Resistance testing is recommended while patients are on therapy. Is there a safe cut off time period during which you could still perform resistance testing? For example, should resistance testing be performed for a patient on ddI+3TC+nelfinavir with a viral.load >50,000 copies/mL who stops his medications on his own 5 days before his visit to the office?
Dr. Brian Conway responds: The reason for the recommendation to perform resistance testing while the patient is still on therapy is that, once drug pressure is removed, the wild type isolates may overgrow any drug resistant viruses that may be present in the circulation. Based on current assays, the resistant virus would not be detected if it accounts for less than 10-20% of the isolates that are being tested. For some drugs such as 3TC, it is possible that resistant isolates may disappear quite quickly, perhaps even within 5 days. This is highly unlikely for ddI and nelfinavir.
It would still be reasonable to order the resistance test on the described patient, and to interpret the result as if therapy had continued. However, a report of susceptibility to 3TC should be interpreted with healthy skepticism, and I would not design the subsequent regimen with the idea that 3TC was still an active drug.
