In Vivo Phototesting

Categories: Medical & Pharmaceutical, Clinical, In Vivo Exposure


The term photosensitivity is used to describe the acute reaction of skin to UV radiation (UVR) and is considered abnormal if the reaction of skin to UVR is either quantitatively or qualitatively outwith that of the normal population.

Photosensitivity diseases are a heterogeneous group of conditions for which detailed clinical assessment is vital.

A further category of abnormal photosensitivity disease are seen in those induced by drugs and chemicals. These include antibiotics (e.g. fluoroquinolones and doxycycline), diuretics (e.g. thiazides) and non-steroidal anti-inflammatory drugs.

Photosensitivity diseases
Chemical composition Diagram

Phototesting Modalities

Example Photosensitive Patient

For routine screening, a xenon arc solar simulator can be used for MED testing. 

  • UVA/ UVB sources used to determine wavelength band giving rise to photosensitivity
  • For a full and precise investigation, the PhotoTest450 must be employed

Minimal erythema dose (MED) in Photosensitive skin

Polymorphic light eruption
NHS 2016, Polymorphic light eruption
  • The induced erythema is graded 22-26 hours following the dosage, based on an established 5-point scale
  • Polymorphic light eruption typically requires 2 hours of exposure to trigger a reaction, which may not develop for up to 24 hours and lasting up to 48 hours
0No difference from surrounding skin
(+)Just perceptible erythema (diffuse mild erythema without defined borders)
+Uniform erythema with sharply defined borders
++Bright red colour and slight induration (edema) on palpation
+++Bright red colour and pronounced induration (edema) raised above the surrounding skin
Urticaria (hives)
NHS 2016, Urticaria (hives)
  • In solar urticaria where immediate photosensitivity is the problem, the MUD (i.e. the dose of irradiation required to elicit just detectible urticaria) is used as the main outcome measure
  • 5—point scale used when testing patients suffering from idiopathic solar urticaria
  • Solar urticaria is usually triggered within just a few minutes of exposure, with lesions appearing within as little as an hour
0No reaction
(+)Just perceptible erythema (minimal urticaria dose; MUD)
+Erythema localised to irradiated area
++Erythema spreading beyond the irradiated area
+++Wheal in some parts of irradiated area