The American Psychiatric Association (APA) has established the symptoms and criteria for diagnosing attention deficit hyperactivity disorder (ADHD). These criteria divide the condition into three basic types based on major symptoms⁵:

• ADHD, predominantly inattentive type
• ADHD, predominantly hyperactive-impulsive type
• ADHD, combined type

In addition, some people are diagnosed with "ADHD, not otherwise specified" when symptoms of inattention, hyperactivity, and/or impulsivity are present but do not fit into one of the three types.

A doctor will use criteria for diagnosing attention deficit hyperactivity disorders to determine whether a child has ADHD. Information used to diagnose the condition includes:

• An interview with the child.
• Medical history, including asking a parent about the child's social, emotional, educational, and behavioral history.
• Physical exam.
• Behavior rating scales or checklists for ADHD, used by parents and teachers to evaluate the child's symptoms.

It can be difficult to determine whether a child's behavior problems are caused by ADHD, other conditions with similar symptoms, or a combination of ADHD and another condition. Several verbal and written tests for associated disorders are used to help with this determination.

In addition, children with ADHD may have difficulty learning to read, write, or do math problems. Testing for learning disabilities will help teachers develop the best educational plan for a child with these difficulties.

Other tests may be done to identify other medical problems that might explain the child's symptoms, such as:

• Hearing or vision impairment. This type of disability often interferes with school achievement.
• Lead exposure. Children who have even small amounts of lead in their bodies can have symptoms similar to ADHD.
• Low red blood cell counts (anemia). This condition can cause low energy and poor concentration. It can be diagnosed with results from a complete blood count (CBC).
• Thyroid disease. Blood tests can help find out if a person has too much or too little thyroid hormone, which also can affect energy and attention. This is more common in adults than children.
• Seizures. Seizures can affect brain function and result in unusual behavior. In rare cases, a person with ADHD symptoms may have an electroencephalogram (EEG) to find out if seizures are occurring.

Parents often question whether ADHD is overdiagnosed. Many doctors and researchers believe that the increase in ADHD diagnoses results from improved detection techniques, especially the standardization of assessment criteria. Current and future research should help in answering this question.

Many adults with ADHD have never been diagnosed or treated. ADHD is a lifelong condition that, left untreated, can lead to low self-esteem, frustration, school or job failure, drug abuse, and depression. To diagnose ADHD in an adult, a doctor may use the Wender Utah Rating Scale (WURS), a written test that consists of 25 questions about childhood difficulties that are often seen with the condition. The scale evaluates the presence and severity of ADHD symptoms during childhood.