I think this report might help. I had to use it myself.
http://www.pearsonassessments.com/N...C-8E4A114F7E1F/0/WISCIV_TechReport_7.pdf

WISC-IV added an Extended IQ, The Technical Report #7, which should be used when there are two or more scores in the 18� and 19�s.
Good Luck:)