Dermatologists Keep Calling These Light Spots a Fungus. On Our Skin, It Often Isn't.

Two things that look identical and are not the same

The word "fungus" almost always means the dermatologist is thinking of tinea versicolor — an overgrowth of Malassezia, a yeast that lives on everyone's skin and sometimes gets out of hand. It leaves lighter or darker patches, and yes, an antifungal is the right answer for it. On the face of it, a reasonable call.

But there is a second condition that looks so much like tinea versicolor that even dermatologists frequently mix them up, and it is seen more often in our skin. It's called progressive macular hypomelanosis, or PMH. The medical literature describes PMH as frequently misdiagnosed and treated inadequately with antifungals or steroids, leaving patients frustrated, and it shows up most in the deeper skin tones, Fitzpatrick four through six — in other words, in us.

Here is the part that matters: PMH is not a fungus. It is thought to be driven by ordinary skin bacteria, the same family involved in acne, settling into the hair follicles and quietly turning down the pigment around them. Antifungals and steroids treat it inadequately. So you can do everything your dermatologist told you, exactly right, for months, and watch it fail — not because skin like ours is stubborn, but because the target was never a fungus in the first place.

How to actually tell them apart, in about a minute

You do not have to guess, and you should not have to spend three months finding out the hard way. There are two quick, cheap, in-office tests that help sort this out, and it is completely reasonable to ask for them by name.

A Wood's lamp — a handheld ultraviolet light, held over the skin in a dark room. Under it, PMH announces itself: a distinctive coral-red glow coming from the hair follicles, described in the literature as a hallmark of the condition and absent in normal skin. That red glow is a bacterial signal. Fungus does not produce it. Tinea versicolor may show a faint yellowish glow or, often, none at all, which is exactly why the lamp is decisive for PMH and only suggestive for fungus. Vitiligo lights up a bright, chalky white. Pityriasis alba, a mild and mostly-childhood condition, barely glows at all. One lamp, one minute, and the most important question is answered: is that red glow there or not.

A KOH scraping — a painless swipe of the surface, looked at under a microscope. If it's tinea versicolor, the yeast is visible. If it comes back clean in a person of color, that is a flag to think PMH. A negative scraping in skin of color is a reason for the doctor to consider PMH.

And there is a third clue you can check yourself, in the mirror, tonight. PMH lives on the trunk: the chest, the back, the stomach, around the midline. It only rarely reaches the face and neck. So if these light spots are only on the face and nowhere on the torso, PMH becomes a weaker bet, and the more likely culprits are tinea versicolor or old hypopigmentation left behind by acne or irritation that healed lighter. If there are matching spots on the chest or back, PMH climbs the list. That single observation tells your dermatologist more than another appointment of squinting will.

The one thing to take with you

If a dermatologist looks at light spots on our skin, says "fungus," and reaches for the prescription pad without ever running a Wood's lamp or a scraping, that is the moment to speak up. Not because they're careless, but because the two conditions that look most alike here demand different treatments, and the one that's more common in us is the one that gets missed. Asking for the lamp and the scraping by name is the difference between another season of the wrong cream and actually treating the thing you have.

Our skin is not harder to diagnose. It has just been studied less, pictured less in the textbooks, and second-guessed more. You knowing the name of the test is part of how that changes.