Precision medicine (PM) is a medical model that proposes the
customization of healthcare, with medical decisions, practices, or
products being tailored to the individual patient.
Precision psychiatry is a sub-field of precision medicine - focusing specifically on individually tailored medical interventions for psychiatric disorders.
The practice of precision psychiatry does not necessarily suggest a different diagnosis for psychiatric disorders such as PTSD or a different diagnostic criteria or process. In fact, the diagnosis could very well remain the same.
Rather, precision medicine and psychiatry are concerned with customizing treatment. To this end, medical testing would need to be more extensive and more accurate / "precise". It is this additional testing, useful for tailoring treatment and monitoring its progress, that is the focus of precision psychiatry:
The major challenge that precision psychiatry faces is that psychiatry
does not yet use measurement to track the equivalent of vitals and
images of the organs of interest, namely the brain and alterations in
peripheral measures such as blood.
A couple of good examples to consider are schizophrenia and depression.
Schizophrenia was traditionally divided into several symptom-based sub-types. This categorization was dropped recently, and early genetic research suggested quite different sub-types that might explain why the original sub-types did not work out. However, sub-types based on genetic profile clusters may also be untenable. Current research has revealed over 100 genetic loci contributing to schizophrenic symptoms, and suggesting the potential for custom treatments, as many medications have different targets. Here we have a single diagnosis of schizophrenia done in the traditional way, where additional genetic testing may guide individual treatment.
Depression - the core clinical symptom of major depressive disorder - has several potential treatment regimens, including a variety of medications, psychotherapies, brain stimulation, and alternative therapies such as light exposure and meditation, each of which appear moderately effective across the patient population. Recently however, some limited research has suggested that brain scans can help predict which therapy would be most effective for each individual. Here again, we have a standard diagnosis based on patient-reported symptoms, where the addition of brain scans may eventually guide individual treatment and monitoring.
In precision oncology for example (another sub-field of precision medicine), the classification of biomarkers and their corresponding treatments has sometimes led to differential diagnoses, and sometimes not. For example, breast cancer is known to have several different receptor biomarkers and corresponding treatments, but is not necessarily classified as different types of breast cancer because the biomarkers don't cluster - they are specific to the patient.