Number Needed to Diagnose, Predict, or Misdiagnose: Useful Metrics for Non-Canonical Signs of Cognitive Status?

Background/Aims: “Number needed to” metrics may hold more intuitive appeal for clinicians than standard diagnostic accuracy measures. The aim of this study was to calculate “number needed to diagnose” (NND), “number needed to predict” (NNP), and “number needed to misdiagnose” (NNM) for neurological signs of possible value in assessing cognitive status. Methods: Data sets from pragmatic diagnostic accuracy studies examining easily observed and dichotomised neurological signs (“attended alone” sign, “attended with” sign, head turning sign, applause sign, la maladie du petit papier) were analysed to calculate the NND, NNP, and NNM. Results: All measures of discrimination showed broad ranges. The range of NND and NNP suggested that these signs were, with a single exception, of value for correctly diagnosing or predicting cognitive status (presence or absence of cognitive impairment) when between 2 and 4 patients were examined. However, NNM showed similar values (range 1–5 patients) suggesting risk of misdiagnosis. Conclusion: NND, NNP, and NNM may be useful, intuitive, metrics in assessing the utility of diagnostic tests in day-to-day clinical practice. A ratio of NNM to either NND or NNP, termed the likelihood to diagnose or misdiagnose, may clarify the utility or inutility of diagnostic tests.

IS THE APPLAUSE SIGN USEFUL FOR DIAGNOSIS OF DEMENTIA AND MCI?

ObjectiveTo examine the diagnostic utility of the applause sign for dementia and MCI in consecutive referrals to cognitive disorders clinic.ResultsOver a 10–month period (January–November 2014) 243 consecutive new outpatients (F:M=124:119; age range 18–91 years, median 61) were administered the applause test (“clap 3 times”). Of the 240 with interpretable results, 46 were diagnosed with dementia (DSM–IV criteria), 66 with MCI (Petersen criteria), and 128 with no cognitive impairment. Using the categorization of applause sign from Luzzi et al. (2011), the percentage of patients with any cognitive impairment who clapped 3–10, and >10 times was 38.9, 60, 70.8, and 87.5 respectively. The null hypothesis that the proportion of patients with cognitive impairment did not differ significantly between groups was rejected (χ2=21.4, df=3, p<0.001). Abnormal scores (>3 claps) had sensitivities for dementia, MCI, and any cognitive impairment of 0.52, 0.24, and 0.36 respectively, and specificities of 0.84, 0.88, and 0.88.ConclusionsThis pragmatic study confirms and extends the results of a previous smaller study (n=100) of applause sign in the cognitive clinic, showing the applause sign to be a high specificity low sensitivity test for cognitive impairment.