Defining ambiguity in molecular differentiation of psoriasis and eczema

We read with great interest the study by Schmitt et al.1 comparing dermatopathology with a NOS2/CCL27-based PCR classifier for differentiating psoriasis from eczema. The work is timely and clinically relevant. However, the manuscript's central claim that molecular testing ‘clearly’ outperforms dermatopathology in diagnostically ambiguous cases would be more convincing if two methodological issues were clarified. First, the operational definition of the ‘ambiguous’ subgroup appears internally inconsistent. In the Methods, ambiguous cases are defined by low inter-rater consensus among dermatopathologists. In the Results, however, the same subgroup is described as cases ‘defined by low rater concordance with PsorX’.1 These are not interchangeable constructs. If ambiguity was determined solely by low dermatopathologist agreement, then the wording in the Results should be corrected because concordance with the index test reflects a different analytic criterion. If, however, concordance with PsorX contributed to subgroup construction, then the comparison in that subgroup becomes at least partially circular: the index test would help define the very subset in which it is subsequently shown to perform best. In diagnostic accuracy studies, this kind of coupling can inflate apparent superiority and should be explicitly avoided or transparently reported.2, 3 Second, the handling of non-binary dermatopathology outputs remains insufficiently specified. Dermatopathologists were allowed six categories, including psoriasiform eczema, eczematized psoriasis, undetermined and diagnosis other than psoriasis or eczema, yet diagnostic performance was ultimately summarized by sensitivity, specificity and accuracy using a fourfold contingency framework.1 The manuscript does not clearly state how ‘other’ or undetermined calls were incorporated into that framework. This omission is not trivial. In figures 1 and 3, non-binary outputs were not rare, particularly in the subgroup presented as diagnostically difficult. Depending on whether such results were excluded, collapsed post hoc or counted as errors, the apparent performance of dermatopathology could change materially. Diagnostic accuracy methodology has long emphasized that intermediate, indeterminate or otherwise non-classifiable test results should be reported and analytically handled in an explicit and reproducible manner rather than silently absorbed into a binary endpoint.2, 4 The clinical implications of this reporting gap are illustrated by the third case in Figure 4. The most informative discordant example may not be the two cases in which molecular testing aligned with subsequent psoriasis-directed treatment response, but rather the patient later found to have mycosis fungoides despite predominantly psoriasis-favouring dermatopathology.1 That case suggests that a low psoriasis-probability molecular result in a psoriasiform biopsy should not be read simply as support for ‘eczema’. Instead, its greatest clinical value may be to interrupt forced binary classification and prompt renewed diagnostic scrutiny, including repeat biopsy or broader work-up, particularly when morphology, therapeutic response and disease course diverge. This distinction matters because mycosis fungoides is a recognized clinical and histologic mimicker of both psoriasis and eczema.5 Accordingly, the manuscript would be strengthened by clarifying which definition of ‘ambiguous’ was actually used, explicitly reporting how all non-binary pathology outputs entered the performance analysis and reframing discordant molecular results as triggers for diagnostic reassessment rather than automatic support for the eczema side of a binary classifier. These clarifications would not weaken the study's message; rather, they would make its conclusions methodologically tighter and clinically more actionable. The author has nothing to report. The author declares no conflicts of interest. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.