The breakthrough could lead to people who may not fully develop dementia until a decade later being given drugs immediately.
This is crucial as experts believe the key to beating the disease lies in identifying it early when treatment is more effective.
Alzheimer's is incurable and currently there are only a few treatments which can ease the ravaging effects - but for a limited time.
Lead researcher Dr Ramon Trullas said that if the initial findings can be replicated by other laboratories, the results will change the way people think about the causes of Alzheimer's, the Daily Express reported.
He said that the discovery may enable to search for more effective treatments that can be administered during the pre-clinical stage.
British experts welcomed the results and called for continued funding for research into early detection.
At present, the only way to accurately diagnose Alzheimer's is during a post-mortem. The relationship between known "biomarkers" with the cause of the disease is unclear. This makes it nearly impossible to diagnose its early stages with any certainty.
But a study at the CSIC Institute of Biomedical Research of Barcelona could change that.
Researchers found that one biomarker is detectable in spinal fluid at least 10 years before signs of dementia appear.
They confirmed that a decrease in mitochondrial DNA (mtDNA) in cerebral fluid may be an indicator for Alzheimer's and should be investigated as a possible cause.
This discovery suggests that the process of the disease starts earlier than previously thought and that mtDNA depletion may be one of the earliest predictors.
The team believes that by finding a way to block the degeneration, clinicians may be able to diagnose and treat the disease before symptoms even appear.
The study is published in the journal Annals Of Neurology.
--ANI (Posted on 15-08-2013)