Qualitative versus Quantitative PCR

In the early 1990's, each laboratory had its own in-house technique for the RT-PCR assay and the reliability of these assays was quite variable. Even as of now, the FDA has not approved any of the RT-PCR assays. However, most laboratories currently use one of the several available diagnostic kits that are automated and designed to reduce the likelihood of contamination. There are two types of RT-PCR, qualitative and quantitative. Qualitative hepatitis C virus RT-PCR provides the greatest sensitivity, meaning that it can measure as few as 100 copies (viral particles) of hepatitis C virus/ml of serum. As the name implies, however, qualitative RT-PCR provides only a positive (presence of hepatitis C virus) or negative (absence of hepatitis C virus) result.

By contrast, quantitative RT-PCR measures the amount of virus. These tests, however, are only accurate within a certain range of viremia (circulating virus in the blood). This means that quantitative assays are not as sensitive as qualitative assays and can only detect as few as 500 copies/ml. Moreover, these assays are less accurate at extremely high viral levels (over 2 million copies/ml). In the past year, there has been an attempt to standardize these various quantitative assays so that the levels of virus that are measured by different assays can be compared. As a matter of fact, results of quantitative RT-PCR are now reported in standard International Units/ml (IU/ml).

Branched chain DNA (bDNA) is the other quantitative technique. It is based on the amplification of the detection signal rather than of the nucleic acid itself. As a result, this test is less prone to contamination and is more accurate when measuring higher levels of the virus as compared to RT-PCR. However, the bDNA assay is not as sensitive as the RT-PCR and is unable to measure levels of virus below 200,000 copies/ml.

Finally, transcription mediated amplification (TMA) is a qualitative technique that is distinct from PCR. This test can measure as few as 2 to 5 copies of virus/ml.