Acne vulgaris is diagnosed based on a medical professional's clinical judgment.[15] The evaluation of a person with suspected acne should include taking a detailed medical history about a family history of acne, a review of medications taken, signs or symptoms of excessive production of androgen hormones, cortisol, and growth hormone.[15] Comedones (blackheads and whiteheads) must be present to diagnose acne. In their absence, an appearance similar to that of acne would suggest a different skin disorder.[28] Microcomedones (the precursor to blackheads and whiteheads) are not visible to the naked eye when inspecting the skin and can only be seen with a microscope.[28] There are many features that may indicate a person's acne vulgaris is sensitive to hormonal influences. Historical and physical clues that may suggest hormone-sensitive acne include onset between ages 20 and 30; worsening the week before a woman's period; acne lesions predominantly over the jawline and chin; and inflammatory/nodular acne lesions.[1]
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The earliest pathologic change is the formation of a plug (a microcomedone), which is driven primarily by excessive growth, reproduction, and accumulation of skin cells in the hair follicle.[1] In normal skin, the skin cells that have died come up to the surface and exit the pore of the hair follicle.[10] However, increased production of oily sebum in those with acne causes the dead skin cells to stick together.[10] The accumulation of dead skin cell debris and oily sebum blocks the pore of the hair follicle, thus forming the microcomedone.[10] This is further exacerbated by the biofilm created by C. acnes within the hair follicle.[45] If the microcomedone is superficial within the hair follicle, the skin pigment melanin is exposed to air, resulting in its oxidation and dark appearance (known as a blackhead or open comedo).[1][10][20] In contrast, if the microcomedone occurs deep within the hair follicle, this causes the formation of a whitehead (known as a closed comedo).[1][10]

Pimples are raised red spots with a white center that develop when blocked hair follicles become inflamed or infected with bacteria. Blockages and inflammation that develop deep inside hair follicles produce cystlike lumps beneath the surface of your skin. Other pores in your skin, which are the openings of the sweat glands, aren't usually involved in acne.
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