Population reference intervals for minimal erythemal doses in monochromator phototesting

Monochromator phototesting, to measure the minimal erythemal dose (MED), is useful in investigating patients with abnormal photosensitivity at different wavelengths. It relies on access to reliable, up-to-date data on the MED in normal individuals. The purpose of this study was to determine MED in normal subjects at different wavebands and compare these with historical controls.

The study group consisted of 415 normal individuals (349 males) of skin types I-III living in Scotland. Age range was 18-83 years (median 31 years). Phototesting was performed using a monochromator at prescribed wavelengths from 295 to 430 nm. All calibrations were traceable to the National Physical Laboratory. Quality systems were maintained to ISO 9001 and ISO 17025 international standards and ultraviolet (UV) measurements accredited by the United Kingdom Accreditation Service (UKAS).

The 95% reference interval (99% confidence interval for this) ranged from 6.8 to 27 mJ/cm(2) at 295 nm to >82,000 mJ/cm(2) at 430 nm.

Results of the current investigation are broadly in agreement with values published 25 years ago by this centre. This validates the phototesting process based on the use of monochromators with attention to careful control of conditions during UV exposure and MED reading, supported by dosimetric calibration.