What Is Minimal Erythema Dose (Med) Testing? How Laser Systems Differ From Phototherapy

Minimal Erythema Dose (MED) testing is a controlled way to measure how much optical energy a patient’s skin can tolerate before developing barely perceptible redness. Clinicians expose small test areas to incremental doses and assess the skin approximately 12 to 18 hours later, when the faint erythema defining the MED is typically visible. Broad-area phototherapy generally begins at 50% to 100% of the MED, whereas targeted medical lasers may begin at 100% to 400% of the MED because they concentrate energy on diseased tissue while sparing healthy skin.

MED testing establishes an individualized erythema threshold, but the starting dose depends on the treatment geometry. Whole-body phototherapy must protect large areas of uninvolved skin, while targeted lasers can deliver supra-erythemogenic doses to selected lesions.

What MED Testing Measures

The Minimal Erythema Threshold

The MED is the lowest dose of light energy that produces barely perceptible erythema. The response is usually assessed after a delayed interval, commonly around 12 to 18 hours, although some device protocols evaluate test sites within 24 hours.

The measurement is expressed as an energy dose, such as a UVB fluence or laser fluence, and varies substantially between patients.

Why Individual Testing Matters

Skin sensitivity to ultraviolet light and laser energy is not uniform. Patient factors, treatment history, anatomical location, and the condition of the skin can all affect the response.

Starting treatment without an appropriate threshold assessment increases the risk of excessive erythema, phototoxicity, burns, and disease exacerbation.

How Clinicians Establish MED

Clinicians typically perform incremental test exposures on a relatively non-sun-exposed area, such as the lower back or buttocks. Each test spot receives a different dose, allowing the practitioner to identify the lowest exposure that produces the defined erythematous response.

The result should be interpreted using the device’s validated protocol rather than assumed from the patient’s visible skin phototype alone.

Why Initial Dosing Differs by Treatment Type

Broad-Area Phototherapy

Broad-area modalities such as narrow-band UVB or PUVA expose both affected and unaffected skin. The maximum practical dose is therefore constrained by the tolerance of the uninvolved skin, not only by the tolerance of the lesions being treated.

Initial dosing commonly begins at 50% to 100% of the MED. More conservative protocols may begin around 50% to 70%, particularly when patient sensitivity is uncertain or the treatment area is extensive.

Targeted Medical Laser Systems

Targeted systems, including dermatological excimer laser platforms, direct concentrated light onto selected lesions. Surrounding healthy skin can be shielded or excluded from exposure.

Because the energy is confined to the treatment target, clinicians may initiate therapy at 100% to 400% of the MED, depending on factors such as plaque thickness, induration, lesion response, and anatomical site.

Thicker areas such as the knees and elbows may require higher starting doses than thinner or more sensitive skin.

The Meaning of Supra-Erythemogenic Dosing

A targeted laser dose above the patient’s measured MED is described as supra-erythemogenic. This does not mean that testing is unnecessary; it means that the therapeutic dose is deliberately higher than the threshold established on a test site because the treatment is confined to the lesion.

The approach relies on careful lesion selection, controlled fluence, shielding, and monitoring for the expected versus excessive erythematous response.

Why Lesion Characteristics Affect the Laser Dose

Plaque Thickness and Induration

Thicker, more indurated plaques may require greater energy delivery to achieve a therapeutic effect through the lesion. The starting dose can therefore be adjusted upward within the system’s clinical protocol.

This adjustment should be based on the lesion and its response, not applied uniformly to every patient or body site.

Anatomical Location

Skin thickness and local sensitivity differ across the body. Extensor surfaces such as the knees and elbows often tolerate or require different dosing from thinner or more sensitive areas.

Anatomical location is therefore an important modifier alongside MED, lesion thickness, and prior treatment response.

Depigmented Lesions Require Additional Judgment

For excimer treatment of hypopigmented conditions such as vitiligo, the initial fluence may need to reflect the degree and color of depigmentation in the lesion, rather than the patient’s surrounding skin phototype.

Completely depigmented patches may begin at relatively low fluences, often around 100 to 150 mJ/cm², depending on the device protocol and clinical assessment. Test spots can be used to identify the lowest dose that produces confluent erythema, after which treatment may begin at the MED or approximately 50 mJ/cm² below it.

Understanding the Trade-offs

Higher Dose Does Not Mean Better Treatment

Targeted lasers permit higher lesion doses, but excessive energy can still cause painful erythema, blistering, burns, post-inflammatory pigment changes, or worsening of the treated condition.

Dose escalation should follow the patient’s observed response at each session rather than a fixed assumption that the highest permissible dose is optimal.

MED Is Not a Universal Treatment Dose

The MED is a reference threshold, not an automatic prescription. A broad-area protocol may start below MED, while a targeted protocol may intentionally exceed it.

The same numerical MED cannot be transferred uncritically between devices, wavelengths, body sites, or disease indications.

Broad-Area Treatment Has a Larger Safety Constraint

Whole-body phototherapy is less selective because uninvolved skin receives radiation. This limits dose escalation and increases the importance of protecting against unnecessary erythema, cumulative photoaging, and long-term radiation-related risks.

Targeted treatment reduces exposure to healthy skin but is most appropriate when disease is sufficiently localized and technically accessible.

Protocols Must Be Device- and Condition-Specific

Excimer lasers, narrow-band UVB units, and other light-based systems differ in wavelength, beam profile, spot size, calibration, and dosing units. A protocol developed for one system should not be copied directly to another.

Clinical judgment, validated manufacturer guidance, and disease-specific treatment protocols remain necessary even when an MED has been measured.

Making the Right Choice for Your Goal

The practical decision is to combine individualized testing with the exposure pattern and characteristics of the target lesion.

If your primary focus is broad-area phototherapy: Begin conservatively, typically at 50% to 100% of the MED, because uninvolved skin limits the safe starting dose.
If your primary focus is targeted laser treatment: Consider 100% to 400% of the MED when appropriate for lesion thickness, induration, and location, while shielding surrounding skin and monitoring the response.
If your primary focus is treating depigmented lesions: Base the initial fluence on the lesion’s degree of depigmentation and use incremental test spots rather than relying only on the surrounding skin phototype.
If your primary focus is safety: Perform protocolized MED testing and adjust subsequent doses according to erythema, pain, pigmentary change, and other clinical responses.

MED testing provides the safety baseline; treatment geometry and lesion-specific judgment determine how that baseline is applied.

Summary Table:

Aspect	Broad-Area Phototherapy	Targeted Medical Laser Systems
Treatment Geometry	Exposes affected and unaffected skin	Concentrates on selected lesions; shields healthy skin
Starting Dose	50% - 100% of MED	100% - 400% of MED
Dose Limiting Factor	Tolerance of uninvolved skin	Lesion thickness, location, and response
Typical Applications	Psoriasis, vitiligo (widespread)	Localized lesions (plaque psoriasis, vitiligo patches)
Key Safety Consideration	Risk of cumulative exposure to healthy skin	Risk of excessive erythema if lesion dose too high

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