What is the independent variable? What level of measurement is it?

Mosfeldt et al. (2012) collected data on 792 patients age 60 or over who were admitted to a hospital in Denmark with a hip fracture between 2008 and 2010. They reported that an elevated creatinine level upon hospital admission for a hip fracture (>90 mmol/L for women and >105 mmol/L for men) is associated with an almost threefold increase in mortality risk.