Diagnosis
The diagnosis of FTT relies on plotting the childs height and weight on a validated growth chart, such as the World Health Organization (WHO) growth charts for children younger than two years old or the U.S. Centers for Disease Control and Prevention (CDC) growth charts for patients between the ages of two and twenty years old. While there is no universally accepted definition for failure to thrive, the following are examples of diagnostic criteria for FTT:

Weight under the 5th percentile among children of the same sex and corrected age;
Weight for length below the 5th percentile among children of the same sex and age;
Length for age below the 5th percentile;
Body mass index for age under the 5th percentile;
Weight for age or weight for length dropping by at least two major percentiles (95th, 90th, 75th, 50th, 25th, 10th, and 5th) on a growth chart;
Weight below 75% of the median weight for age;
Weight below 75% of median weight for length; or
Weight velocity less than the 5th percentile.After diagnosis, the underlying cause of FTT must be evaluated by a medical provider through a multifaceted process. This process begins with evaluating the patients medical history. The medical provider will ask about complications during pregnancy and birth, health during early infancy, previous or current medical conditions of the child, and developmental milestones that have been reached or not reached by the child. The childs feeding and diet history, including overall caloric intake and eating habits, is also assessed to help identify potential causes of FTT.
In 1907, Bavarian psychiatrist Alois Alzheimer was the first to identify and describe the characteristics of progressive dementia in the brain of 51-year-old Auguste Deter. Deter had begun to behave uncharacteristically, including accusing her husband of adultery, neglecting household chores, exhibiting difficulties writing and engaging in conversations, heightened insomnia, and loss of directional sense. At one point, Deter was reported to have "dragged a bed sheet outside, wandered around wildly, and cried for hours at midnight." Alzheimer began treating Deter when she entered a Frankfurt mental hospital on November 25, 1901. During her ongoing treatment, Deter and her husband struggled to afford the cost of the medical care, and Alzheimer agreed to continue her treatment in exchange for Deters medical records and donation of her brain upon death. Deter died on April 8, 1906, after succumbing to sepsis and pneumonia. Alzheimer conducted the brain biopsy using the Bielschowsky stain method, which was a new development at the time, and he observed senile plaques, neurofibrillary tangles, and atherosclerotic alteration. At the time, the consensus among medical doctors had been that senile plaques were generally found in older patients, and the occurrence of neurofibrillary tangles was an entirely new observation at the time. Alzheimer presented his findings at the 37th psychiatry conference of southwestern Germany in Tübingen on April 11, 1906; however, the information was poorly received by his peers.