There are too many things to mention that can cause a slightly high alkaline phosphatase. Quest Labs ( the one I USED to use puts an upper limit of 125 U/L on it) so yours is ALMOST normal.
You can ask for a GGT level to determine if the raised level is caused by bone or by your liver/gall bladder but I think the refinement is probably unnecessary. Usually severe liver disease is accompanied by levels FAR higher than yours.
I agree with your doctor...just watch it re-think the situation in 3 months.
Yes, it COULD very well be the niacin. Do you have any numbers on the AP that were done BEFORE your started niacin? What happened to your HDL with and without the niacin...a boost in HDL is the MAIN reason we all take it.
(I get an alkaline phosphatase of 40-50 with 500 mg. slow acting niacin./day.)
I just saw your other post with the HDL drop with niacin use mentioned. In that case you have NO further use for niacin so you are right in stopping it...there's no good reason to take it again.