Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1375 page)

   Samples with low viral load may be untypable. Sequencing methods are less effective than hybridization methods in genotyping samples with mixed genotypes.

HEPATITIS C VIRUS (HCV) RNA, QUANTITATIVE VIRAL LOAD: MOLECULAR ASSAY

   Definition

   The HCV viral load assay quantifies HCV RNA in the plasma of HCV-infected individuals. The HCV test is standardized against the first WHO International Standard for HCV RNA for nucleic acid amplification technology assays (NIBSC code 96/790).

   
Normal range:
Not detected when the result is below the level of detection of the assay.

   Use

   
Methods

   Branched DNA assay (bDNA; Siemens): A signal amplification technology that detects the presence of specific nucleic acids by measuring the signal generated by branched, labeled DNA probes; a reliable method that provides consistent results in the higher range of the assay.

   Real-time PCR: Reverse transcription followed by amplification and quantification of the targeted DNA molecule; generally offers both a wider range of quantification and a lower limit of detection than the bDNA method.

   Used in the management of HCV-infected individuals undergoing antiviral therapy.

   Limitations

   PCR inhibitors in the specimen may lead to underestimation of viral quantitation or false-negative results in rare cases. However, current technologies include internal controls which if not amplified during the PCR, will not give a result rather than a false-negative result.

HEPATITIS D VIRUS (HDV; DELTA HEPATITIS) ANTIBODY

   Definition

   HDV is a subviral agent that is dependent on the HBV virus for its life cycle; therefore, HDV infection cannot occur in the absence of HBV infection.