Does the presence of anxiety affect the validity of a screening test for depression in the elderly?

Introduction: Depression in the elderly is frequently detected by screening instruments and often accompanied by anxiety. We set out to study if anxiety will affect the ability to detect depression by a screening instrument. Objective: To validate the short Zung depression rating scale in Israeli elderly and to study the affect of anxiety on its validity. Design: The short Zung was validated against a psychiatric evaluation, in a geriatric inpatient and outpatient service. The overall validity was determined, as well as for subgroups of sufferers and non-sufferers of anxiety. Setting: An urban geriatric service in Israel. Patients: 150 medical inpatients and outpatients, aged 70 years and older. Measures: Psychiatric evaluation of modified Anxiety Disorders Interview Schedule for DSM-IV as criterion standard for anxiety and depression and short Zung instrument for depression. Results: By criterion validity, 60% suffered from depression. The overall validity of the short Zung was high (sensitivity 71.1%, specificity 88.3%, PPV 90.1%, NPV 67.1%). The validity for those not suffering from anxiety was good (sensitivity 71.1%, specificity 90.2%, PPV 84.4%, NPV 80.7%). In those with anxiety, sensitivity, specificity and PPV were high (71.2%, 77.8%, 94.9% respectively), although the specificity was less than in non-suffers. However major difference was in the NPV rate being much lower (31.8%). Conclusion: The short Zung, an easily administered instrument for detecting depression, is also valid in the Israeli elderly. However, anxiety limits the usefulness of this instrument in correctly ruling out depression. The clinician must be aware, therefore, that those suffering from anxiety may score negatively for depression on a screening instrument, such as the short Zung.