I did notice that his ANC would always be lower 2 weeks after his LP's.  With regard to his platelets, I did notice at times they'd get a little lower at his 2 week blood draw after LP, but not significant.  We would never get blood drawn one week as he was on Dex that week as well and that would mess with his counts.   I do have every record of his counts though, and I find it interesting that I can find patterns too.  I think each child is different and that maybe that is how your son reacts to that chemo.  Even though the IT MTX that they get in LP's isn't supposed to cross the blood/brain barrier, some does, of course, and that is the reason why they don't get their oral dose that week.  Apparently, the IT MTX is a very strong dose and it is very probable that would mess with their counts...