Phototesting: 4 Key Tests To Diagnose Photosensitivity

Phototesting includes a range of diagnostic investigations to assess whether a patient’s skin condition is triggered or worsened by ultraviolet (UV) radiation exposure. These tests help confirm photosensitivity, identify the specific wavelengths involved, and diagnose conditions like sunscreen allergies.

Introduction

Phototesting is crucial in photodermatology for patients presenting with rashes or eruptions that appear or intensify after sun exposure. It encompasses multiple procedures tailored to the clinical suspicion, aiding in distinguishing between phototoxic, photoallergic, and idiopathic photosensitive disorders. Full evaluation often combines patient history, examination, and targeted tests, leading to precise management strategies.

Common indications include polymorphic light eruption (PMLE), solar urticaria, chronic actinic dermatitis, drug-induced photosensitivity, and contact photoallergies from sunscreens or medications. By replicating UV exposure in controlled settings, clinicians can provoke characteristic reactions, confirm diagnoses, and recommend protective measures.

Photopatch tests

Photopatch testing specifically evaluates photocontact allergies, where allergens in topical products like sunscreens react with UVA light to cause dermatitis. Two sets of potential photoallergens are applied to the back: one set remains unirradiated (control for contact allergy), while the other receives UVA exposure (typically 5 J/cm²).

Common allergens tested include chemical UV filters (e.g., benzophenones, octocrylene), fragrances, and nonsteroidal anti-inflammatory drugs (NSAIDs) like ketoprofen, which are frequent culprits in photoallergic reactions. Readings are taken at day 2 and day 4 post-application. A positive photoallergic response shows reaction only on the irradiated side, while bilateral positives indicate pure contact allergy. Stronger reactions on the irradiated side suggest combined allergies.

If multiple positives occur, referral for comprehensive patch testing is recommended to uncover co-sensitizations. This test guides sunscreen selection, advising avoidance of implicated filters and favoring mineral-based options like titanium dioxide or zinc oxide.

Phototests

Phototests measure the skin’s minimal erythema dose (MED) across UV spectrum bands to define photosensitivity thresholds. Monochromator light testing uses a xenon arc lamp filtered to deliver precise wavelengths (UVB 290–320 nm, UVA 320–400 nm, visible light), mimicking natural sunlight spectrum.

Serial dilutions expose small back patches, with readings at 24 hours for erythema. Normal MED varies by skin type: fair skin ~20–40 mJ/cm² UVB, darker skin higher. Abnormal thresholds indicate broadband photosensitivity (e.g., solar urticaria) or action spectrum-specific issues (e.g., UVA in chronic actinic dermatitis).

These tests quantify sensitivity, essential for conditions like xeroderma pigmentosum or erythropoietic protoporphyria, where precise avoidance is critical. They also baseline phototherapy dosing.

Photoprovocation tests

Photoprovocation involves repeated sub-erythemal UV exposures over days to replicate natural eruptions in patients with normal MEDs. Typically, 5 cm² back patches receive broadband UVB/UVA daily for 2–5 days, provoking PMLE (papules, vesicles), actinic prurigo, or photoaggravated eczema.

Morphology guides diagnosis: PMLE shows delayed eczematous or papular rashes peaking 24–48 hours post-exposure. Biopsy with immunofluorescence confirms if needed. False negatives occur in 10–40% due to seasonal variation; late winter testing maximizes sensitivity. Positive tests validate photosensitivity and test treatments like antimalarials.

Laboratory investigations

Laboratory tests complement phototesting by identifying underlying causes:

• Photoporphyrin screen: Detects porphyrias via elevated protoporphyrin (e.g., protoporphyria).
• Antinuclear antibodies (ANA): Screens for lupus erythematosus photosensitivity.
• Extractable nuclear antigens: Ro/La antibodies in subacute cutaneous lupus.
• Full blood count, renal/liver function: Rules out systemic disease.
• Skin biopsy: Histology/immunofluorescence for PMLE (lymphocytic infiltrate) or solar urticaria.

These identify metabolic or autoimmune drivers, refining diagnosis.

What to expect during phototesting

Phototesting induces controlled reactions: redness, itch, or rash, resolving in 1–2 weeks. Patients avoid topicals/medications pre-test (steroids/antihistamines 48 hours prior). Tests last 1–2 days, with follow-up readings. Painless but may itch; emollients manage symptoms.

Diagnosis

Rarely diagnostic alone, phototesting correlates with history/exam. Classical solar urticaria (wheals in minutes) or chronic actinic dermatitis (low MEDs) confirm directly. Equivocal results need repeat testing or biopsy. Integrates with photopatch for allergies.

Patient counselling

Post-testing, counselling covers diagnosis, triggers,

photoprotection

(clothing, shade, broad-spectrum SPF50+), and management (topicals, hardenings, systemic agents). Tailored: simple for PMLE, intensive for actinic dermatitis. Multidisciplinary for pediatric/congenital cases like xeroderma pigmentosum.

Emphasize compliance; follow-up monitors progress.

Frequently Asked Questions (FAQs)

Is phototesting painful?

No, but it may cause temporary redness, itching, or rash lasting 1–2 weeks.

Who needs phototesting?

Patients with suspected UV-aggravated rashes: PMLE, solar urticaria, drug reactions, sunscreen allergies.

How accurate are these tests?

Highly specific for allergies/urticaria; photoprovocation false negatives 10–40% for PMLE.

What if tests are negative?

Does not exclude photosensitivity; consider history, repeat in season, or biopsy.

Can phototesting guide sunscreen choice?

Yes, photopatch identifies allergens to avoid.

Phototoxicity vs Photoallergy

Feature	Phototoxicity	Photoallergy
Onset	Hours (dose-dependent)	Days (sensitization required)
Mechanism	Toxic (non-immune)	Immune (T-cell mediated)
Spread	Exposed areas only	May spread beyond
Examples	NSAIDs, tetracyclines	Sunscreen filters, fragrances

Distinction aids management.

This comprehensive approach ensures thorough photodermatology evaluation, improving outcomes through targeted interventions.