Quantification of Pathogens

Abstract

In characterizing a pathogen, priority is often given to the assessment of antibiotic susceptibility (Chapter 8); indeed, this feature can be clinically more important than identification itself. However, in some contexts the need is primarily for quantitative data, i.e. information on the number of cells/virions per unit volume in the specimen — and on the changes which occur in this parameter; such information can be used, for example:

• To monitor the progression of viral infection by measurement of viral load in serial specimens.

• To evaluate the efficacy of chemotherapy by direct observation of its effect on virus/cell numbers.

• To test the efficacy of new antiviral agents.

• In chronic hepatitis C: to assess the probable response to therapy with IFN-α (interferon-α) — a long-term response commonly being associated with lowlevel viraemia (e.g. Yamada et al, 1995) and with certain genotypes of the hepatitis C virus.

• To assess infectiousness; for example, in hepatitis B, high-level viraemia is associated with ready transmission of virus via needle-stick accidents etc. — transmission in low-level viraemia is more likely via large volumes of blood (e.g. transfusion).

• To assess the gut concentration of Oxalobacter formigenes in the context of cystic fibrosis/hyperoxaluria (see later).