Drug-Induced Photosensitivity Workup

Updated: Sep 18, 2020
  • Author: Alexandra Y Zhang, MD; Chief Editor: Dirk M Elston, MD  more...
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Laboratory Studies

To exclude porphyria cutanea tarda, assess urine porphyrin levels, which are elevated in porphyria cutanea tarda and within the normal range in pseudoporphyria and drug-induced photosensitivity. Determine antinuclear antibody (ANA) and anti-Ro (SS-A) antibody levels.


Other Tests

Photopatch testing is an important tool in the diagnosis of photoallergic contact dermatitis. Suspected photoallergens are applied to the back in 2 sets. One set is removed after 24 hours and irradiated with 5-10 J/cm2 UV-A. Both sets of patch tests are evaluated for a positive reaction after 48 hours. Erythema, edema, and/or vesiculation at an irradiated site indicate a positive reaction. A positive reaction at both sites is interpreted as an allergic contact dermatitis. A positive reaction at the unirradiated site with a stronger one at the irradiated site should be interpreted as both allergic dermatitis and photoallergic contact dermatitis reaction to the same compound.

Phototesting with UV-A; UV-B; and, sometimes, visible light is helpful in diagnosing photosensitivity disorders. This test is performed by treating small areas of skin on the back or inner aspect of the forearms with gradually increasing doses of light. The minimum dose of light required to produce uniform erythema over the entire irradiated site after 24 hours is called the minimum erythema dose (MED). Patients with phototoxic reactions have a reduced MED to UV-A or, in some instances UV-B.


Histologic Findings

In acute phototoxic reactions, necrotic keratinocytes are observed. If the reaction is severe, the necrosis is panepidermal. In addition, epidermal spongiosis with dermal edema and a mixed infiltrate consisting of lymphocytes, macrophages, and neutrophils may be present. Blue-gray pigmentation associated with phototoxic reactions results from increased melanin in the dermis or deposition of the drug or its metabolites within the skin.

The histologic features of a lichen planus–like phototoxic reaction are essentially indistinguishable from idiopathic lichen planus. However, increased amounts of spongiosis and necrotic keratinocytes may be present.

The histologic features of a subacute cutaneous lupus erythematosus (SCLE) – like reaction reveal an interface dermatitis that is indistinguishable from non–drug-induced SCLE. Like porphyria cutanea tarda, pseudoporphyria causes a subepidermal blister at the level of the lamina lucida. A characteristic feature of both pseudoporphyria and porphyria cutanea tarda is festooning, which refers to the irregular configuration of the dermal papillae in the floor of the bulla.

Photoallergic reactions are histologically similar to contact dermatitis. Epidermal spongiosis with a dermal lymphocytic infiltrate is a prominent feature. However, the presence of necrotic keratinocytes is suggestive of photoallergy rather than allergic contact dermatitis.