My son had similar test results on his WISC IV with exception to the vocabulary (18) and letter-number sequencing (19). However, according to above grade level testing he is highly mathematically gifted. His psychologist will actually be testing him with the Stanford-Binet this summer to try to get an accurate mathematical IQ. He just finished 3rd grade and will be taking a 3 month pre-algebra course through Johns Hopkins this summer. You may want to discuss the differences in testing with your child's psychologist.
