Is misdiagnosis possible?
Yes, misdiagnosis is a probability with all illness. In general, you will find an approximate 5% misdiagnosis rate amongst other illnesses. A systematic review from 2005 (Stone et al BMJ. 2005; 331: 989;doi:10.1136/bmj.38628.466898.55) concluded the misdiagnosis rate for FND had averaged around 5% since 1970 for Conversion Disorder. Keeping in mind, studies can be interpreted many different ways and different inclusion and exclusion scenarios can impact the results.
It is important that functional diagnoses derive from positive signs not negative test results. It is also imperative that not all symptoms are labeled functional and that assumptions are not made that functional symptoms are not co-existing or the result of organic illness.
Some in the medical community mistake medical uncertainty with “all in your head”. It is vital doctors not equate functional symptoms with “all in your head”and not insinuate one means the other.
This poor practice of assuming medical uncertainty always means a psychogenic cause often effects tens of millions of patients throughout the US and UK alone.
Rare disease patients are at highest risk because of the vast unfamiliar symptoms stemming from the 6,800 known rare diseases. This number is sure to climb as new research and technology emerges.
Initially receiving incorrect diagnoses is a common experience for many rare disease patients. Of the total number of survey participants, 41% received at least one such misdiagnosis before obtaining the correct one. This results in inappropriate treatment.
Rare disease patients and organizations strongly feel there is a direct correlation between diagnostic delay and a misdiagnosed psychiatric diagnosis. “Patients who initially received a false psychological or psychiatric diagnosis experienced longer delays in diagnosis, suggesting that this type of diagnosis introduced yet an even greater barrier for patients before the quest for correct diagnosis could be resumed”, according to a 2009 rare disease report by EURORDIS. 
The report also states: “Respondents affected by TS and CF reported delays four times longer if they initially received a psychological or psychiatric misdiagnosis and MFS patients reported delays ten times longer in this same situation. In the case of psychological or psychiatric misdiagnosis, respondents reported delays 2.5 to 14 times longer than those who initially received a correct diagnosis.”