August 2020 More information regarding the accuracy of the scratch collapse test for carpal tunnel syndrome.
Areson D et al: Accuracy of the Scratch Collapse Test for Carpal Tunnel Syndrome in Comparison With Electrodiagnostic Studies. Hand (N Y) 2020 Jul 23;1558944719895786. doi: 10.1177/1558944719895786. Online ahead of print.
For the June Journal Article Review, I discussed an article by Montgomery et al describing the accuracy of the scratch collapse test (SCT).
Coincidentally, a similar article, by Areson et al, appeared this week in a different journal. The discussion here will briefly compare the two studies and will have more meaning if you (re)familiarize yourself with the Montgomery article and the performance of the scratch collapse test.
From patients who were attending a hospital electrodiagnostic clinic, Montgomery et al tested 92 subjects who had a mix of upper extremity nerve complaints. A physiatry resident and an electrodiagnostic technician independently performed the SCT on all patients for both carpal and cubital tunnel syndromes, and then one of two technicians then performed electrodiagnostic studies (EDS). When the results of the SCTs were compared against the EDS standards for CTS, the sensitivities (accurately identifying those who indeed had the condition determined by the EDS) the examiners ranged from 0 to 14%, and the specificities (accurately diagnosing those who did indeed not have the condition) ranged from 68 to100%.
In the just published study by Areson et al, one investigator, who was blinded, tested 40 patients for CTS using the SCT, and then the patients received an EDS from an investigator who did not know the results of the SCT. They found that the SCT had a sensitivity of .48 and a specificity of .59, neither of which was statistically correlated with the EDS results. Their findings were not dissimilar to the accuracy of previously reported results for other diagnostic tests such as Tinel, Phalen, and Durkan. The results of both the Montgomery and Areson studies, the first ones about the SCT to incorporate double blinding, are at odds with the original report of the SCT from 2008, which indicated a sensitivity of .64 and a specificity of .99. Areson et al also note that previous investigators repeated the SCT rather than performing the test just once, which could introduce observer bias. They also pointed out that it was entirely subjective of what constituted a proper collapse on testing.
Areson and colleagues, although concluding that the SCT used in isolation is not an accurate test for CTS, point out that their results cannot necessarily be generalized to other compression neuropathies. They also observed that all three of their patients who had EDS-confirmed cervical radiculitis and CTS had a positive SCT. So the final value of the SCT in accurately diagnosing compression neuropathies awaits further studies, including ones looking for double crush syndrome. In the meantime, my opinion voiced in the June Report on Hands is unchanged—I do not rely on the SCT.