7.1 Malabsorption of drugs

Malabsorption of antimycobacterial drugs with all first line therapies as well as ethionamide and cycloserine has been reported in co-infected persons.

Absorption of drugs may be less in those patients with a low CD4 count, whether it be due to HIV enteropathy or other specific HIV related gut diseases resulting in sub therapeutic serum and drug levels and consequently associated with treatment failure and drug resistance. Although some studies show lower peak concentrations of rifampicin and ethambutol as well as lower AUC compared with controls, there are other data suggesting that rifampicin is well absorbed in HIV patients even those with AIDS or with diarrhoea. ^61-67

7.2 Therapeutic drug monitoring (TDM)

TDM of TB drugs: [BII]

Based on the limited amount of available data TB drug therapeutic monitoring might be useful (but is often not very helpful) in:

• patients who are at high risk of malabsorption of their TB drugs,
• in those who are responding inadequately todirectly observed therapy with first line drugs
• in patientsbeing treated for multi-drug resistanttuberculosis.
• In those who are on non-standard TB regimens or taking non-standard doses

One of the problems with monitoring anti-mycobacterial drugs in HIV positive patient is that the kinetics of absorption is not predictable.  It is therefore difficult to know when to measure a peak serum level; and it is probably best to assess this in the individual patient by checking levels at more than one time point post dose eg 1, 2 and 4 hours. Decision re dosing may be difficult as there can be long delays in results returning to the physician. ^61-67

TDM of HIV drugs:[BII]

TDM may be relevant for PI and NNRTI especially when regimens are complex, when no formal PK data are available to guide the physician and when virological failure occurs.