Q. My doctor recently ordered a SPECT scan for me. If the results say normal, does this mean I do not have Alzheimer's disease? Some have told me that it is not an inclusive test. I was wondering what your opinion is?

A. The results of a SPECT scan are certainly very helpful in making a diagnosis of Alzheimer's disease, but they are not the final word. Basically, imaging studies like SPECT and PET are measures of blood flow to various parts of the brain, or measures of how the brain uses sugar to produce energy. Decreased activity in certain brain regions on SPECT or PET often points to some form of chronic brain disease, or dementia. However, the doctor must also consider the patient's actual mental capabilities, impairment in memory, ability to do calculations, use language correctly, carry out complex executive tasks, activities of daily living, etc.

One study (Muller et al, 1999) found a good correlation between results on SPECT and results from a pen-and-paper test called the Mini-mental state exam (MMSE). The MMSE asks the patient a number of questions testing orientation, memory, etc. and is used as a screening test for dementia. However, this same study looked at the abnormal SPECT pattern that is supposed to be typical of Alzheimer's Disease, and found that while it was present in about half of patients with probable Alzheimer's, it was also seen in 25% of the control subjects, who did not have the disease.

So, in this study, SPECT was only partially sensitive and not entirely specific for Alzheimer's Disease. The bottom line is that a normal SPECT does not guarantee that a patient is free of (early) Alzheimer's, nor does an abnormal SPECT mean that the patient absolutely has the disease. The SPECT is just one piece of the puzzle. The diagnosis of dementia still rests on a careful history, physical examination, laboratory tests, mental status exam, and sometimes additional tests, such as an MRI of the brain (a sort of X-ray) and specialized psychological testing.

A number of biochemical and genetic tests may also be useful, but again, is not the final word. Please speak with your doctor if you have further questions, and consider contacting the Alzheimer's Association for further information (800-272-3900, or http://www.alz.org/). I hope things work out well for you regardless of your diagnosis. Certainly, new research is making the outlook for Alzheimer's much brighter than it ever was.

March 2001

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