A study comparing CSF and serum BDNF measurements in a sample of AD patients found that they were not correlated (r2 = 0.03, n = 27); however, a study comparing blood and BDNF levels in pigs and rats found significant positive correlation between the two in both pigs and rats (r2 = 0.41, 0.44, respectively) [13], [34]. Here, BDNF is linked to Alzheimer disease.