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Table 1 Advantages and disadvantages of genotypic and phenotypic resistance assays

Advantages Disadvantages

Genotypic resistance assays

Have a wider availability, requiring biosafety level 1 facilities

Are less expensive

Display a shorter turnaround time

(typically 1-2 weeks)

Detect sentinel mutations that are linked to reduced clinical response but not necessarily to reduced phenotypic susceptibility

Phenotypic resistance assays

Measure the effective susceptibility to all (also new) drugs Measure effects of mutational interactions

Require plasma samples in general with viral load >1000 RNA copies/mL Exhibit difficulties in detecting minor variants Exhibit a sensitivity that is strongly influenced by virus genetic variability

The relevance of some mutations remains still unclear Some (complex) mutation patterns may be difficult to interpret

Require highly specialized laboratory facilities, typically biosafety level 2 or 3

Require plasma samples in general with viral load >1000 RNA copies/mL

Exhibit difficulties in detecting minor variants

Exhibit a sensitivity that is strongly influenced by virus genetic variability

Are more expensive (in general twice as much as genotyping)

Have longer turnaround time (typically 2-4 weeks)

The reproducibility range of the assay extends for some drugs above clinically relevant cut-offs

The clinical relevance of drug susceptibility levels is not always clear phenotypic resistance results. This turnaround time can be an important factor in some clinical situations, where immediate results are needed, e.g., postexposure prophylaxis and primary HIV-1 infection.

Sequencing has the advantage of detecting all known resistance-related mutations—mutations considered as markers for developing resistance patterns (e.g., K70R or Q151M in the RT gene)—or substitutions that are considered to be associated with a lower genetic barrier to resistance development (e.g., the reversal mutations T215A/C/D/S in the RT gene). Phenotypic assays measure the effective susceptibility, resulting from known, but also unknown, resistance-related mutations. However, they do not detect the presence of the latter two sets of mutations. Although these mutations are associated with reduced clinical response, they are not always linked with reduced phenotypic susceptibility.

In addition, the current phenotypic assays have difficulties in detecting resistance to some nucleoside reverse transcriptase inhibitors. These drugs compete with the natural deoxynucleoside triphosphates (dNTPs) for binding to the reverse transcriptase and incorporating into the growing DNA chain. The phenotypic assays use activated cells with high intracellular dNTP pools that do not always reflect the relevant in vivo cell conditions. The high intracellular dNTP pools lead to resistance levels that cannot be reliably measured because they are within the reproducibility range of the assays.

Another issue in the interpretation of genotypic results is whether the mutations constituting a part of a set of multiple mutations exist on a single clone or are spread across multiple clones of variants. Phenotypic resistance assays resulting in low-level resistance may be caused by a small change in susceptibility of the whole virus population or a mixture of resistant and susceptible virus. The linkage of mutations could have important clinical implications, but at the moment this issue cannot be routinely elucidated with the current resistance assays.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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