What can be detected by the Cepheid Expert Flu test in patients suspected of having influenza, particularly in vulnerable populations such as the elderly, young children, and those with compromised immune systems?

What the Cepheid Xpert Flu Test Detects

The Cepheid Xpert Flu assay is a rapid molecular test that detects and differentiates influenza A, influenza A 2009 H1N1, and influenza B viruses in approximately 70 minutes using nasopharyngeal swabs or nasal aspirate-wash specimens. 1

Specific Viral Targets

The Xpert Flu test provides multiplexed PCR detection for three distinct targets:

• Influenza A virus (general detection of all influenza A subtypes) 1
• Influenza A 2009 H1N1 (specific identification of the pandemic H1N1 strain) 1
• Influenza B virus 1

This three-target approach allows clinicians to not only confirm influenza infection but also differentiate between influenza types and identify the specific H1N1 pandemic strain. 1

Test Performance Characteristics

The Xpert Flu assay demonstrates superior sensitivity compared to rapid antigen tests, with performance approaching that of reference RT-PCR methods:

• Influenza A sensitivity: 97.3-100% depending on specimen type, compared to only 62-72% for rapid antigen tests 2, 1
• Influenza B sensitivity: 80.7-100%, substantially higher than the 48-55% sensitivity of rapid antigen tests 2, 3
• Specificity: 100% for all targets (influenza A, H1N1, and influenza B) 2, 1

The American Thoracic Society classifies the Xpert Flu/RSV XC as a multiplex PCR assay with 3 targets for influenza A/B plus respiratory syncytial virus. 4

Approved Specimen Types

The test is validated for use with:

• Nasopharyngeal swabs (preferred specimen for older children and adults) 4, 1
• Nasal aspirate-wash specimens 1
• Mid-turbinate nasal swabs (acceptable alternative) 4

The Infectious Diseases Society of America recommends nasopharyngeal specimens as superior to all other upper respiratory tract specimens for detecting influenza viruses. 4

Clinical Context and Timing

Specimens should be collected as soon as possible after illness onset, preferably within 4 days of symptom onset, when viral shedding is highest. 4, 5

For optimal detection:

• Immunocompetent adults: Collect within 5 days of symptom onset, as viral shedding decreases rapidly after this period 5
• Young children: May shed virus for up to 10 days, extending the detection window 5
• Immunocompromised patients: Can shed virus for weeks to months, allowing for extended testing periods 5

Advantages Over Rapid Antigen Tests

The Xpert Flu assay addresses critical limitations of rapid influenza diagnostic tests (RIDTs):

• Sensitivity is 86-100% versus only 10-70% for RIDTs 6
• Provides results in approximately 70 minutes, bridging the gap between rapid antigen tests (10-20 minutes) and traditional RT-PCR (2+ hours) 1
• Minimal hands-on time with automated result interpretation 1
• Near-absence of non-interpretable results 3

The Infectious Diseases Society of America recommends using rapid molecular assays like Xpert Flu over RIDTs in outpatients to improve detection of influenza virus infection. 4

Clinical Applications

This test is particularly valuable for:

• Point-of-care diagnosis in emergency departments and urgent care settings where rapid, accurate results influence immediate clinical decisions 7
• Hospitalized patients requiring prompt diagnosis for infection control measures and antiviral treatment decisions 4
• High-risk populations including immunocompromised patients, elderly individuals, young children, and pregnant women where accurate diagnosis is critical 4, 5

Important Limitations

• Does not detect other respiratory pathogens beyond influenza A, H1N1, and influenza B 4
• Not approved for testing on bronchoalveolar lavage fluid in most versions, limiting utility in mechanically ventilated patients with lower respiratory tract infections 4
• Influenza B sensitivity may be slightly lower (80.7% in one study) compared to influenza A detection 3

For hospitalized patients, the Infectious Diseases Society of America recommends RT-PCR or other molecular assays as the primary diagnostic test, with the Xpert Flu assay serving as an acceptable rapid molecular option. 4, 6