What does the Cepheid Xpert hepatitis C virus (HCV) viral load assay measure?

What the Cepheid Xpert HCV Viral Load Assay Measures

The Cepheid Xpert HCV viral load assay is a quantitative nucleic acid test that measures hepatitis C virus RNA levels in blood specimens to diagnose active HCV infection, confirm viremia, and monitor treatment response. 1

Primary Measurement and Clinical Purpose

The Xpert HCV Viral Load test quantifies HCV RNA using real-time PCR technology with the following key characteristics:

• Detects and quantifies HCV RNA (the viral genetic material) in serum, plasma, or capillary whole blood specimens 2, 3, 4
• Confirms active viral replication and distinguishes current infection from past resolved infection (where antibodies remain positive but virus is absent) 1
• Provides quantitative results reported in International Units per milliliter (IU/mL), the WHO-standardized unit for HCV viral load 2, 5

Technical Performance Specifications

The assay demonstrates excellent analytical sensitivity and accuracy:

• Lower limit of detection: 5 IU/mL, making it highly sensitive for detecting low-level viremia 1
• Turnaround time: 60-105 minutes depending on the specific cartridge version (original VL test ~105 min; newer VL Finger-Stick test ~60 min) 1, 3
• Strong correlation with reference laboratory methods (r = 0.97-0.99) when compared to Abbott RealTime HCV and Roche Cobas platforms 2, 5
• Accurate across all HCV genotypes, with no significant genotype-dependent performance variation 2, 4, 5

Clinical Applications in HCV Management

Diagnosis of Active Infection

• Confirms viremia after positive HCV antibody screening, as antibody tests alone cannot differentiate active from resolved infection 1
• Enables point-of-care diagnosis in decentralized settings where traditional laboratory infrastructure is unavailable 1, 5
• Particularly valuable for immunocompromised patients who may have false-negative antibody results despite active infection 1

Treatment Monitoring

• Establishes baseline viral load before initiating direct-acting antiviral therapy 2, 3
• Monitors on-treatment response to assess virological suppression during therapy 3, 6
• Confirms sustained virological response (SVR) by detecting absence of virus after treatment completion 1, 6
• Detects treatment failure or reinfection in high-risk populations such as people who inject drugs 3

Specimen Types and Testing Flexibility

A major advantage of the Xpert platform is its versatility:

• Serum and plasma specimens: Standard testing using 1 mL sample volume 2, 4
• Capillary whole blood (finger-stick): The VL Finger-Stick version uses only 100 μL of capillary blood, eliminating venipuncture barriers for patients with difficult venous access 3
• Dried blood spot (DBS) specimens: Whole blood collected on DBS can be tested, expanding access in resource-limited settings 4

Advantages Over Traditional Laboratory Testing

The Xpert HCV Viral Load assay addresses critical gaps in HCV care delivery:

• Rapid same-day results enable immediate clinical decision-making and treatment initiation, improving linkage to care 3, 5
• Minimal operator training required compared to complex laboratory-based PCR platforms 1, 2
• Leverages existing GeneXpert infrastructure already deployed for tuberculosis and HIV testing in many resource-limited countries 1
• Point-of-care capability allows decentralized testing in community health centers, prisons, and harm reduction programs 1, 5

Important Clinical Caveats

Near-Limit-of-Quantification Performance

• Discordant results may occur when HCV RNA levels are very low (typically <100 IU/mL), particularly during early treatment response (weeks 1-4 of direct-acting antivirals) 3
• The highest HCV RNA level missed in validation studies was 68 IU/mL, occurring in patients under treatment 3
• If clinical suspicion remains high with a negative result during treatment monitoring, repeat testing or use of an alternative platform may be warranted 3

Quantitative Correlation Considerations

• The original Xpert HCV VL test (using 100 μL capillary blood diluted in 1 mL buffer) yields results approximately 1.3 log₁₀ IU/mL lower than laboratory tests using 1 mL plasma, reflecting the smaller plasma volume (~55 μL actual plasma) 3
• The newer VL Finger-Stick test shows much closer agreement with laboratory methods (mean difference only 0.10 log₁₀ IU/mL) 3
• Use the same assay platform for serial monitoring to avoid variability between different testing methods 2, 5

What the Test Does NOT Measure

It is critical to understand the limitations:

• Does not measure HCV antibodies—it detects only viral RNA, not the immune response 1
• Does not determine HCV genotype—separate genotyping is required for treatment selection 5
• Does not assess liver damage or fibrosis stage—viral load does not correlate with disease severity, and fibrosis assessment requires liver biopsy or non-invasive fibrosis testing 1
• Does not replace HCV core antigen testing in all scenarios, though HCV RNA is generally preferred when costs are equivalent 1