(NEXSTAR) – What started out as a "tiny little brown line" on one woman's fingernail led to a rare, potentially life-threatening medical condition, one that she's now trying to help others catch early.

Lauren Koltcz said she noticed the mark on her left thumbnail last May, and thought to herself, "Well, that's odd."

While rare and easily missed, the "splinter"-like line shouldn't be ignored, experts warn.

"If you have a dark-pigmented vertical streak going down your nail, this should be checked," Dr. Lindsey Zubritsky, a Mississippi-based dermatologist warned her 1.6 million TikTok followers.