The traditional method of assessing minimal phototoxic dose (MPD) prior to photochemotherapy with psoralen–ultraviolet A (PUVA) is inconvenient and cannot directly determine PUVA start doses. A handheld minimal erythema dose UVB tester can be modified by fitting a TL-10 UVA compact fluorescence lamp (CFL).
To determine whether MPD testing is possible with a CFL and to calculate a fixed factor to convert observed MPD to PUVA-equivalent MPD.
Patients had two sets of MPD tests performed on symmetrical, contralateral sites on the lower back. MPD test results from a panel of PUVA lamps were compared with MPD from the modified handheld tester. Additionally, a questionnaire survey was completed by 43 U.K. phototherapy units to assess routine practice concerning MPD testing prior to PUVA therapy.
Thirty-seven patients with psoriasis were recruited. Boston phototypes in the 31 with conclusive MPD reactions were: I, four; II, 11; III, 12; and IV, four. The handheld MPD results were linearly related to the PUVA panel MPD results as follows: PUVA MPD = 0·48 × handheld MPD + 0·17 J cm−2. The measured PUVA MPD was 0·48 of the handheld MPD, not 0·15 as predicted by the published PUVA action spectrum.
The modified MPD tester is a convenient and safe method for PUVA MPD testing, overcoming many problems of the ‘traditional method’. The difference between the PUVA and TL-10 lamps was lower than predicted from published studies. This suggests that formal re-evaluation of the erythema action spectrum for PUVA is now needed.