinal pathogens have become available. They include genogroup I and genogroup II norovirus. The sensitivity of these assays for norovirus is in the same range as RT-qPCR. These platforms include FilmArray Gastrointestinal Panel, xTAG Gastrointestinal Pathogen Panel, and Verigene Enteric Pathogens Test.
Rapid commercial enzyme immunoassays (EIAs) that detect norovirus antigen in stool samples are also available. However, these kits have poor sensitivity (50 to 75%), and are, in general, not recommended for testing single samples from sporadic cases of gastroenteritis. These assays can be used for preliminary identification of norovirus when testing multiple specimens during outbreaks. However, samples that test negative should be confirmed by a second technique, such as RT-qPCR. Thus, EIA kits should not replace RT-qPCR during outbreak investigations.
Genetic characterization of noroviruses detected in stool and environmental samples can be very useful in epidemiologic investigations by linking cases, suggesting a common source, or identifying new emerging virus strains. Norovirus can be genotyped by sequence analysis of a RT-PCR product amplified from a partial region of both the polymerase gene and capsid gene (region B-C) in a single reaction for either genogroup I or genogroup II viruses.
All laboratories participating in CaliciNet, a national laboratory surveillance network for norovirus outbreaks coordinated by CDC, use dual typing for norovirus. The sequences obtained are compared to CaliciNet reference sequences for typing. An example of dual typing nomenclature is the GII.P16-GII.4 Sydney norovirus strain that has caused about half of all norovirus outbreaks in the United States in recent years (see CaliciNet data).