The specimen tested was discovered in a university basement late last century after a search by rival teams of academics.
"It is extremely satisfying to place this last piece in the medical puzzle that Auguste Deter, the first ever Alzheimer patient, presented us with," said Professor Manuel Graeber, from the University of Sydney.
"It is not only of historical interest, however, as it ends any speculation about whether the disease is correctly named after Alois Alzheimer.
"Alzheimer's ability to recognise this dementia more than a century ago provides compelling support for specialisation in medicine. Alzheimer was a founding father of neuropathology, an important medical speciality that is still underrepresented," Graeber said in a statement.
The study was published in Lancet Neurology.
For decades the more than 200 slides that Alzheimer prepared from Deter's brain were lost. Then in 1992, after Graeber uncovered new information pointing to their location, two teams of medical researchers began a dramatic race to find them.
The slides were examined and confirmed beyond doubt that Deter was suffering from Alzheimer's disease, with large numbers of amyloid plaques and neurofribrillary tangles in the brain that are hallmarks of the disease.
Until now a more sophisticated DNA analysis of the small amount of fragile material in single slides has not been possible.
"We found a mutation whose ultimate effect is the formation of amyloid plaques. These plaques, which form between nerve cells and seem to suffocate them are the key diagnostic landmark of the disease," researchers said.
