Virtual phenotype vs phenotype

A randomised study in Spain showed that a similar number of drugs defined as active were prescribed to 260 patients who received phenotypic or virtual phenotype testing, but by as-treated analysis (which excludes patients who may have dropped out of the study due to virologic failure or adverse events), significantly more of those in the virtual phenotype group had viral load below 400 copies/ml at week 24.1

The GenPherex study, conducted in Italy, demonstrated equivalence between phenotypic and virtual phenotype testing in 106 patients randomised to switch therapy on the basis of resistance testing. Participants had been exposed to an average of nine drugs. Approximately 60% of patients in each arm were sensitive to at least three drugs in the new regimen, and after twelve months similar proportions in each arm had viral load below 400 copies/ml.2

The open-label PhenGen study involving 303 people has shown that virtual phenotype and phenotype were equivalent. After six months of treatment, 55% of the virtual phenotype group had undetectable viral load compared to 53% of the phenotype group and mean reductions in viral load were nearly identical.

Whilst the Virtual Phenotype lags behind other resistance testing systems in terms of evaluation of its use in patient management, it is already in use in some settings, such as the Chelsea and Westminster Hospital, London.

Virtual phenotyping was also compared with rules-based algorithms in a retrospective analysis of ACTG 372B/D. When the data were controlled for baseline viral load and number of new drugs received, the virtual phenotype at baseline was significantly more predictive of virological failure than rules-based interpretation of genotypic data.3


  1. Perez-Elias MJ et al. Realvirfen Study Group Phenotype versus virtual phenotype for choosing antiretroviral therapy after failure: a prospective, randomized study., 2004
  2. Mazzotta F et al. Real versus virtual phenotype to guide treatment in heavily pre-treated patients: 48 week follow-up of Genotipo-Fenotipo di Resistenzia (GenPheRex) trial. JAIDS 32: 268-280, 2003
  3. Hammer S et al. Relationship of phenotypic and genotypic resistance profiles to virological outcome in a trial of abacavir, nelfinavir, efavirenz and adefovir dipivoxil in patients with virological failure receiving indinavir (ACTG 372). Antivir Ther 4: S45, 1999
Community Consensus Statement on Access to HIV Treatment and its Use for Prevention

Together, we can make it happen

We can end HIV soon if people have equal access to HIV drugs as treatment and as PrEP, and have free choice over whether to take them.

Launched today, the Community Consensus Statement is a basic set of principles aimed at making sure that happens.

The Community Consensus Statement is a joint initiative of AVAC, EATG, MSMGF, GNP+, HIV i-Base, the International HIV/AIDS Alliance, ITPC and NAM/aidsmap

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.

NAM’s information is intended to support, rather than replace, consultation with a healthcare professional. Talk to your doctor or another member of your healthcare team for advice tailored to your situation.