This is done by looking at how dark the melasma appears; dermal melasma appears darker than epidermal melasma under the Woods lamp. Severity
The severity of facial melasma may be assessed by colorimetry, mexametry, and the melasma area and severity index (MASI) score. Differential diagnoses
Melasma should be differentiated from freckles, solar lentigo, toxic melanoderma, Riehls melanosis, post-inflammatory hyperpigmentation, friction melanosis, ochronosis (endogenous and exogenous), cutaneous erythematosus lupus. Additionally, it should not be confused with phytophotodermatosis, pellagra, endogenous phototoxicity, nevus of Ota, café au lait macules, seborrheic keratosis, Civattes poikiloderma, acquired bilateral nevus of ota-like macules (Horis nevus), periorbital hyperpigmentation, erythrose pigmentaire peribuccale of Brocq, erythromelanosis follicularis faciei, facial acanthosis nigricans, and actinic lichen planus.Also, cases of drug-induced pigmentation have been reported, caused by amiodarone, or hydroquinone-induced exogenous ochronosis (see ochronosis treatment)

Treatment
Assessment by a dermatologist can help guide treatment. Treatments to hasten the fading of the discolored patches include:

Topical depigmenting agents, such as hydroquinone (HQ) either in over-the-counter (OTC - 2%) or prescription (4%) strength. HQ inhibits tyrosinase, an enzyme involved in the production of melanin. Tretinoin, a retinoid, increases skin cell (keratinocyte) turnover. This treatment is not used during pregnancy due to risk of harm to the fetus. Azelaic acid (20%) is thought to decrease the activity of melanocytes. Tranexamic acid by mouth has shown to provide rapid and sustained lightening in melasma by decreasing melanogenesis in epidermal melanocytes.
Birth-control pills and hormone replacement therapy also can trigger melasma. The discoloration usually disappears spontaneously over a period of several months after giving birth or stopping the oral contraceptives or hormone treatment.Genetic predisposition is also a major factor in determining whether someone will develop melasma. People with the Fitzpatrick skin type III or greater from African, Asian, or Hispanic descent are at a much higher risk than others. In addition, women with a light brown skin type who are living in regions with intense sun exposure are particularly susceptible to developing this condition.The incidence of melasma also increases in patients with thyroid disease. It is thought that the overproduction of melanocyte-stimulating hormone  brought on by stress can cause outbreaks of this condition. Other rare causes of melasma include allergic reaction to medications and cosmetics. Addisons disease
Melasma suprarenale (Latin - above the kidneys) is a symptom of Addisons disease, particularly when caused by pressure or minor injury to the skin, as discovered by FJJ Schmidt of Rotterdam in 1859. Diagnosis
Types
The two different kinds of melasma are epidermal and dermal. Epidermal melasma results from melanin pigment that is elevated in the suprabasal layers of the epidermis.Dermal melasma occurs when the dermal macrophages have an elevated melanin level. Melasma is usually diagnosed visually or with assistance of a Woods lamp (340 - 400 nm wavelength). Under Woods lamp, excess melanin in the epidermis can be distinguished from that of the dermis.