Toenail Fungus or a Bruise? Signs to Watch For

A dark or discoloured toenail could be a bruise, a fungal infection, or occasionally something more serious. Here is how to tell, and when to get it checked.

October 2, 2026 · 6 min read · By the FungalNailTest.com team

A toenail that suddenly looks different can be unsettling. Is it a fungal infection? A bruise from stubbing your toe or a long run? Or something that needs a doctor's attention? These three possibilities can look surprisingly alike, especially in the early stages.

This guide explains the typical differences, a simple way to keep track at home, and the warning signs that mean you should see a healthcare provider promptly.

What a bruised toenail looks like

When a toe is injured, blood can collect under the nail. Doctors call this a subungual hematoma. Typical features include:

A red, purple, dark brown or black patch under the nail.
It appears soon after a known injury: a stubbed toe, a dropped object, or a tight shoe on a long hike.
It may be painful or throbbing at first, especially if a lot of blood is trapped.
Over time, the dark area moves forward as the nail grows and is eventually trimmed off.

Not every bruise comes from a single big injury. Repeated small knocks, from running, hiking, sports or shoes that are too short or narrow, can also cause dark discolouration and thickening, particularly in the big toenail. Runners sometimes call this "runner's toe".

What toenail fungus looks like

A fungal nail infection usually develops gradually, with no clear injury to point to. According to Mayo Clinic, it often begins as a white or yellow-brown spot under the tip of the nail, and as it goes deeper the nail may discolour, thicken and crumble at the edge (Mayo Clinic). Common features:

White, yellow or yellow-brown discolouration, rather than deep purple or black.
Thickening, brittleness or crumbling, sometimes with debris under the nail.
It usually starts at the tip or edges and slowly spreads back towards the cuticle.
It doesn't grow out on its own, and may slowly get worse or spread to other nails.
It may come with athlete's foot on the surrounding skin.

Mayo Clinic also notes that discolouration from a bacterial infection tends to be green or black, which is another reason a dark nail isn't automatically a bruise.

Can it be both?

Yes. A nail that has been injured can be more vulnerable to infection, and trauma is listed among the risk factors for fungal nail infections by the Canadian Dermatology Association. So it's possible to start with a bruise and later develop a fungal infection in the damaged nail, or to have an old fungal infection and then injure the same toe.

A simple way to tell: watch whether it moves

One of the most useful things you can do at home is track the discoloured area over a few weeks:

Take a clear photo of the nail in good light. Include a ruler or coin for scale if you can.
If it's easy to do, make a tiny scratch or mark on the nail surface at the edge of the discoloured area (near the cuticle side).
Take another photo every couple of weeks.

A bruise should move forward with the nail as it grows, with clear, healthy nail appearing behind it. Toenails grow slowly, so this takes patience. Fungal discolouration, by contrast, tends to stay in place or slowly spread, rather than growing out.

Warning signs: don't wait on these

Most dark toenails are bruises, but rarely a dark area under the nail can be a sign of subungual melanoma, a type of skin cancer that develops under the nail. The Canadian Dermatology Association notes that people may delay seeing a doctor because they mistake the discolouration for a bruise (Canadian Dermatology Association). See a healthcare provider promptly if you notice:

A brown or black streak running lengthwise in the nail, especially in the big toe or thumb.
A streak that gets wider or darker over time.
Discolouration with no injury you can remember.
A "bruise" that doesn't heal or grow out with the nail.
Darkening of the skin around the nail or cuticle.
A lump, sore or bleeding in or around the nail.

Also see a healthcare provider promptly if you have severe pain or pressure under a nail after an injury, a possible broken toe, or signs of infection such as spreading redness, warmth, swelling or pus. If you have diabetes, poor circulation or a weakened immune system, get any foot or nail change checked early.

Our test does not check for melanoma

A fungal nail test looks for fungal DNA only. It cannot rule out melanoma, a bruise or any other condition. If you have a dark streak or spot under a nail that you can't explain, see a healthcare provider in person, whatever any test shows.

Quick comparison

Bruise: follows an injury; red, purple or black; may hurt at first; grows out with the nail.
Fungus: gradual; white, yellow or yellow-brown; thick, crumbly; starts at the edges; doesn't grow out on its own.
Get checked promptly: a dark streak, no known injury, changes that spread to the skin, or a "bruise" that doesn't move.

When testing can help

If your nail is thick, yellowish or crumbly and you're not sure whether fungus is to blame, testing can save you months of guessing and money spent on treatments you might not need. Our at-home kit lets you clip a sample, mail it back in the prepaid envelope, and get a qPCR DNA result, reviewed by a licensed Canadian podiatrist with over 50 years of experience, within 4–14 days of your sample arriving.

A negative result can help point you towards other causes like trauma, but it isn't a diagnosis. Read our guide to understanding your results to see what each result means and what to do next.

Frequently asked questions

How can I tell a bruised toenail from fungus?

A bruise usually follows an injury, looks red, purple or black, and moves forward as the nail grows. Fungus tends to start gradually as white, yellow or yellow-brown thickening at the tip or edges and doesn't grow out on its own.

How long does a bruised toenail take to grow out?

Toenails grow slowly, so a bruise can take many months to reach the free edge and be trimmed off. Big toenails take the longest. Tracking with photos every few weeks helps you see whether the dark area is moving.

When should I see a doctor about a dark toenail?

See a healthcare provider promptly for a brown or black streak with no clear injury, a streak that widens or darkens, darkening of the skin around the nail, a lump or bleeding, severe pain after injury, or any foot change if you have diabetes.

Can a fungal nail test check for melanoma?

No. A fungal nail test only looks for fungal DNA. It cannot rule out melanoma, a bruise or other conditions. Unexplained dark streaks or spots under a nail need an in-person check.

When does testing for fungus make sense?

If the nail is thick, yellowish or crumbly and you're unsure of the cause, a mail-in qPCR test can show whether fungal DNA is present. Results are reviewed by a licensed Canadian podiatrist. Share the report with your healthcare provider.

Sources

Important

This article is general information, not medical advice, and is not a substitute for a healthcare provider. Nail changes can have many causes. If you have diabetes, poor circulation, a weakened immune system, pain, swelling or signs of infection, see a healthcare provider promptly.