Although depression is by no means a silent disease, it is seriously underdiagnosed. Experts estimate that only 34% of people with depression seek help, and only one-third of those who have major depression get the help they need.
 
When people do reach out for help, doctors typically diagnose depression by asking about feelings and experiences. They may also use screening tools and look for possible medical causes by performing a physical exam and sometimes ordering lab tests.
 
A physical exam and medical history may offer clues that point to depression caused by medication or an underlying illness. In these cases, blood tests or x-rays may confirm the problem. Often, when people are unable or unwilling to recognize their own depression, their initial complaints are medical. Headaches, stomach problems, sexual difficulties, and lack of energy are among the more common medical complaints.
 
If your symptoms suggest depression and medical causes seem unlikely, your doctor will be interested in hearing whether you’ve had any feelings of sadness or hopelessness and whether you’ve noticed any changes in your appetite, sex drive, or sleep patterns. He or she may also ask these questions:  

• Have you or anyone in your family ever suffered from depression or another mental disorder? If so, how was it treated?
• Do you get satisfaction and pleasure from your life?
• Do you ever have thoughts about suicide or have you attempted suicide?
• Do you drink alcohol? If so, how often and how much?
• Do you use any drugs such as marijuana, cocaine, crack, or heroin to get high or relax? If so, which drugs and how often?

Screening Tools

To help identify depression, doctors may use such screening tools as these:  

• Self-report scales, which present you with a checklist of symptoms to fill out. These scales may pick up some symptoms or subtle mood changes that otherwise might not be identified.
  
• Scales completed by a clinician, which are slightly better at detecting depression than self-reports.
  
• An interview by a doctor or therapist.

Because you may minimize symptoms or may not even be aware of them, your doctor or therapist may want to speak to someone close to you. Where a child or teen is concerned, the doctor may interview parents and, when possible, teachers or a guidance counselor.

When Other Tests May Be Useful

There is no lab test to determine whether you have a "chemical imbalance" in the brain. Doctors who want to determine whether someone is depressed generally order tests only when they note a potential health problem during a physical exam or medical history.
 
Most doctors do not embark on a battery of lab tests for many reasons. Tests can be costly and may not be covered by insurance. In some cases, results point the doctor in the wrong direction, leading to more tests and unnecessary anxiety. Usually, self-reports of symptoms combined with the clinical skills of a doctor or therapist are enough to begin treatment of depression or bipolar disorder.
 
Sometimes, however, you may need more tests to confirm a diagnosis, tease out information, or distinguish depression from other psychological or neurological problems. Your doctor may ask you to take any of the following:  

• Psychological tests, during which you answer questions, respond to pictures, or perform tasks like sorting cards or drawing pictures. These tests can give your doctor a better sense of your coping mechanisms, your temperament, or your ability to organize and plan.
  
• Tests that look at the brain, such as an EEG or MRI, which can help identify causes of dementia or some rare causes of depression. Both tests are painless. During an EEG, electrodes taped to your scalp pick up electrical signals. An MRI uses magnets, a radio wave transmitter, and a computer to pick up small changes in energy in hydrogen molecules in your brain and process the data to make a detailed scan of your brain.
  
• Tests for biological causes of depression, such as a blood test to check thyroid function.

Advances in Brain Imaging

Not all that long ago, the brain could be viewed only during an autopsy or neurosurgery. Fortunately, newer technology offers a variety of noninvasive ways to study the living, working brain.
 
Magnetic resonance imaging (MRI), for example, generates a colorful, three-dimensional computer image that accurately depicts structures in the brain. A variant of this brain scan, called functional MRI (fMRI), tracks swift, small metabolic changes that take place when a region of the brain responds during various tasks. For example, fMRI can show the expansion of blood vessels and changes in temperature that typically occur in the brain when a person exercises.
 
Other types of brain scans, such as positron emission tomography (PET) or single photon emission computed tomography (SPECT), also track brain activity. PET can zero in on the metabolism of blood sugar, which is an indicator of brain activity. PET and SPECT can map the brain in other ways as well �?for example, by measuring the distribution and density of neurotransmitter receptors in certain areas.
 
Another method, called quantitative electroencephalography, takes things a step further. As with a conventional electroencephalogram (EEG), electrodes placed on the scalp measure electrical activity. But during a quantitative EEG, researchers also calculate values that correlate with activity in specific regions of the brain, allowing scientists to map the brain’s structure and activity. For example, this technology can be used to see what parts of the brain respond when a patient takes a drug.
 
Brain imaging enables investigators to research:  

• which regions of the brain respond to various tasks or stimuli
• how activity in certain areas of the brain or the size of specific brain structures correlates with depression or other mental illnesses
• how various treatments affect neurotransmitter activity and distribution
• how treatments affect certain biological markers of depression.

One fMRI study published in the Journal of Neuroscience found that the hippocampi of 24 women who had a history of depression were 9%�?3% smaller than those of women who did not. The more bouts of depression a woman had, the smaller the hippocampus. Other brain-scan research has found that depressed people generally have less activity in the prefrontal cortex, a region of the brain vital to judgment and planning.
 
Ultimately, these sophisticated methods may reveal how nerve pathways work and interact, helping define the roles of specific neurotransmitters in mood disorders. By showing how the brain responds to medications and other forms of therapy, new imaging techniques might help improve treatment.