I am not aware of any relationship between incubation time and appearance of IgM antibody. IgM responses can vary for patient to patient. A negative result does not completely rule out a recent primary CMV infection. However, I do not believe this variability is associated with length of incubation. I think it is more related to an individual's immune function.

Because CMV IgM antibody may persist for many months after a primary infection, its detection in a single serum specimen is of limited value in determining the timing when infection occurred.