High serum creatinine... please help?

My mother is a rheumatoid arthritis patient. She has been suffering with it since the past 7 years. It is not too bad as of now. She has inflammation on winter mornings. She takes methotraxate once every week. Recently she went for blood test and found out that she had elevated levels of serum creatinine. What could be the reason and what she do to reduce it? She is slightly overweight but not so much that it seems like a threat. She is 56 years old and is currently a homemaker.