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The article by Hachinski et al.1 ends with the conclusion, “An upgoing thumb sign appears to be a sensitive and reliable indicator of brain involvement. It is easy to evaluate and is compatible and confirmatory of the patient's symptoms and signs.” However, to what extent is this test reliable or confirmatory (i.e., can clinicians rely on it to meaningfully enhance their diagnostic certainty)? Using the data from table 1,1 the calculated positive likelihood ratio of the upgoing thumb test is 2.07, and the negative likelihood ratio is 0.21. These values are far from the cutoffs of 10 and 0.1, respectively, that normally serve to rule out or rule in a diagnosis.2 In fact, positive likelihood ratio values between 1 and 2 are unlikely to change the posttest probability, and values between 2 and 5 imply only a small change in the posttest probability.3 Indeed, employing the numbers reported in table 1,1 the pretest probability was 0.68, and in the presence of an upward going thumb, it changed to a posttest probability of 0.82, a modest increase in diagnostic certainty.
Footnotes
Author disclosures are available upon request (journal{at}neurology.org).
- © 2018 American Academy of Neurology
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