it seems to me you have the right approach [edit: using sum], although you don't need glimmix (or nlmixed), i guess you'd use proc genmod, and if you decide to include the repeated measures you can do a GEE model with the 'repeated' statement. Re gee modelling, this is not the best example, but the first suggestion in a google search, shows gee modelling for longitudinal count data, a little analogous to what you have with (i think) fixed timepoints: http://journals.sagepub.com/doi/pdf/10.1177/1094428104263672
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