Diagnosing DLB can be challenging because of the wide range of symptoms with differing levels of severity in each individual. DLB is often misdiagnosed or, in its early stages, confused with AD. As many as one in three diagnoses of DLB may be missed. Another complicating factor is that DLB can occur along with Alzheimer's disease (AD); autopsy reveals that most people with DLB have some level of changes attributed to AD in their brains, which contributes to the wide-ranging variety of symptoms and diagnostic difficulty. Despite the difficulty in diagnosis, a prompt diagnosis is important because of the serious risks of sensitivity to antipsychotics and the need to inform both the person with DLB and the person's caregivers about those medications' side effects. The management of DLB is difficult in comparison to many other neurodegenerative diseases, so an accurate diagnosis is important. The 2017 "Fourth Consensus Report" established diagnostic criteria for probable and possible DLB, recognizing advances in detection since the earlier "Third Consensus" (2005) version. The 2017 criteria are based on essential, core, and supportive clinical features, and diagnostic biomarkers. The essential feature is dementia; for a DLB diagnosis, it must be significant enough to interfere with social or occupational functioning. The four core clinical features (described in the "Signs and symptoms" section) are fluctuating cognition, visual hallucinations, REM sleep behavior disorder, and signs of parkinsonism. Supportive clinical features are marked sensitivity to antipsychotics; marked autonomic dysfunction; nonvisual hallucinations; hypersomnia; reduced ability to smell; false beliefs and delusions organized around a common theme; postural instability, loss of consciousness and frequent falls; and apathy, anxiety, or depression. Direct laboratory-measurable biomarkers for DLB diagnosis are not known, but several indirect methods can lend further evidence for diagnosis.The indicative diagnostic biomarkers are: reduced dopamine transporter uptake in the basal ganglia shown on PET or SPECT imaging; low uptake of 123iodine-metaiodobenzylguanidine shown on myocardial scintigraphy; and loss of atonia during REM sleep evidenced on polysomnography. Supportive diagnostic biomarkers (from PET, SPECT, CT, or MRI brain imaging studies or EEG monitoring) are: lack of damage to medial temporal lobe; reduced occipital activity; and prominent slow-wave activity. Probable DLB can be diagnosed when dementia and at least two core features are present, or when one core feature and at least one indicative biomarker are present.