Given the number of separate conditions in the schizophrenia spectrum disorders categories, it probably seems as though it would be easy for a doctor to make a diagnosis. However, there are many potential causes of delusions, hallucinations and loss of reality. Just knowing that someone is suffering from these symptoms doesn't help to know why they are suffering. Information on the proper cause of symptoms is important if an accurate diagnosis is to be made. Doctors will rely on logical thinking and diagnosis rules that help them to separate one condition from another when they are not sure of the cause of psychotic symptoms.
The basic rule for diagnosis is that physical, medical and temporary conditions that could create the symptoms must be ruled out before it is assumed that there is an ongoing mental illness present.
Substance abuse or the interaction of multiple medications can cause the symptoms, as can various medical conditions. Doctors will wait to make a permanent diagnosis until after people have had a medical exam and have been under observation for a long enough period of time so that any substances taken can clear the body. They will also generally try to view past medical records that might explain the symptoms. They may request tests looking for evidence of potential causes. Special care must be taken to separate schizophrenia from pre-existing conditions, including other mental disorders. A diagnosis of schizophrenia can be made only when other causes of psychosis have been ruled out.
In some cases, diagnosis is particularly tricky because multiple causes come together to create a confusing result. For example, a person with a hidden vulnerability for schizophrenia may get involved with stimulant drug abuse. They may show up at the hospital in a fully psychotic state (hallucinating, delusional or losing touch with reality). In this case, drugs may have caused the symptoms to begin, but they would not necessarily be responsible for continuing them. In triggering initial symptoms, they might also serve as a stressor strong enough to trigger the vulnerability to schizophrenia. That condition would then take over and cause further ongoing psychotic symptoms. Originally, a diagnosis of substance/medication-induced psychotic disorder would be appropriate. But over time, the diagnosis would need to shift to something like schizophrenia or a related condition. Only time and a documented history of treatment can point towards an accurate diagnosis.