To the Editor.
—To assess the specificity of the Hachinski upgoing thumb sign,1 I studied 12 normal subjects for the presence of the sign. The group consisted of six male and six female-subjects. All but one were right-handed, and all were aged between 7 and 71 years (mean age, 37.3 years). I used Hachinski's instructions and figure as the reference standard for a positive sign with an estimated angle between thumb and forefinger greater than 60°.In only two of 12 subjects was the upgoing thumb sign absent bilaterally. In one subject, there was an asymmetric upgoing thumb. The remaining nine subjects had bilateral upgoing thumb signs. Thus, the false-positive rate was 83%, and the specificity was 17%. Even with a sensitivity of 100% (no false-negative responses), the posterior probability of corticospinal tract damage after finding an upgoing thumb would be only 1.2 times the prior probability, the ratio
Mahler ME. Specificity of the Upgoing Thumb. Arch Neurol. 1993;50(3):239. doi:10.1001/archneur.1993.00540030007003
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