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Why screening accuracy matters
Advanced practice nurses use screening tools every day, but a positive result doesn't always mean the patient has the condition. How likely a positive screen is to be correct depends on both the tool's accuracy and how common the condition is in the population being screened. This paper works through that relationship using the Patient Health Questionnaire-9 (PHQ-9) for depression.
The PHQ-9
The PHQ-9 scores each of the nine DSM-IV criteria for major depression from 0 (not at all) to 3 (nearly every day), for a total of 0 to 27. In its validation study, 6,000 patients in 8 primary care and 7 obstetrics–gynecology clinics completed it, and 580 were reinterviewed by a mental health professional. Scores of 5, 10, 15 and 20 represented mild, moderate, moderately severe and severe depression. A score of 10 or more had a sensitivity of 88% and a specificity of 88% for major depression (Kroenke et al., 2001).
Key measures
- Sensitivity: the proportion of people with the condition who test positive.
- Specificity: the proportion of people without the condition who test negative.
- Positive predictive value (PPV): the proportion of positive tests that are true positives.
- Negative predictive value (NPV): the proportion of negative tests that are true negatives.
Sensitivity and specificity are properties of the test. Predictive values change with prevalence (Celentano & Szklo, 2019).
Worked example 1: a general primary care clinic (prevalence 10%)
Of 1,000 patients screened, 100 have major depression.
| Depression | No depression | Total | |
|---|---|---|---|
| PHQ-9 ≥10 | 88 (true positives) | 108 (false positives) | 196 |
| PHQ-9 <10 | 12 (false negatives) | 792 (true negatives) | 804 |
| Total | 100 | 900 | 1,000 |
- PPV = 88 ÷ 196 = 44.9%
- NPV = 792 ÷ 804 = 98.5%
Worked example 2: a clinic for patients with chronic pain (prevalence 20%)
Of 1,000 patients, 200 have major depression.
| Depression | No depression | Total | |
|---|---|---|---|
| PHQ-9 ≥10 | 176 | 96 | 272 |
| PHQ-9 <10 | 24 | 704 | 728 |
| Total | 200 | 800 | 1,000 |
- PPV = 176 ÷ 272 = 64.7%
- NPV = 704 ÷ 728 = 96.7%
The prevalences are illustrative.
Interpreting the results
With the same test, fewer than half of positive screens in the general clinic are true cases, but nearly two-thirds are in the higher-prevalence clinic. In both settings, a negative screen is very reassuring. This is why a positive PHQ-9 is a reason for a diagnostic interview, not a diagnosis. It also explains why screening low-risk populations generates many false positives that need follow-up time and resources.
Implications for population health practice
- Always follow a positive screen with a clinical assessment before diagnosing or treating.
- Consider the population's prevalence when planning how many follow-up appointments a screening program will need.
- Use the PHQ-9's severity scores to monitor response to treatment over time, which is one of its strengths.
- Remember that screening is only worthwhile when there is a system to treat and follow up the people it identifies.
References
Celentano, D. D., & Szklo, M. (2019). Gordis epidemiology (6th ed.). Elsevier.
Kroenke, K., Spitzer, R. L., & Williams, J. B. W. (2001). The PHQ-9: Validity of a brief depression severity measure. Journal of General Internal Medicine, 16(9), 606–613. https://doi.org/10.1046/j.1525-1497.2001.016009606.x
