Diagnostic Testing for Norovirus Infection
Reverse transcription-polymerase chain reaction (RT-PCR) is the recommended diagnostic test for suspected norovirus infection due to its superior sensitivity and specificity compared to other available methods. 1
Specimen Collection and Handling
Optimal Specimen Type
- Whole stool specimens are strongly preferred over rectal swabs due to higher viral loads present in stool 1
- Collect 10-50 mL of whole stool in a clean container 1
- Vomitus can also be tested but has unknown sensitivity compared to stool 1
Timing of Collection
- Collect specimens as early as possible in the illness course
- Ideally obtain specimens during acute phase (within 48-72 hours after symptom onset) when viral shedding is highest 1
- Specimens collected up to 7-10 days after onset may still yield positive results 1
Storage and Transport
- Refrigerate specimens at 39°F (4°C) if testing will occur within 2-3 weeks 1
- For longer storage, freeze at -4°F (-20°C) or -94°F (-70°C) 1
- Transport specimens individually bagged and sealed, on ice or frozen refrigerant packs in insulated containers 1
Diagnostic Methods
Recommended Primary Method: RT-PCR
- RT-PCR is the gold standard for norovirus detection with sensitivity of 92-93% and specificity of 100% 1, 2
- Real-time RT-PCR (RT-qPCR) provides quantitative results that can help distinguish clinically significant infections from asymptomatic shedding 3
- A cycle threshold (Ct) value of 31 has been identified as the optimal cut-off for attributing illness to norovirus 3
Alternative Method: Enzyme Immunoassays (EIAs)
- EIAs have lower sensitivity (36-80%) compared to RT-PCR but high specificity (83-100%) 1, 4, 5
- Not recommended for sporadic cases due to poor sensitivity 1
- May be useful for preliminary screening in outbreak settings when:
Outbreak Investigation
- Collect specimens from at least 5 ill persons for optimal diagnostic yield 1
- Testing more specimens increases sensitivity (from 44.1% with 3 samples to 76.9% with 5 samples) 4
- An outbreak is laboratory-confirmed as norovirus when stool or vomitus specimens from at least 2 ill persons test positive 1
Interpretation of Results
Positive RT-PCR
- High viral load (low Ct value <26.5) strongly suggests norovirus as the causative agent 4, 3
- Consider clinical correlation for specimens with Ct values near the cut-off of 31 3
Positive EIA
- In outbreak settings with multiple positive specimens, likely indicates norovirus as the causative agent 1
- For sporadic cases, confirm with RT-PCR due to limited sensitivity 1, 4
Negative Results
- If clinical suspicion remains high, consider:
- Testing additional specimens
- Using more sensitive methods (RT-PCR if EIA was initially used)
- Evaluating for other causes of gastroenteritis 6
Common Pitfalls and Caveats
- Rectal swabs have significantly lower sensitivity (65.6%) than whole stool specimens 6
- EIAs may miss certain genotypes (GI.8, GII.10, GII.16) and specimens with low viral loads 4
- RT-PCR may detect asymptomatic shedding (up to 16% of healthy individuals) 3
- Delayed specimen collection may result in false negatives as viral shedding decreases over time 1