Generally, a high serum creatinine level means that the kidneys are not working well. Causes include kidney failure (acute or chronic), kidney infection, drug toxicity, dehydration, high blood pressure (hypertension), and high sugar levels (diabetes). It is possible to have acute renal failure and still have normal-sized kidneys. Renal atrophy may result from chronic renal disease or infection, as well as other conditions such as narrowing of renal arteries or obstruction of urinary tract.
Would be interested to know more about your mother and what her doctors are telling her.