Abnormal-response yield and waveband prioritisation in monochromator phototesting for suspected photodermatoses

Monochromator phototesting is a specialist investigation to assess abnormal skin response to defined ultraviolet and visible wavebands in patients with suspected photodermatoses. Monochromator phototesting spans ultraviolet B (UVB), ultraviolet A (UVA) and visible light (VL) and is used in specialist UK photodiagnostic centres. Abnormal-response yield varies between individuals and diagnostic groups. However, the procedure is time- and resource-intensive and limited to specialist centres.

To identify the wavebands and waveband combinations that captured the greatest proportion of abnormal monochromator responses, and to explore whether these findings could inform future evaluation of limited-waveband approaches within specialist phototesting pathways.

This retrospective single-centre analysis included 668 phototesting results from 552 individuals collected between 2020 and 2025 at the Scottish Photobiology Service, NHS Tayside, Dundee, UK. Records prior to 2020 were reviewed in some selected patients to distinguish persistently negative monochromator tests from previously documented abnormal responses that had resolved before the study period. Assessed wavebands were 305 ± 5 nm (UVB), 335 ± 27 nm (UVB + UVA), 365 ± 27 nm (UVA), 400 ± 27 nm (UVA + VL) and 430 ± 27 nm (VL).

Of 552 individuals, 353 received a final clinical diagnosis of photodermatosis and 199 did not. Across individuals with photodermatoses, 335 nm showed the highest single-wavelength abnormal-response yield (203/353, 57.5%), followed by 365 nm (194/353, 55.0%), 305 nm (169/353, 47.9%), 400 nm (108/353, 30.6%) and 430 nm (46/353, 13.0%). Selected two- and three-waveband combinations increased abnormal-response yield, with 305 + 365 nm showing the highest two-waveband yield (239/353, 67.7%) and 305 + 365+400 nm showing the highest selected three-waveband yield (259/353, 73.4%). In complete-case paired analysis, abnormal-response rates differed significantly across wavelengths (Cochran's Q = 200.4, df = 4, p < 0.001). Exploratory subgroup analyses showed distinct wavelength-response profiles across CAD, PLE and SU: CAD showed high yields at 305 and 335 nm, PLE showed lower monochromator abnormal-response yields across selected wavelengths and combinations, and SU showed greater longer-wavelength involvement.

In this exploratory single-centre study, around one quarter of individuals with a final photodermatosis diagnosis in our service did not demonstrate abnormal responses on narrow-waveband monochromator phototesting. Limited waveband monochromator combinations showed differing abnormal response yields across photodermatoses, capturing most abnormal responses in chronic actinic dermatitis and many in solar urticaria, but under-detecting polymorphic light eruption. These findings support waveband phototesting across the ultraviolet and visible spectrum in specialist phototesting services. Broadband and provocation-based photodiagnostic approaches remain important, particularly for conditions such as polymorphic light eruption.