According to the police, in this test, in around 1 year, there were 57099 people who were told that they might have a brain disease called dementia. 1892 of these people knew they had dementia because a doctor had tested them.
I'm having trouble understanding the sentence in bold. The translation is my best guess. My understanding of 分かる is a bit shaky. Does this sentence suggest that the 1892 people already knew they had dementia before they took the police test, or does it imply that those people went to the doctor after the police test and were then confirmed as having dementia? (putting aside the fact that a person diagnosed with dementia might still not know they have dementia).