You need to acknowledge and discuss the reason for missing. Some people - young women mostly, intentionally restrict insulin therapy to lose weight. It's called diabulimia. They manage their glucose levels right before the blood draw so that their glucose is normal on the labs, but the HA1c doesn't lie.
Dexamethosone suppresses ACTH = Pituitary Adenoma Dexamethosone fails to suppress ACTH = Ectopic ATCH (ex - Small Cell Lung Cancer)