There is no single blood test or brain scan that diagnoses depression. Instead, testing for depression relies on standardized questionnaires, a clinical interview, and sometimes lab work to rule out other conditions. The process typically starts with a short screening tool your doctor can give you in minutes, and a formal diagnosis requires at least five specific symptoms lasting two weeks or longer.
What Screening Questionnaires Look Like
The most widely used screening tool is the PHQ-9, a nine-question survey you can complete in under five minutes. Each question asks how often you’ve been bothered by a specific problem over the past two weeks, and you rate each one from 0 (not at all) to 3 (nearly every day). Your total score falls into one of five ranges:
- 0 to 4: No depression
- 5 to 9: Mild depression
- 10 to 14: Moderate depression
- 15 to 19: Moderately severe depression
- 20 to 27: Severe depression
A score of 10 or higher is generally the threshold that prompts your provider to dig deeper. The PHQ-9 is a screening tool, not a diagnosis by itself. Think of it like a thermometer: it tells you something is off, but not exactly what’s causing it. Many primary care offices now hand you this questionnaire on a clipboard during routine visits, which is exactly what national guidelines recommend. The U.S. Preventive Services Task Force advises screening all adults 19 and older for depression, including pregnant and postpartum individuals and older adults, even when they aren’t showing obvious symptoms.
Screening Tools for Specific Age Groups
Standard adult questionnaires don’t always capture how depression shows up in teenagers or older adults, so modified versions exist for both groups.
For adolescents, the PHQ-A adapts the standard PHQ-9 with language that fits a younger person’s experience. It adds “irritable” alongside “depressed” and “hopeless” as a mood descriptor, since irritability is a more common sign of depression in teens than persistent sadness. It also reframes concentration problems around schoolwork, reading, or watching TV, and includes a question about whether the teen has felt depressed or sad most days over the past year.
For older adults, the Geriatric Depression Scale (GDS) uses a simple yes-or-no format instead of scaled ratings, which works better for people who fatigue easily or have mild cognitive difficulties. The short form has 15 questions about the past week. Scores of 0 to 4 are considered normal, 5 to 8 suggest mild depression, 9 to 11 moderate, and 12 to 15 severe. Any score above 5 warrants a more thorough evaluation, and a score of 10 or higher almost always indicates depression.
The Nine Symptoms Doctors Look For
A formal diagnosis of major depressive disorder requires five or more of nine specific symptoms to be present during the same two-week period. At least one of the five must be either a persistently depressed mood or a loss of interest or pleasure in activities you used to enjoy. Those two are considered the core symptoms.
The remaining seven symptoms are:
- Appetite or weight changes: significant increase or decrease without intentional dieting
- Sleep problems: insomnia or sleeping far more than usual
- Physical restlessness or slowness: feeling agitated or moving noticeably slower than normal
- Fatigue or loss of energy: feeling drained even after rest
- Difficulty thinking or concentrating: trouble making decisions or staying focused
- Feelings of worthlessness or excessive guilt: self-blame that feels disproportionate to the situation
- Thoughts of death or suicide: recurring thoughts about dying, or specific plans or attempts
These symptoms also need to cause real impairment in your daily life, whether that means struggling at work, withdrawing from relationships, or being unable to handle basic tasks you normally manage fine.
What Happens During a Clinical Interview
If a screening tool flags potential depression, your provider will follow up with a more detailed conversation. This clinical interview goes well beyond the questionnaire. Expect questions about when your symptoms started, whether anything triggered them, how they’ve changed over time, and how much they’re interfering with your daily routine.
You’ll likely be asked about your family history of depression or other mental health conditions, since genetics plays a significant role in vulnerability. Substance use comes up too, because alcohol and certain drugs can both cause and mask depressive symptoms. Your provider will also assess whether you’ve had thoughts of harming yourself, which can feel uncomfortable but is a routine and important part of the evaluation.
In clinical settings, providers sometimes use a structured rating scale called the Hamilton Depression Rating Scale during this interview. It covers 17 areas, including mood, guilt, different types of sleep disruption, anxiety, physical symptoms, and changes in weight or sexual function. Unlike the PHQ-9, which you fill out yourself, this scale is scored by the clinician based on your answers and their observations. It helps measure severity more precisely and track whether treatment is working over time.
Blood Tests That Rule Out Other Causes
Several medical conditions produce symptoms that look almost identical to depression. Feeling exhausted, foggy, unmotivated, and gaining weight could point to an underactive thyroid rather than a mood disorder. Before settling on a depression diagnosis, your provider may order blood work to check for these physical causes.
Common tests include thyroid hormone and TSH levels (to check for hypothyroidism or hyperthyroidism), vitamin B12, folic acid, iron, and vitamin D levels (to detect nutritional deficiencies that affect mood and energy), and blood sugar levels (to rule out diabetes or low blood sugar). These conditions are treatable on their own, and if one of them is driving your symptoms, addressing it directly can resolve what feels like depression without ever needing antidepressants or therapy.
Not everyone who screens positive for depression will need blood work. Your provider makes that call based on your symptoms, medical history, and whether anything in your presentation suggests a physical cause.
What Happens After a Positive Screen
A positive depression screening sets a specific sequence in motion. Federal quality standards require that a follow-up plan be documented within two calendar days of the visit where you screened positive. That follow-up takes different forms depending on how severe your symptoms are and what resources are available.
Your provider may refer you to a psychiatrist, psychologist, clinical social worker, or mental health counselor for a more thorough evaluation. In some cases, they’ll start treatment directly, whether that means beginning psychotherapy, recommending a behavioral health program, or discussing medication if the situation warrants it. Group therapy, support groups, and depression management programs also count as valid follow-up pathways.
One important distinction: the initial screening itself isn’t the same as a full diagnostic evaluation. If you score high on the PHQ-9 at your annual physical, that result alone won’t lead to a prescription. Your provider needs a complete picture before recommending any specific treatment. The screening is the starting point, not the finish line.
How to Get Tested
The simplest path is to ask your primary care doctor for a depression screening at your next visit. You don’t need a referral to a specialist just to get screened. Many clinics now include it as part of standard intake paperwork, so you may have already been screened without realizing it.
If you want a sense of where you stand before that appointment, the PHQ-9 is freely available online and takes about three minutes. It won’t give you a diagnosis, but it gives you a concrete score to bring to your doctor, which can make starting that conversation easier. A score of 10 or above is a strong reason to follow up with a provider who can do a full evaluation, order any necessary blood work, and help you figure out the right next step.