Is It Really a Fungal Nail?
About half of abnormal nails are not infected. How fungal infection differs from footwear damage, psoriasis and lichen planus — and the streak that needs checking.
Part of the Complete Fungal Nail Infection Guide.
Key fact: Fungal infection accounts for about half of all nail disease. The other half is something else — most often psoriasis or repeated trauma from footwear — and neither responds to antifungal treatment, however long you persist with it.
Why getting this right matters
Antifungal tablets are taken for months and carry real, if uncommon, risks. Lacquer is applied for up to a year. Neither does anything at all if the nail is not infected — and in the meantime, whatever is actually wrong goes untreated.
There is also the rare but serious reason: a melanoma arising under a nail looks, to most people and to plenty of clinicians, like a fungal nail or an old bruise.
What fungal infection actually looks like
- Starts at the free edge or a side and works backwards toward the base, rather than appearing all over at once
- Yellow, white or brown discolouration, often streaky rather than uniform
- Crumbly debris under the nail, lifting it from the bed
- Thickening of the nail plate
- Asymmetrical — one or two nails, and frequently one foot worse than the other
- Athlete's foot alongside it in a large proportion of cases — scaly, itchy skin between the toes or on the sole
That last point is the single most useful clue. Fungal nails rarely arrive without the skin being involved somewhere, and finding athlete's foot alongside makes the diagnosis considerably more likely.
Trauma from footwear
The commonest imitator, and easily missed because there is no memorable injury — just years of shoes that are slightly too short, or running.
| Points towards trauma | Points towards fungus |
|---|---|
| Affects the nails that take the pressure — big toe, or the longest toe | Can affect any nail, and spreads between them |
| Symmetrical, both feet, same nails | Often asymmetrical |
| Thickening without much discolouration | Discolouration and crumbling debris |
| Stable over years | Progresses through the nail and to others |
| No athlete's foot | Athlete's foot often present |
Trauma and fungus also coexist: damaged nails are easier to infect, so a thickened traumatic nail can become genuinely infected later.
Nail psoriasis
Psoriasis affects nails in a large proportion of people who have it, sometimes before any skin patches appear. Its signature features are different:
- Pitting — small dents in the nail surface, like pinpricks. Fungal infection does not cause this
- "Oil drop" or salmon patches — orange-pink discolouration under the nail with a defined edge
- Onycholysis — the nail lifting from the bed, often with a distinct border
- Multiple nails, often fingernails as well as toenails
- Psoriasis elsewhere — scalp, elbows, knees, or behind the ears, which is worth looking for
Psoriatic nails can also become secondarily infected with fungus, which is one of several reasons laboratory confirmation earns its place.
Lichen planus
Less common, but it damages nails in a distinctive way: thinning and ridging along the length of the nail, splitting, and in some cases scarring that destroys the nail permanently. It can affect many nails at once and progresses relatively quickly.
It matters because the window for treatment is limited — once scarring has occurred the nail does not recover — so nails deteriorating rapidly across several digits deserve assessment rather than a year of lacquer.
The dark streak that needs checking
A new brown or black band running lengthwise down a nail should be assessed. Features that raise concern: a single nail affected; a band wider than about 3mm; an irregular or blurred edge; the band getting wider, particularly towards the base; pigment extending onto the skin around the nail (Hutchinson's sign); and any accompanying splitting or destruction of the nail.
- Thumb, index finger and big toe account for the large majority of cases
- A bruise moves. Blood under a nail grows out with it and shifts towards the tip over weeks. A melanoma stays put, anchored at the base
- No injury remembered is more concerning, not less — though many people invent one retrospectively
- Multiple bands across several nails, symmetrical and long-standing, are usually benign, particularly in people with darker skin
The reason this appears in a fungal nail guide is that it is repeatedly treated as one. Months of antifungal lacquer on a pigmented streak is time that matters.
The less common causes
- Bacterial infection — particularly pseudomonas, which turns the nail green
- Chronic paronychia — inflammation of the skin fold around the nail, common in people whose hands are often wet
- Yellow nail syndrome — slow-growing, thickened yellow nails, sometimes with chest or lymphatic problems
- Warts and other growths under or beside the nail
- Medication effects and general illness, which usually affect all nails together
How it is confirmed
Nail clippings, taken as far back as possible, along with debris from underneath, sent for microscopy and culture. Practical points:
| Point | Why |
|---|---|
| Sample before treatment | Antifungal treatment already started makes the result unreliable. If you have been using a lacquer, expect to be asked to stop for a period first |
| Expect a wait | Culture takes several weeks. This is the test, not a delay |
| A negative is not final | Sampling misses infection reasonably often, and may be repeated where the picture strongly suggests fungus |
| It matters most before tablets | Oral antifungals are a months-long course with monitoring. Local practice on when samples are required varies |
What to do next
- Look at the skin as well as the nail — athlete's foot alongside makes fungal infection much more likely
- Look at the other nails, and at fingernails as well as toenails
- Look for psoriasis elsewhere — scalp, elbows, knees
- Get any dark streak assessed rather than treated
- Get confirmation before committing to tablets, and see treatments compared for what each option involves
Frequently Asked Questions
How can I tell if my nail is fungal or damaged?
Fungal infection usually starts at the free edge or side and works back, causes discolouration with crumbly debris, is often asymmetrical, and frequently comes with athlete's foot. Damage from footwear tends to affect the nails taking the pressure, is symmetrical on both feet, causes thickening without much discolouration, and stays stable over years.
Can nail psoriasis look like a fungal infection?
Yes, and it is the commonest medical imitator. Psoriasis causes pitting, which fungal infection does not, along with orange-pink oil drop patches under the nail and lifting of the nail with a defined border. It often affects several nails including fingernails, and psoriasis may be visible on the scalp, elbows or knees.
Do I need nail clippings sent to a lab?
It is recommended before starting oral antifungal tablets, because about half of abnormal nails are not fungal and tablets are a months-long course. Samples must be taken before antifungal treatment starts, and culture takes several weeks. Local practice varies on when testing is required.
Is a black line in my nail dangerous?
It needs assessing rather than treating. Concerning features are a single affected nail, a band wider than about 3mm, an irregular edge, widening over time, pigment extending onto the surrounding skin, and nail splitting. A bruise grows out and moves towards the tip; a melanoma stays anchored at the base.
My nail treatment has not worked after a year. Why?
The commonest explanation is that the nail was never fungal. Other possibilities are that the infection reached the growing base where lacquer does not penetrate, that athlete's foot kept reinfecting it, or that the course was stopped early because appearance had not improved yet.
References
- NHS. Fungal nail infection. nhs.uk
- NHS. Athlete's foot. nhs.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summary: Fungal nail infection. cks.nice.org.uk
- DermNet. Fungal nail infections. dermnetnz.org
- National Institute for Health and Care Excellence. Suspected cancer: recognition and referral (NG12). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. An abnormal nail is not always a fungal infection, and a new dark streak in a nail should always be assessed. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.