Editorial |

Testing a Test:  Critical Appraisal of Tests for Diagnosing Scabies

Joerg Albrecht, MD; Michael Bigby, MD
Arch Dermatol. 2011;147(4):494-497. doi:10.1001/archdermatol.2011.59.
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Anyone who has ever tried to scrape the foot of a crying and kicking infant will welcome the possibility of using simple packing tape instead of a scalpel to find mites. Given the problem of trying to find mites, eggs, or scybala, especially in children, many practitioners diagnose scabies clinically1,2 and treat even if neither mite nor products can be found.

The article by Walter et al3 in this issue of the Archives has the potential to substantially change the way we approach the diagnosis of scabies. It singles out the packing tape test as the most useful test for the diagnosis of scabies. It should be stressed that the evaluated test used packing tape and not office tape, which is not as strong. Packing tape was also used by Katsumata and Katsumata,4 who first described the test. In addition, the tape remains on the skin for a surprising 30 seconds, which may be time-consuming if many lesions are assessed. For the practitioner, the issue is not the strength of the tape but the strength of the evidence presented and the implications of this study for clinical practice. In clinical appraisal terms, are the results valid and important, and can they be applied to your setting?

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Figure 1.

Pretest and posttest probabilities of scabies illustrated with a Fagan nomogram.The pretest probability for both examples was 0.36. Example 1 (bold line): The positive likelihood ratio for dermoscopy was 1.52. If a patient's dermoscopy result were positive, then his or her posttest probability of scabies would be 0.46. Example 2 (gray line): The negative likelihood ratio for dermoscopy was 0.37. If a patient's dermoscopy results were negative, then his or her posttest probability of scabies would be 0.17.

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