Chronic actinic dermatitis (CAD) is an uncommon, eczematous photosensitive 
eruption affecting predominantly elderly men and to which drug-induced 
photosensitivity may sometimes appear clinically identical. This retrospective 
study compares the monochromatic irradiation results in 11 patients with CAD and 
14 patients with drug-induced photosensitivity, to assess whether such testing 
is useful in the differentiation of these two conditions. Thus, the action 
spectra of the drug photosensitivity patients were plotted and compared with 
those of 12 nonphotosensitive control patients: 10 patients were found to be 
photosensitive in the UVA range; the implicated drugs included quinine, 
sparfloxacin, amiodarone, doxycycline, mefenamic acid, nalidixic acid, 
fenbrufen, diclofenac, enalapril, diltiazem and prochlorperazine maleate. One 
patient on doxycycline was photosensitive in both the UVA and UVB ranges. The 
remaining three patients were not tested until after discontinuation of their 
drug and their light tests were then normal. In the CAD group, five patients 
were photosensitive in the UVA, UVB and visible light ranges and six were 
photosensitive in the UVA and UVB ranges. Comparison of the mean minimal 
erythema dose responses then demonstrated dissociation of the drug-induced from 
the CAD group in the UVB region; the result was statistically significant. This 
suggests that UVA-sensitivity dissociated from UVB-sensitivity is a relative 
indicator of drug-induced photosensitivity and monochromatic irradiation testing 
may therefore be helpful in the differentiation of these two disorders.