Phytophotodermatitis Workup

Updated: Nov 04, 2021
  • Author: William P Baugh, MD; Chief Editor: William D James, MD  more...
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Laboratory Studies

Phytophotodermatitis (PPD) is a clinical diagnosis. All laboratory data obtained are used to support the diagnosis and to exclude other diseases in the differential diagnosis.

Serum psoralen levels may be checked if unknown ingestion of psoralens is suspected.

Porphyrin levels may be obtained to rule out porphyria cutanea tarda.

If the clinical picture does not clearly distinguish between photoallergic and phototoxic dermatitis, performing a photopatch test may be prudent. The photopatch test should include testing for sunscreens because they may be a cause of allergic contact dermatitis associated with sun exposure.



A skin biopsy may be performed to determine the clinical diagnosis if it remains in question and to help distinguish between allergic contact, photoallergic, and photodrug reactions.


Histologic Findings

Epidermal hyperkeratosis, with or without parakeratosis, is observed. Scattered necrotic keratinocytes (apoptotic cells) are found in the epidermis. Sunburn cells (cells with pyknotic nuclei, increased volume, and pale staining cytoplasm) are also found in the epidermis. Slight spongiosis is observed in the epidermis. Minimal inflammatory cell infiltrate consisting of neutrophils (predominant cell type early on), lymphocytes (less common), and macrophages and melanophages (late) is present. Subepidermal blistering and extravasation of erythrocytes may or may not be present. Pigment incontinence with melanophages is observed in the papillary dermis. The dermis shows some edema and enlargement of vascular endothelial cells.

Both light microscopy and transmission electron microscopy in animal models show keratinocyte necrosis and vacuolization within 24 hours. Within 72 hours, intraepidermal and subepidermal blistering is visible. [30, 31]