How is depression diagnosed? The answer isn't cut-and-dried; a wide range of tests are available to determine what your symptoms mean.

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If you think you might be suffering from depression, the only way to know for sure is to see a doctor or mental health professional. Once you’ve made that important first step, your first question may well be: “How is depression diagnosed?”

The process starts with a doctor or mental health professional giving you a comprehensive assessment to determine whether you are indeed suffering from depression. Some of the things that might happen during that assessment are described below.

How Depression Is Diagnosed? Screening Questionnaires

You doctor will first ask you to describe your symptoms in detail. He or she also may use screening questionnaires or rating scales to help clarify whether depression is at the root of your problems.

Guidelines from the American Psychiatric Association recommend that people with major depressive disorder complete a rating scale—either administered by their doctor or self-reported—to assess the type, frequency, and severity of their symptoms. Using a rating scale to evaluate symptoms can help doctors and mental health professionals tailor the treatment plan specifically to you. They can also be used later on to determine whether treatment is working.

Rating Scale Guidines

Rating scales may include any of the following:

• Patient Health Questionnaire (PHQ 9): This self-test contains nine items that assess depression symptoms. It can help diagnose depression and then evaluate the severity to help your doctor select the most appropriate treatment and monitor your progress while you are being treated.
• Quick Inventory for Depressive Symptomatology Self-Report (QIDS SR): This is a 16-item self-test that is translated into multiple languages. It is used to assess a broad range of depression symptoms, and it can be used to track treatment response.
• Beck Depression Inventory (BDI): This classic depression self-test contains 21 questions that assess the intensity of depression. Each item includes a list of four statements that refer to one symptom of depression. Statements are listed by increasing severity.
• Center for Epidemiologic Studies-Depression Scale (CES-D): The CES-D has been around since the 1970s, and it remains one of the most widely used methods for assessing the symptoms of depression. On this test, you will be asked to check off which of 20 statements you have felt in the past one to seven days. Examples of statements on the scale include, “I felt that everything I did was an effort,” or “I felt that people dislike me.”
• Hamilton Rating Scale for Depression (HRSD), also known as the Hamilton Depression Rating Scale (HDRS) or HAM-D: A psychologist or psychiatrist administers this 21-question scale to assess the severity of depression in people who already have been diagnosed. Each item ranks the severity of symptoms such as depressed mood, feelings of guilt, and difficulty in work and activities.
• Zung Self-Rating Depression Scale: The Zung scale is a 20-question assessment. The user checks off how often he or she has experienced certain symptoms, such as feeling downhearted, having crying spells, or having trouble sleeping.

How Is Depression Diagnosed? Physical Exams and Diagnostic Tests

In addition to having a detailed interview, you may need a physical examination and diagnostic tests to rule out illnesses that cause symptoms of depression, such as a thyroid problem, stroke, central nervous system tumor, head injury, or multiple sclerosis.

People with typical signs and symptoms of depression usually do not need to have a detailed physical work-up, but those with complex medical problems or whose symptoms do not neatly fit the diagnosis of depression (for example, they have fatigue and weight loss without sadness or loss of interest) should have a physical exam.

A blood test is used to rule out a number of issues that may be be contributing to your health woes.

Diagnostic tests may include:

• Blood tests: The following blood tests can detect medical conditions that can cause depression symptoms:
• Blood sugar (glucose) levels
• Complete blood count (CBC)
• Levels of hormones, such as thyroid hormone
• Levels of inflammation markers, such as erythrocyte sedimentation rate (ESR)
• Levels of minerals, such as calcium and iron
• Levels of vitamins such as B12, folate, and vitamin D
• Liver and kidney function tests

Although blood tests can help pinpoint medical conditions, they can’t diagnose depression. However, several groups of researchers have identified promising genetic markers in the blood that may help differentiate individuals with and without depression.

In addition to genetic markers, scientists have also studied other biomarkers found in the blood, such as indicators of how well the body manages inflammation, maintains neuronal health, and balances the stress response. Researchers are even working on blood tests to help determine the risk that someone with bipolar disorder will commit suicide.

• Imaging tests: Imaging tests such as an MRI or computed tomography (CT) scan of the brain may be ordered to rule out tumors, bleeding, or neurological disorders, such as multiple sclerosis. Imaging tests also may include a carotid ultrasound to check for blocked arteries in the neck that supply blood to the brain. Most people with depression symptoms do not require brain-imaging tests. These will only be ordered if your doctor suspects the symptoms have an underlying neurological cause.
• Electrophysiological tests: Certain forms of tests of electrical waves in the brain or heart are sometimes used to evaluate depression:
• Electrocardiogram (ECG) is used to diagnose heart problems
• Electroencephalogram (EEG) is used to rule out a seizure disorder (epilepsy)
• Neuropsychological testing: Neuropsychological testing may help to diagnose cognitive and memory complaints. They may also be used if attention deficit/hyperactivity disorder (ADHD) or certain other mental health conditions that have a neurological basis are believed to play a role in your symptoms.
• Sleep study: A polysomnography, or sleep study, may help uncover reasons for interrupted sleep or unusual daytime drowsiness, such as obstructive sleep apnea, restless leg syndrome, or narcolepsy. These conditions may contribute to or mimic the symptoms of depression.
• Medication or drug testing: Tests may be ordered to check blood levels of certain medications, or to look for drugs and other substances that may cause or worsen depression symptoms.
• Testing for infections: Your doctor may order tests to check for certain types of infections that can affect brain function and behavior, such as HIV, syphilis, or Lyme disease.

Once you’re diagnosed with depression, the next step is to get treatment. Even mild depression is unlikely to go away if you ignore it, and it may well worsen over time. Untreated depression often leads to greater problems in your life and can take a major toll on your physical health.