Depression is measured through a combination of standardized questionnaires, clinical interviews, and professional observation. There is no single blood test or brain scan that can diagnose it. Instead, measurement relies on tracking specific symptoms, their severity, and how long they’ve lasted. The tools range from simple nine-question surveys you can complete in a few minutes to structured clinical interviews that take half an hour with a trained professional.
The PHQ-9: Most Common Screening Tool
The Patient Health Questionnaire-9 (PHQ-9) is the most widely used depression screening tool in primary care. It asks how often over the past two weeks you’ve experienced nine specific problems: loss of interest or pleasure in activities, feeling down or hopeless, sleep difficulties, low energy, appetite changes, feelings of worthlessness or failure, trouble concentrating, noticeably slowed or restless movement, and thoughts of self-harm or death.
Each item is scored from 0 (not at all) to 3 (nearly every day), producing a total between 0 and 27. The score ranges break down like this:
- 1 to 4: Minimal symptoms
- 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 the threshold that typically prompts further evaluation. At that cutoff, the PHQ-9 correctly identifies about 88% of people who do have major depression and correctly rules it out in about 86% of people who don’t. It’s a screening tool, not a diagnosis on its own, but it’s remarkably useful for a questionnaire that takes under five minutes to complete. Many clinicians also use it to track changes over time, repeating it at follow-up visits to see whether a score is trending up or down.
What a Clinical Diagnosis Requires
A formal diagnosis of major depressive disorder follows criteria from the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). The core requirement: at least five out of nine specific symptoms must be present most of the day, nearly every day, for at least two consecutive weeks. At least one of those five symptoms must be either depressed mood or loss of interest in activities.
The nine symptoms overlap closely with the PHQ-9 questions, because the PHQ-9 was designed to mirror them. They include depressed mood, loss of interest or pleasure, significant weight or appetite changes, sleep problems, observable changes in physical movement (either slowing down or becoming agitated), fatigue, feelings of worthlessness or excessive guilt, difficulty thinking or concentrating, and recurrent thoughts of death or suicide. The symptoms also need to represent a clear change from how you functioned before, and they can’t be better explained by another medical condition or substance use.
This is why screening questionnaires alone don’t equal a diagnosis. A clinician applies these criteria through a conversation that explores the timeline, severity, and context of your symptoms.
Telling Depression Apart From Grief
One of the trickier measurement challenges is distinguishing clinical depression from normal grief, since they share symptoms like sadness, sleep disruption, and appetite loss. The DSM-5 highlights several practical differences. In grief, self-esteem typically stays intact; in depression, feelings of worthlessness and self-loathing are common. Grief usually comes in waves mixed with positive memories of the person who died, while depression tends to produce a pervasive, unrelenting unhappiness. Perhaps the most telling distinction: a grieving person can usually be comforted by friends, family, or meaningful connection. A person with major depression typically cannot.
Other Questionnaires and Scales
The PHQ-9 isn’t the only option. The Beck Depression Inventory (BDI-II) is a 21-item self-report questionnaire with scores ranging from 0 to 63. Its severity categories are 0 to 13 (no depression), 14 to 19 (mild), 20 to 28 (moderate), and 29 to 63 (severe). It covers a broader range of symptoms than the PHQ-9 and is commonly used in therapy settings and research studies.
The Hamilton Rating Scale for Depression (HAM-D) works differently from both. It’s not a self-report tool. Instead, a trained clinician conducts a 20- to 30-minute interview and rates your symptoms based on what they observe and what you describe. This makes it more thorough but also more resource-intensive and dependent on the interviewer’s skill. It has been the standard in clinical research for decades, particularly in studies testing whether treatments work. A shorter six-item version focuses on core symptoms like depressed mood, guilt, loss of interest in work and activities, slowed movement, anxiety, and general physical symptoms, and has actually shown greater sensitivity to changes over time than the full 17-item version.
For children and adolescents, the Children’s Depression Inventory (CDI-2) is designed for ages 7 to 17. It uses age-appropriate language and accounts for the fact that depression in young people often looks like irritability rather than sadness.
Why There’s No Blood Test for Depression
Researchers have identified dozens of biological markers associated with depression, including elevated stress hormones, higher levels of inflammatory proteins in the blood, reduced volume in brain regions involved in memory and emotional regulation, disrupted sleep-wake cycles, and lower levels of key chemical messengers like serotonin and dopamine. Stress hormones can now be measured from hair or fingernails to estimate chronic levels, not just a single-moment snapshot.
The problem is that none of these markers are specific or consistent enough to work as a standalone diagnostic test. Someone with depression might show elevated stress hormones while another person with equally severe depression does not. These markers are more useful in research for understanding the biology of depression and predicting treatment response than for diagnosing any individual person. For now, the most reliable measurement still depends on carefully assessing what you’re experiencing and for how long.
Digital Tracking and Passive Measurement
A newer approach uses smartphone and wearable data to passively monitor patterns linked to depression. GPS data can track how much you move around during the day and how often you leave home. Sleep sensors on wearables can measure sleep duration and disrupted wake-sleep rhythms. Researchers have found that changes in mobility, location patterns, and sleep quality can flag potential depressive episodes, sometimes before the person recognizes the shift themselves.
These tools aren’t replacements for clinical assessment, but they offer something questionnaires can’t: continuous, objective data collected without requiring you to remember how you felt over the past two weeks. They’re particularly promising for tracking people already diagnosed with depression, where catching a relapse early matters most.
How to Use These Tools Yourself
If you’re trying to measure where you stand right now, the PHQ-9 is freely available online and takes about three minutes. Score yourself honestly based on the past two weeks, not your worst day or your best. A score under 5 suggests minimal symptoms. A score of 10 or above is a meaningful signal worth bringing to a healthcare provider.
If you’re tracking depression over time, whether on your own or alongside treatment, retaking the same questionnaire at regular intervals (every two to four weeks) gives you a concrete way to see whether things are improving, stable, or getting worse. A change of 5 or more points on the PHQ-9 generally reflects a clinically meaningful shift. Writing down your scores creates a record that’s far more reliable than trying to recall how you felt weeks or months ago, and it gives your provider useful information if you do seek care.