Drug level testing

Sometimes treatment failure occurs when drug levels in the body fall too low and are not able to fully suppress HIV. This may happen for a variety of reasons, for example due to drug interactions or simply individual variations in how a drug is processed. Persistent vomiting or diarrhoea, which may be a side-effect of some drugs, can also lead to low drug levels. Conversely, drug levels may also rise too high, causing more severe side-effects .

Inadequate drug levels were a more common problem with unboosted first-generation protease inhibitors, which had poor bioavailability. But boosting with a small dose of ritonavir raises blood levels of other protease inhibitors and helps keep them from falling too low between doses.

Low drug levels are a potential concern with once-daily dosing if adherence is a problem, since the virus has a full 24 hours to start replicating if a dose is missed.

Drug-level testing, known as therapeutic drug monitoring, is useful for determining whether there are adequate concentrations in the blood. Although large randomised controlled trials have not yet validated the benefits of drug monitoring in terms of clinical outcomes, drug-level testing can be beneficial in some clinical settings. Examples of these are in the treatment of children; pregnant women; and people with impaired liver or kidney function, in order to avoid toxicities; when managing drug interactions; or in cases of virological failure in an adherent person.

Therapeutic drug monitoring is limited in many settings by a shortage of laboratories that can perform the tests properly and experts who can interpret them accurately.