Human noroviruses cannot be grown in cell culture. Therefore, diagnostic methods focus on detecting viral RNA or antigen. Most public health and clinical virology laboratories can test for norovirus using real-time reverse transcription-polymerase chain reaction (RT-qPCR) assays.
RT-qPCR assays are the preferred laboratory method for detecting norovirus. These assays are very sensitive and can detect as few as 10 to 100 norovirus copies per reaction. They use different primers to differentiate genogroup I and genogroup II norovirus. RT-qPCR assays are also quantitative and can provide estimates of viral load. The assays may be used to detect norovirus in stool, vomitus, foods, water, and environmental specimens.
Conventional RT-PCR Assays for Genotyping
Conventional RT-PCR followed by sequence analysis of the RT-PCR products is used for norovirus genotyping. Typically, a partial region of the capsid gene, such as region D, is used by laboratories participating in CaliciNet, a national laboratory surveillance network for norovirus outbreaks coordinated by CDC.
Rapid commercial assays, such as enzyme immunoassays (EIAs), that detect norovirus antigen have recently been developed. However, these kits have poor sensitivity (50%) and are not recommended for diagnosing norovirus infection in sporadic cases of gastroenteritis. The RIDASCREEN Norovirus 3rd Generation EIA was recently cleared by Food and Drug Administration 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 molecular methods during outbreak investigations.
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