What should be noted if analyzing the combination of lab-producing RNA-Seq data and open RNA-Seq data in GEO

Question

[Zhilong Jia]

I have some RNA-Seq data from blood samples for a disease, while no controls. If I would like to use controls from GEO, GTEx or TCGA, what should I pay attention to concerning the analysis of deferentially expressed gene? Such as normalization? Thank you.