Hi, it is great that you were doing well on the previous plan. I would think about the change as an “experiment” to see if you would do as well on the same dose twice a month instead of three times a month. The answer to that change is “NO”. You did not do as well and you should go back to what worked better for you.
This does not mean that you cannot try another change in the future, but it is better to make a change from a place where things are working. Sometimes changes do not work when in other situations they might have worked. For example, if someone has an acute infection, many of us get worse, but if we have a really stable month, we are more likely to react the same as other stable months.
Most people seem to have a lower threshold and if the level gets lower than this, they get symptoms. When you get immunoglobulin, the body immediately starts using it and losing it. For most people, the half life of immunoglobulin is between 21 and 29 days, meaning that if you got 40 g once a month, just before the next infusion, the amount “left” would be only about 20 g (over baseline). Since you need three doses of 40 g in a month, your threshold is likely 32-35ish above baseline. As Lauren points out, this could be given as 50-55 g every two weeks (which works out as less immunoglobulin). Many get monthly immunoglobulin, but then you need an even higher dose at that time to stay about the needed threshold (for example 70-80 g once a month and at the end of the month, you would be at 35-40 g over baseline (if you are a person with a 28 day half-life for immunoglobulin). There has to be a formula for this. In reality, there are differences in rates used and lost based on the amount infused but it can be calculated in you if the doctor checks pre (trough) values, and peak values of immunoglobulin in you. Most doctors just go by what works since people are not perfect metabolic machines. The whole point is next time if you are trying to space to every two weeks, then try a higher dose each of those times.