How can i know where the infection is coming from? 4 weeks post large incisional hernia repair with infected fluid drainage. mesh or tissue?

Possibly both: It is exceedingly hard to determine if a post-operative infection spares any prosthetic material that was implanted, including mesh; if the infection approaches the material, we assume it is infected. Due to the risk of treatment failure if infected foreign material is left in place, it is generally recommended by id to remove any prosthetic material in these situations.

Tough to tell: It depends on the technique of repair. Sometimes a ct scan is helpful. Your surgeon should know the diagnostic algorithm.