What causes fragile veins and difficulty accessing for lab work & iv's?

Age/ chronic use: Older people , history of chronic diseases requiring recurrent access over years, female gender, obesity, and very thin skin with steroids use.

Heredity, age: Some are just born with small veins, and aging thins the skin and there is easy bruisability, and the veins become fragile. Obesity tends to hide veins making iv/venipuncture difficult. Try using heat, patting of area lightly with the hand as means to plump up veins for access.