The PHQ-9 measures the frequency of nine core depressive symptoms over the previous two weeks: 1) Little interest or pleasure in doing things, 2) Feeling down, depressed, or hopeless, 3) Trouble falling/staying asleep or sleeping too much, 4) Feeling tired or having little energy, 5) Poor appetite or overeating, 6) Feeling bad about yourself or that you are a failure, 7) Trouble concentrating on things, 8) Moving/speaking slowly or being fidgety/restless, 9) Thoughts of self-harm. For each item, you rate frequency from 0 (not at all) to 3 (nearly every day). The total score ranges from 0-27, with higher scores indicating greater depression severity. The tool also includes a functional impairment question assessing how difficulties have impacted work, home, or relationships.

Extensive research validates the PHQ-9's accuracy. Studies show it identifies major depression with 88% sensitivity (correctly identifying those with depression) and 88% specificity (correctly identifying those without depression). A score of 10 or higher has 88% sensitivity and specificity for major depression. For moderate depression, sensitivity is 94% and specificity 80%. The PHQ-9's diagnostic accuracy equals or exceeds longer instruments and shows strong correlation with clinician assessments (r=0.84). Importantly, the PHQ-9 is a screening tool, not a diagnostic tool—it identifies likely depression but requires clinical evaluation for formal diagnosis. Its reliability across different administrations is excellent (test-retest reliability = 0.84).

PHQ-9 scores are interpreted as: 0-4 = No significant depression, 5-9 = Mild depression (monitor, may not require treatment), 10-14 = Moderate depression (consider treatment options), 15-19 = Moderately severe depression (treatment recommended), 20-27 = Severe depression (immediate treatment strongly recommended). However, interpretation is nuanced: A score of 10+ suggests probable major depression requiring clinical evaluation. Any positive response (score ≥1) to item 9 (suicidal thoughts) warrants immediate professional attention regardless of total score. Functional impairment (the additional question) also guides treatment decisions—even mild symptoms causing significant impairment may need intervention.

Yes, the PHQ-9 is an excellent monitoring tool. Research shows a 5-point score reduction indicates clinically meaningful improvement. Treatment response categories are: Remission = score <5, Response = ≥50% reduction from baseline, Partial Response = 25-49% reduction, No Response = <25% reduction. Tracking scores every 2-4 weeks provides objective evidence of treatment effectiveness. The PHQ-9 is sensitive to change, making it ideal for monitoring psychotherapy progress, medication efficacy, or lifestyle intervention effects. Many clinicians use it to adjust treatment plans—if scores don't decrease by 25-50% after 4-8 weeks of treatment, different approaches may be needed. Self-monitoring with PHQ-9 can empower individuals in their recovery journey.

While highly valuable, PHQ-9 self-screening has limitations: 1) Self-report bias (over- or under-reporting symptoms), 2) Not accounting for medical conditions that mimic depression symptoms, 3) Not assessing duration criteria (DSM-5 requires 2+ weeks of symptoms), 4) Missing contextual factors (recent loss, trauma, substance use), 5) Cultural variations in symptom expression, 6) Literacy or comprehension issues. The PHQ-9 doesn't diagnose other mental health conditions (anxiety, bipolar disorder) that may co-exist. It's most effective as part of comprehensive assessment including clinical interview. Online calculators should supplement, not replace, professional evaluation. Those with scores ≥10, suicidal thoughts, or significant impairment should seek clinical assessment.