Subungual Hematoma vs Fungus: How to Tell the Difference (Before You Panic)

Subungual hematoma (bruise): History of trauma or pressure + moves forward as the nail grows + smooth nail surface
Nail fungus (onychomycosis): No clear injury + stays in place or spreads backward + thick, crumbly texture with keratin debris
Melanoma (rare but serious): No trauma + vertical dark stripe + pigment spreading onto surrounding skin (Hutchinson’s sign)

If the dark spot moves, it’s usually a bruise. If it doesn’t move, texture and skin changes decide the next step.

Subungual Hematoma vs Fungus: How to Tell the Difference

How can you tell if a dark spot under your toenail is a bruise or fungus?

Watch behavior, not just color.
A subungual hematoma is blood trapped under the nail plate from the vascular nail bed. Because it’s locked into the keratin plate, it moves forward with nail growth.
Nail fungus (onychomycosis) is an infection. It causes onycholysis (lifting), thickening, and keratin debris, and it usually stays fixed or creeps back toward the cuticle.

This growth test solves most anxiety in one month.

Differential Diagnosis of the Dark Nail (Benign vs. Malignant)

From a clinical point of view, any dark nail requires a differential diagnosis.

Benign causes
- Subungual hematoma (bruise)
- Onychomycosis (fungal infection)
Malignant cause that must always be excluded
- Subungual melanoma

Bruises and fungus are common and manageable. Melanoma is rare, but missing it has serious consequences. Good content and good care always rule it out first.

Understanding Subungual Hematoma (Bruise Under the Nail)

This YouTube video below by Larry B. Mellick, MD explains subungual hematoma emergencies and when trephination becomes necessary. He outlines the procedure, safety steps, and expected outcomes. These insights stress the importance of timely intervention to relieve pain and prevent nail damage.

A subungual hematoma forms when trauma ruptures capillaries in the vascular nail bed, trapping blood in the subungual space beneath the nail plate.

Common “I don’t remember hurting it” stories

Long hikes or downhill running (toe repeatedly jams the toe box)
Runner’s toe after races or treadmill sessions
Pressure from ski boots or rigid cycling shoes
Chronic compression from steel-toed work boots
Dropping something light that didn’t seem important

Chronic micro-trauma explains why many people swear nothing happened.

Transverse vs. longitudinal pigment (key logic)

Hematoma: transverse (horizontal), blotchy, or amorphous
Melanoma: longitudinal melanonychia, a vertical stripe

A bruise is physically trapped in keratin. As the nail matrix pushes out new nail, the bruise must migrate forward. The clear nail at the base slowly increases. That’s basic nail physics.

Color changes during healing (normal, not scary)

Red or maroon → fresh blood
Purple or black → clotted blood
Brown or gray → aging blood
Serous crust: small dry flake left behind as it grows out

That last one confuses many people. A serous crust is just dried blood residue, not fungus.

Does a bruised nail always hurt?

No. Early pressure is common. Older hematomas are often painless. The nail surface usually stays smooth and normal thickness.

Splinter hemorrhages

You may see tiny, thin, vertical red-brown lines that look like wood splinters. These are splinter hemorrhages—small capillary bleeds from friction or pressure. They’re common with work boots and sports shoes. As long as they grow out, they’re benign and not melanoma.

Nail trephination: why DIY puncturing is dangerous

This YouTube video below by Larry B. Mellick, MD explains nail trephination and when clinicians use it. He describes the technique, indications, and aftercare basics. These insights clarify safe management of painful nail hematomas.

Severe pressure from a fresh hematoma can be relieved with nail trephination, a sterile medical procedure.

The distance between the nail plate and the distal phalanx (toe bone) is only a few millimeters. A DIY puncture can introduce Staphylococcus bacteria directly to bone, causing osteomyelitis, a surgical emergency. This is why “bathroom surgery” is never worth the risk.

Understanding nail fungus (onychomycosis)

Onychomycosis is caused by dermatophytes that digest keratin.

Instead of blood, fungus produces subungual hyperkeratosis—a chalky buildup of keratin debris that pushes the nail plate upward, leading to onycholysis and gradual deformity.

Signs pointing to fungus

Yellow, white, or cloudy-brown color
Progressive thickening
Brittle or crumbly texture
Lifting with debris underneath
Usually painless until advanced

Fungus does not grow out cleanly.

The texture + scratch test

Gently run a clean fingernail across the surface.

Smooth and glassy: pigment is under the nail (bruise or melanoma)
Rough, pitted, chalky: fungal damage to the nail plate

Texture often answers what color can’t.

Subungual hematoma vs fungus: side-by-side

Feature	Subungual Hematoma	Nail Fungus
Primary cause	Trauma	Fungal pathogen
Pigment direction	Transverse / blotchy	Irregular or spreading
Movement	Moves forward	Stays or spreads backward
Nail thickness	Normal	Thickened
Nail bed surface	Smooth	Keratin debris
Pain	Early pressure	Usually painless

The red flag that matters most: subungual melanoma

Rare, but critical to recognize.

Warning signs

Dark pigment that does not move
Vertical brown or black stripe
Pigment spreading onto surrounding skin
Nail splitting or destruction without trauma

🚨 Hutchinson’s sign

Hutchinson’s sign means pigment has extended into the periungual skin (cuticle or side folds).

A bruise is strictly subungual. It will never stain the skin.
If pigment looks like ink bleeding into paper, seek evaluation urgently.

What a specialist looks for

Dermatologists often use dermoscopy.

Hematoma: circumscribed purple-black globules (blood spots)
Melanoma: parallel ridge pattern and irregular pigment networks

These are real diagnostic terms you may hear in clinic.

The 30-second acetone swab test

Some dark nails are just external staining (pseudochromonychia).

Hair dye
Sock lint
Chemicals or polish residue

Quick check:

Dab a Q-tip with acetone
Gently rub the nail surface

If pigment wipes off, it’s not a bruise or fungus.

Multiple nails changing at once? Think systemic

If dark bands appear on multiple fingers or toes simultaneously without trauma, consider systemic causes such as Addison’s disease or medication effects (including chemotherapy agents like hydroxyurea). This needs a broader medical review.

Time-stamp strategy

Because toenails grow only ~0.1 mm per day, small changes are hard to see.

Mark the top edge of the pigment with a fine-tip marker
Or take a photo next to a ruler
Recheck in 4 weeks

A fixed reference point makes movement obvious and lowers anxiety.

Does a bruised toenail always fall off?

No. Nails usually shed only when 25–50% or more of the nail bed is involved. Smaller hematomas often grow out fully.

Can I put antifungal cream on a bruise?

It won’t hurt, but it won’t help. Antifungals can’t penetrate trapped blood.

Recovery window

Toenails grow about 1 mm per month.

Small hematoma: 3–4 months
Large hematoma: 6–9 months to trim away

Checking weekly fuels worry. Monthly checks are enough.

When should you see a doctor?

Get evaluated if:

The spot doesn’t move after 4–6 weeks
The nail thickens or crumbles
Pigment spreads onto skin
A vertical dark stripe appears
Several nails change together

Guidance often aligns with standards from the American Academy of Dermatology (AAD).

Final Thought

A dark spot under a nail is scary, and that reaction is human.
Most cases are either a bruise that grows out or a fungus that slowly changes texture. Watch movement. Feel texture. Respect red flags. With the right clues, panic turns into clarity—and clarity leads to the right next step.