Q and A


Do I have 9 hours a day without meds if the half-life is 15 hours?


I am on the PIVOT study. I have read that the half-life of boosted darunavir is approx 15 hours.

As I take my meds once a day, does this mean that for 9 hours I am at risk?

The reason I ask is that I had a viral load blip in January after 2 years on meds (only 1 year on PIVOT) and wondered if this is due to the meds not being in my system long enough?

I did have a TDM (therapeutic drug monitoring) blood check, but this was inconclusive and it was never repeated.


Thank you for your question.

A viral load blip is very common and people on 3 drugs also get viral load blips. If your viral load became undetectable again after the blip then this is nothing to worry about. For more information please follow this link.

You are right that darunavir with low-dose ritonavir has a half life of approximately 15 hours. This increases by 30% in the presence of food. But the half life means the time it takes for the concentration to reduce from half it’s maximum level. Most drugs would still be well above the minimum level needed after one half-life. It takes 5 half lives (ie 75 hours for darunavir/r) before a drug is considered to have left the body entirely. Drugs that are used once-daily, are only approved based on still having active levels at 24 hours after dosing.

The concentration of the half life is not directly related to the minimum concentration needed to suppress at least 90% of the virus, called the EC90. It is this number that is the most important thing to focus on. It means that after 15 hours there will still be enough darunavir in your body to suppress the virus.

A single blip may be related to drug levels, but this is unlikely given you have been undetectable for so long. It depends on how high your viral load went, but this can also commonly be a lab error. Viral load tests are not so accurate at very low or very high levels.

The PIVOT study is looking at whether blips are more common in people using boosted PI monotherapy. If the blip is confirmed it is recommended in the study to add back two nukes to your combination..

To find out more please read this section of our training manual as it explains using graphs which is easier to understand as it is visual.


  1. Charlotte Walker


    This is good news as it shows that you are just having a viral load blip which is very common and many people get these. For more information about viral load blips please follow this link.

    Best wishes

  2. Chris

    Thanks for this.

    My re-test confirmed I was undetectable again. However, my routine appointment 2 weeks ago has come back and I am yet again detectable. More tests booked :(

    It is strange that I have managed to maintain undetectable levels for 2 years without blips. Now I am just worried. Am considering asking for Truvada to be reintroduced if I blip again.

    Thanks again for all your responses.


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