"Don't know" sign: description and evaluation of its diagnostic accuracy for cognitive impairment.

Patients in neurology clinics are sometimes not aware of the reason for the consultation, and we have called this circumstance the “Don’t know” sign (DKS). Our objective was to define this new sign and its modalities and to evaluate its prevalence and its diagnostic accuracy for cognitive impairment (CI) in comparison to other observation-based signs. A cross-sectional prospective study included all new outpatients evaluated by the authors at neurology consultation. We recorded observation-based signs. The Global Deterioration Scale (GDS) was used to assess the cognitive status of patients, based on clinical history, caregiver interview, and cognitive test results. We analyzed the prevalence and the diagnostic accuracy for CI of DKS, “head turning sign,” “attending with,” verbal repetition, and combinations, calculating sensitivity (Se), specificity (Sp), positive predictive value (PPV), and negative predictive value (NPV). We enrolled 673 consecutive patients (62% female) with a mean ± SD age of 59.3 ± 20.2 years. DKS was positive in 94 patients (14%) and was strongly associated with GDS score. DKS had a Se of 0.41, Sp of 0.98, PPV of 0.89, and NPV of 0.79 for CI diagnosis. The presence of at least two positive observation signs yielded a Se of 0.50, Sp of 0.97, PPV of 0.86, and NPV of 0.81. DKS is frequently observed in neurology outpatients. It has low sensitivity but high specificity and PPV for CI diagnosis. It does not require additional consultation time, and its use can be recommended in combination with other observation-based signs.