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

Area of Science:

• Dermatology
• Photobiology
• Medical Diagnostics

Background:

• Monochromator phototesting assesses abnormal skin reactions to specific ultraviolet (UV) and visible light (VL) wavebands.
• This specialist investigation is crucial for diagnosing photodermatoses but is resource-intensive and limited to specialized centers.
• Abnormal responses vary significantly among individuals and diagnostic groups.

Purpose of the Study:

• To determine which specific UV and VL wavebands and their combinations yield the highest proportion of abnormal monochromator phototesting responses.
• To explore if these findings can guide the development of more efficient, limited-waveband phototesting strategies within specialist care pathways.

Main Methods:

• A retrospective analysis of 668 phototesting results from 552 individuals diagnosed with or suspected of having photodermatoses between 2020 and 2025.
• Investigated specific wavebands: 305 nm (UVB), 335 nm (UVB+UVA), 365 nm (UVA), 400 nm (UVA+VL), and 430 nm (VL).
• Included review of prior records to differentiate persistent negative tests from resolved abnormal responses.

Main Results:

• Among 353 individuals with photodermatoses, 335 nm showed the highest abnormal response yield (57.5%), followed by 365 nm (55.0%) and 305 nm (47.9%).
• Combinations like 305 nm + 365 nm (67.7%) and 305 nm + 365 nm + 400 nm (73.4%) increased diagnostic yield.
• Distinct wavelength-response profiles were observed: Chronic actinic dermatitis (CAD) showed high yields at 305/335 nm, solar urticaria (SU) at longer wavelengths, and polymorphic light eruption (PLE) had lower yields.

Conclusions:

• Approximately 25% of photodermatosis patients in this study did not show abnormal responses with narrow-waveband testing.
• Limited waveband combinations effectively capture most abnormal responses in CAD and SU but under-detect PLE.
• Findings support comprehensive waveband phototesting across the UV and visible spectrum, while acknowledging the continued importance of broadband and provocation tests for conditions like PLE.