What is the impact when there is a lack of research on a diagnosis when clinicians are trying to diagnose a patient?

When there is a lack of research on a specific disorder, patients suffer, as they are not able to receive the most tailored and up-to-date treatment. As the article from the National Institute of Mental Health in the link below states, diagnoses in the DSM (Diagnostic and Statistical Manual of Mental Disorders), which clinicians use to diagnose mental disorders, are based on clusters of symptoms. Disorders are not based on objective measures from the laboratory. In other words, this is like diagnosing medical conditions based on symptoms such as fever rather than on laboratory measurements. 
Refining the research about diagnoses will ensure validity--that is, making sure that the diagnosis is a condition, not just a set of symptoms, and that it has measurable differences from other conditions. In addition, additional research about diagnoses will help clinicians make more accurate diagnoses and ensure that people who should be diagnosed are diagnosed. If patients receive correct diagnoses, they can then access proper treatment. For example, the diagnosis of Autism Spectrum Disorder (ASD) is currently made based on a set of symptoms. However, if research showed ways to diagnose autism based on objective measurements (such as brain scans, for example), clinicians could be sure that people who were diagnosed definitely had that disorder and could receive effective treatments. 
https://www.nimh.nih.gov/about/directors/thomas-insel/blog/2013/transforming-diagnosis.shtml