Scientists have a growing understanding of which Alzheimer's patients are more likely to outlive their peers with the disease. In 2004, a University of Washington study funded by the National Institute on Aging (a branch of the National Institutes of Health) identified several factors that influence life expectancy:

• Gender. Women in the study tended to live longer than men -- an average of six years after diagnosis, compared with four years for men.
• Age at diagnosis. People diagnosed with Alzheimer's in their 70s had longer survival times than those older than 85. The male-female difference shrank among those who were older when they first developed the disease. Newly diagnosed 85-year-old women in the study had a median life expectancy of 3.9 years, compared with 6 years for unaffected women of the same age. A man diagnosed with Alzheimer's at 85 had a life expectancy of 3.3 years in the study, compared with 4.7 years for a man of that age without the disease.
• Severity of symptoms. The more significant the impairment at the time of diagnosis, the shorter the probable number of years left. People over 85 who wander or have trouble walking, for example, are among those with the poorest survival rates.

Also showing diminished survival rates were study subjects who scored the worst on a commonly used memory and cognition (thinking skills) test, the Mini-Mental State Exam (MMSE).This test asks subjects to do a series of simple tasks, including answering questions about the date and place, remembering three objects, and counting backward by increments. An MMSE score refers to the number of correct answers given out of a total of 30. A five-point drop in score over the course of a year was linked to decreased survival rates among patients in the study.

However, simply scoring poorly doesn't mean a person doesn't have long to live. Sometimes a poor MMSE score is the first indication of cognitive trouble, which may indicate the very start of Alzheimer's and therefore a longer life expectancy than predicted for someone who's had memory problems for years.

Note: There are multiple factors that can influence an MMSE score. The test is not an Alzheimer's diagnostic test -- in fact, no such test exists.

• Other health problems. Survival was also poorest among those aged 85 and older who had histories of diabetes, congestive heart failure, or a past heart attack.

What often happens in people with Alzheimer's disease is that their general health suffers when, in the late stages, they forget or find it difficult to eat, don't sleep properly, lose motor skills, and develop bedsores. This sets the stage for infection, such as pneumonia, or the worsening of other chronic ailments. It's those factors, not the Alzheimer's disease itself, that ultimately result in most deaths.

Bear in mind that the facts above represent group averages. Your parent, of course, is an individual with a unique health history. And Alzheimer's is a disease whose pace can vary widely from person to person. Still, having even a rough sense of what to expect can provide a glimmer of useful light at a difficult time.