The process of applying knowledge of resistance patterns to choose initial and ongoing drug therapy, is known as "sequencing". Sequencing choices are based on drug efficacy (effectiveness), tolerability (side effects), safety, convenience, and the patients disease stage. A patients HIV treatment regimen may be changed due to drug toxicity and regimen failure. Drug toxicity means that some drug side effects make it difficult for patients to take the drugs. The goals of HIV therapy are:
There are three types of regimen failure virologic failure, immunologic failure, and clinical failure. Virologic failure occurs when HIV can still be detected in the blood 48 weeks after starting treatment, or if HIV is again detected in blood that had previously decreased the viral load to an undetectable level. Immunologic failure occurs when the CD4 count decreases below a baseline count or does not increase above the baseline count within the first year of therapy. Clinical failure occurs when the patient experiences an HIV-related infection or has a decline in physical health, despite at least 3 months of anti-HIV treatment.
Virologic failure is the most common type of regimen failure. People with virologic failure, who do not change to a more effective drug regimen, usually progress to immunologic failure within several years. Immunologic failure may be followed by clinical failure.