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.
Avapritinib, sold under the brand name Ayvakit among others, is a medication used for the treatment of advanced systemic mastocytosis and for the treatment of tumors due to one specific rare mutation: it is specifically intended for adults with unresectable or metastatic gastrointestinal stromal tumor (GIST) that harbor a platelet-derived growth factor receptor alpha (PDGFRA) exon 18 mutation.Common side effects include edema (swelling), nausea, fatigue/asthenia (abnormal physical weakness or lack of energy), cognitive impairment, vomiting, decreased appetite, diarrhea, hair color changes, increased lacrimation (secretion of tears), abdominal pain, constipation, rash and dizziness.Avapritinib is a kinase inhibitor. Medical uses
Avapritinib is indicated for the treatment of adults with unresectable or metastatic gastrointestinal stromal tumors (GIST) harboring the platelet-derived growth factor receptor alpha (PDGFRA) exon 18 mutation, including PDGFRA D842V mutations.Avapritinib is also indicated for the treatment of adults with advanced systemic mastocytosis, aggressive systemic mastocytosis, systemic mastocytosis with an associated hematological neoplasm, and mast cell leukemia (MCL). History
The U.S. Food and Drug Administration (FDA) approved avapritinib in January 2020. The application for avapritinib was granted fast track designation, breakthrough therapy designation, and orphan drug designation. The FDA granted approval of Ayvakit to Blueprint Medicines Corporation.Avapritinib was approved based on the results from the Phase I NAVIGATOR clinical trial involving 43 subjects with GIST harboring a PDGFRA exon 18 mutation, including 38 subjects with PDGFRA D842V mutation. Subjects received avapritinib 300 mg or 400 mg orally once daily until disease progression or they experienced unacceptable toxicity. The recommended dose was determined to be 300 mg once daily.