First I wonder what constitutes "Passing" the MMSE. Within the 30 questions there are questions that test 8 different types of cognition. There are 5 questions each for orientation to time and place. They vary from the broadest to the narrowest indications of time and place. The patient may be able to tell you what year it is but not what month or what day. There are three questions related to how information registers such as repeating names. There are 5 question on attention and calculation. Two are these are spelling "world" backwards and counting backwards from 100 by 7s. Three questions are related to recall. They get a list of words and have to repeat them back at some point later. One is repetition or repeating a phrase back. Two relate to language as in naming a pencil. Six questions relate to complex commands like drawing the clock.
In the early stages, on a good day, with an intelligent patient, yes they can pass the test and still have dementia. Mom did. She scored a 27 which is considered passing but she was well into her dementia at that point.... and I knew it despite "passing" the MMSE test. If your loved one doesn't make 30/30 I would suggest further cognitive testing. Even with a 27 on the MMSE I insisted Mom go for further cognitive testing and from those test it was obvious she had major cognitive decline. You have to remember that the MMSE is just a quick test. A complete cognitive testing will last 6 to 8 hours. The MMSE is just one of several test that can be used. Also different types of dementia test differently. Early Alzheimer's patients will usually miss the orientation to time and place questions while Vascular dementia seems to do worse on other questions. The MMSE is just one tool used to determine if further testing is needed. It is not a definitive test. It is the best quick test that can be given by a GP though
There are proposed changes that everybody over a certain age be given a MMSE as part of the routine physical. This will give a base line. If Dad make a 30 every time he goes and suddenly drops to a 27 then you know there was probably a cognitive decline. If he went in and made a 27 with no previous test, you don't know what it might have been before. Maybe he couldn't do the math of counting backwards by 7 all along, or did he lose that ability recently because of cognitive decline?
If your loved one has symptoms of dementia, the MMSE is the first test but should not be combined with symptoms and behaviors to make a determination of the need of further testing to come to an actual diagnosis.