Researchers have acknowledged that most of the cognitive tests fail to predict whether someone has Alzheimer's disease or vascular dementia.
Both Alzheimer's disease and vascular dementia affect learning and memory, behaviour and day-to-day function.
The researchers suggest when older people are confused and forgetful, doctors should base their diagnoses on many different types of information, including medical history and brain imaging.
Of the 118 different tests that were used only two were able to adequately differentiate between Alzheimer's and vascular dementia.
The Emotional Recognition Task (the ability to identify facial expressions in photographs and match emotional expressions to situations, at which people with Alzheimer's were better) and Delayed Story Recall (at which people with vascular dementia were better), were the only tests that appeared to reliably tell the two groups apart.
Many commonly used tests-such as Digit Span (repeating a set of numbers forward, backward), verbal fluency (generating words by first letter or category, such as animals), drawing tasks and more - were unable to distinguish between dementia types. While these tests may assist in diagnosing dementia, they do not adequately discriminate between Alzheimer's disease and vascular dementia," wrote the authors.
They suggest that all cognitive tests should be used cautiously and only in conjunction with other information (imaging, medical history) when diagnosing patients.
The study appears in the journal Neuropsychology.