When you're ill, your doctor may run a battery of tests to help figure out what's causing you to feel unwell. In the case of common infections, a simple blood test can make the diagnosis and guide your doctor in prescribing the right treatment.

When you have symptoms of mental illness, however, diagnosis and treatment can be a far more complex process. Promising research is being done, but we don't yet have a simple blood test that can diagnose depression.

Initial Steps

If you have symptoms of depression and go to your doctor, the process of getting the correct diagnosis and finding a treatment that works for you may take several steps and require the expertise of more than one type of healthcare provider.

To begin, your regular doctor will ask you about your symptoms, look for observable signs of your overall health, and refer to your past medical history. They will also ask about your family's health history, including mental health.

Your doctor can use blood tests to rule out medical conditions that may cause symptoms of depression or even be an underlying cause of the condition.

Depending on the results of the blood tests, the next step may be getting a referral to another type of doctor, such as a psychiatrist or another mental health provider.

Professionals trained in mental health can confirm a diagnosis of depression by differentiating it from other psychiatric conditions, such as bipolar disorder, anxiety disorders, and obsessive-compulsive disorder (OCD).

Once a diagnosis is made, your provider can use all of this information, as well as findings from your primary care doctor, to inform the creation of a treatment plan.

Possible Blood Tests

After your appointment with your primary care physician, they may order one or more blood tests that can be used to rule out other health conditions that can cause symptoms of depression such as fatigue, low mood, and weight changes.

Complete Blood Count (CBC)

A CBC looks at the various types of cells found in blood and counts how many there are. The test can be used to check for anemia or infection, both of which can lead to fatigue.﻿﻿

Thyroid Function Panel

Thyroid tests check the blood for levels of hormones produced by the thyroid gland. If the thyroid gland is under- or overactive, mood symptoms can result.﻿﻿

Creatinine and Blood Urea Nitrogen (BUN)

Creatinine and BUN levels reflect how well the kidneys are working. Not only can kidney disease lead to symptoms similar to depression, but it’s also important for doctors to know if kidney function is impaired before prescribing an antidepressant.﻿﻿ When the kidneys aren’t working well, they may not be able to metabolize certain medications properly.

Liver Function Panel

If the liver is inflamed or damaged, there are often changes that are detectable with a simple blood test. Liver disease can cause symptoms similar to depression, such as lethargy. If a person’s liver isn’t working well, it can also indicate alcohol misuse, which can co-occur with or cause depression.﻿﻿

Like with the kidneys, it’s also important to know how well the liver is working before prescribing medications, as the organ’s ability to metabolize drugs properly can influence how they work.

Fasting Blood Glucose

The amount of sugar in your blood after an overnight fast can be used to detect diabetes. While the exact link is unclear, depression and diabetes often occur together. Several studies have also indicated that people with diabetes have an increased risk of developing depression.﻿﻿ Your doctor may also check your blood glucose levels before prescribing you a medication to treat depression.

Cholesterol

Cholesterol is not known to be specifically linked with depression, but your levels do affect your overall health. Blood tests can detect if you have high cholesterol levels, which increases your risk for health problems associated with plaque buildup in your arteries. Your doctor might want to check your cholesterol before prescribing certain antidepressants, which may affect your levels.

Calcium and Magnesium Levels

While it’s rare, having calcium and/or magnesium levels that are too high or too low can cause psychiatric illness.﻿﻿

Folate and Vitamin B12 Levels

Low levels of folate (vitamin B9) or vitamin B12 can indicate a condition called pernicious anemia, which often causes symptoms of depression.﻿﻿ In some cases, fatigue and lethargy may set in even before tests show a deficiency.

What Tests Results Mean

When the results of your blood work come back, there may be a clear “next step” for you to take. For example, if you have low levels of vitamin B12, your doctor may recommend you start taking a vitamin supplement or receive injections.

If you are diagnosed with a medical condition such as hypothyroidism or diabetes, you may find your depression symptoms start to get better as soon as you begin treatment for the underlying condition. They may even resolve once the condition is managed.

When blood tests indicate you have high cholesterol, your doctor may recommend making some lifestyle changes. You may find that adjusting your diet and getting regular exercise help ease your depression symptoms, too.

The results of your blood tests may prompt your doctor to ask you about your alcohol and drug use as well. If you are using substances or dealing with addiction, it's crucial that you are honest with your doctor. Getting support and treatment for addiction is part of addressing depression.

For your safety, as well as the efficacy of treatment, your doctor needs to know if you are using drugs or alcohol, as it may influence the medications they prescribe.

When All the Tests Are Normal

If all the blood tests your doctor orders come back normal, they will likely feel much more confident that your depression is not being caused by an underlying medical condition. Once another cause has been ruled out, the focus can be turned toward finding the most effective treatment.

Your doctor may start by prescribing an antidepressant for you to try, referring you to a therapist within the healthcare system or community, or both. They may refer you to a psychiatrist for a more thorough assessment or to prescribe medication.

Treatment for depression can be as complex as the condition itself. What works well for one person may not work at all for someone else. You may also need to use more than one treatment, such as psychotherapy and antidepressants, at the same time.

You may need to modify your depression treatment or start something completely new. For example, as you get older, changes in your life and your body can influence how well certain treatments work for you.

Depression Tests of the Future

While they are not yet in common use by most health care providers, in the realm of psychological research there is much interest in the potential of biomarkers to help diagnose depression and other psychiatric conditions.﻿﻿

There are many factors that determine whether someone develops depression in their lifetime, including genetics and environment.

Research also continues to show how the brain and the body are inextricably connected; the health of one influences the health of the other.

Some studies have been looking for a potential connection between levels of inflammation in the body and depression.﻿﻿ Others are investigating how the gut microbiome might influence mental health.﻿﻿

A study published in 2013 explored the possibility of a connection between elevated cortisol levels in young adults (particularly males) and depression. In 2015, a study at UC San Diego proposed changes in a specific gene linked to the X chromosome may contribute to mental illness in women.﻿﻿

One day, we may be able to screen a person’s genetic information, measure the levels of inflammatory markers in their blood, and look for changes in the structure of their brain to determine their risk for depression, diagnose the condition, and find the most effective treatment—but we’re not there yet.﻿﻿

If you or a loved one are struggling with depression, contact the Substance Abuse and Mental Health Services Administration (SAMHSA) National Helpline at 1-800-662-4357 for information on support and treatment facilities in your area. For more mental health resources, see our National Helpline Database.

A Word From Verywell

For the time being, the most effective way to diagnose and treat depression is to be assessed by a doctor and mental health professional and establish an ongoing and supportive relationship them.