The condition has been described since at least the 4th century BCE by Hippocrates and possibly as early as 1500 BCE in the Egyptian Ebers Papyrus. Signs and symptoms
Hallucinations
A hallucination is defined as sensory perception in the absence of external stimuli. Hallucinations are different from illusions and perceptual distortions, which are the misperception of external stimuli. Hallucinations may occur in any of the senses and take on almost any form. They may consist of simple sensations (such as lights, colors, sounds, tastes, or smells) or more detailed experiences (such as seeing and interacting with animals and people, hearing voices, and having complex tactile sensations). Hallucinations are generally characterized as being vivid and uncontrollable. Auditory hallucinations, particularly experiences of hearing voices, are the most common and often prominent feature of psychosis. Up to 15% of the general population may experience auditory hallucinations (though not all are due to psychosis). The prevalence of auditory hallucinations in patients with schizophrenia is generally put around 70%, but may go as high as 98%. Reported prevalence in bipolar disorder ranges between 11% and 68%. During the early 20th century, auditory hallucinations were second to visual hallucinations in frequency, but they are now the most common manifestation of schizophrenia, although rates vary between cultures and regions. Auditory hallucinations are most commonly intelligible voices. When voices are present, the average number has been estimated at three. Content, like frequency, differs significantly, especially across cultures and demographics.
Excluding medical illnesses associated with psychosis is performed by using blood tests to measure:

Thyroid-stimulating hormone to exclude hypo- or hyperthyroidism,
Basic electrolytes and serum calcium to rule out a metabolic disturbance,
Full blood count including ESR to rule out a systemic infection or chronic disease, and
Serology to exclude syphilis or HIV infection.Other investigations include:

EEG to exclude epilepsy, and an
MRI or CT scan of the head to exclude brain lesions.Because psychosis may be precipitated or exacerbated by common classes of medications, medication-induced psychosis should be ruled out, particularly for first-episode psychosis. Both substance- and medication-induced psychosis can be excluded to a high level of certainty, using toxicology screening. Because some dietary supplements may also induce psychosis or mania, but cannot be ruled out with laboratory tests, a psychotic individuals family, partner, or friends should be asked whether the patient is currently taking any dietary supplements.Common mistakes made when diagnosing people who are psychotic include:
Not properly excluding delirium,
Not appreciating medical abnormalities (e.g., vital signs),
Not obtaining a medical history and family history,
Indiscriminate screening without an organizing framework,
Missing a toxic psychosis by not screening for substances and medications,
Not asking their family or others about dietary supplements,
Premature diagnostic closure, and
Not revisiting or questioning the initial diagnostic impression of primary psychiatric disorder.Only after relevant and known causes of psychosis are excluded, a mental health clinician may make a psychiatric differential diagnosis using a persons family history, incorporating information from the person with psychosis, and information from family, friends, or significant others. Types of psychosis in psychiatric disorders may be established by formal rating scales.