Diagnosis and treatment of skin infections

What it is

That means a skin scraping and microscopy, a nail clipping and culture, or a Wood lamp examination before any long course is started. Treating by guess is the commonest error here, and the specific local version of it is a corticosteroid or an unlabeled mixed cream applied to a fungal rash, which changes its shape, spreads it, and delays the diagnosis by months.

What this page covers

• Skin scraping, KOH microscopy, fungal culture, nail clipping and Wood lamp examination before treatment
• Tinea corporis and tinea cruris
• Tinea pedis, treated together with the groin and nails as one recurring source
• Tinea capitis in children, with household and pet screening
• Steroid-modified tinea (tinea incognito) from unlabeled mixed creams — and the rule that no lightening or laser treatment starts on a fold before a scraping
• Tinea versicolor, with a maintenance plan discussed at the first visit
• Cutaneous candidiasis and intertrigo, with diabetes screening where it recurs
• Onychomycosis, confirmed in the laboratory before any oral course
• Malassezia (pityrosporum) folliculitis
• Bacterial folliculitis, furuncles and abscesses, including incision, drainage and culture
• Impetigo, with household infection-control measures
• Erythrasma, pitted keratolysis and trichomycosis
• Cellulitis and erysipelas, with the entry point treated and the threshold for emergency referral stated
• Viral warts, plantar warts and molluscum contagiosum
• Herpes simplex, including antiviral prophylaxis before laser, peel, needling or perioral filler
• Herpes zoster (shingles), with same-day ophthalmology referral for eye or nasal-tip involvement
• Scabies, with same-day treatment of the entire household and domestic staff
• Head lice, handled without stigma

Who it suits

• Someone whose rash has been treated for months with one cream after another — including a mixed cream with no name on the container — and has widened rather than settled. Bring everything you have used, in its own container
• A household that is itching: an itch that worsens at night, in you and in someone else at home. Scabies is treated as a household, not as one person
• A parent bringing a child: a scaly scalp patch with broken hairs, honey-colored crusts around the nose and mouth, pearly bumps, or head lice. Children are treated here, and the household is examined at the same visit
• Anyone pregnant or breastfeeding, and anyone whose patient is a young child: say so at the start of the visit rather than after the prescription is written. It changes which medicine is chosen, not whether you are treated
• Thick, built-up crusts on the hands, feet or scalp in someone elderly, frail, or on treatment that lowers immunity, with little itch or none at all. That form of scabies is intensely contagious and is often missed for months, because it comes with no itch worth mentioning while it passes to everyone around; bring it to the clinic rather than treating it at home, because it is handled on its own pathway
• Not you if you want a prescription without an examination and without a sample, or a diagnosis from a photograph on WhatsApp. The scraping, the nail clipping or the culture is what chooses the medicine; without one, treatment is a guess, and what a guess costs most is time. Nor if you are expecting one visit to end it: fungal courses run long, warts usually need repeat visits, and in scabies and lice what decides the outcome is treating the whole household rather than the prescription

When we advise delaying or not treating

• An oral antifungal course in pregnancy, or while trying to conceive: we defer it. Nail fungus does no harm by waiting and the nail grows out slowly whatever we do, so after delivery is a sound time rather than a compromise. In breastfeeding, each medicine is decided one at a time with your doctor — not on the assumption that it is allowed, and not on the assumption that it is forbidden. An infection that genuinely does need treating during pregnancy is managed alongside the obstetrician looking after you. Say it before the prescription is written, not after
• Oral ivermectin for scabies in pregnancy, in breastfeeding, and in small children. Topical permethrin is what is used instead in those groups, and it is applied to everyone in the household on the same day in any case — the tablet is not a way around treating everyone. If you are breastfeeding, say so, because the instructions for applying it around the chest are different
• Itraconazole where there is heart failure, and in anyone whose medicine list has not been gone through first. It interacts with a long list of ordinary medicines — some cholesterol tablets, some heart-rhythm medicines, some sedatives and others — and a few of those combinations are dangerous rather than merely inconvenient. Bring every tablet, supplement and herbal preparation you take, including the ones you would not think to mention
• Terbinafine where there is existing liver disease, and any oral antifungal at all without the blood test and the medicine review. The laboratory establishes that a fungus is there; the blood test and the medicine list decide whether this particular tablet is safe for you. That is the second half of the decision, and the page states only the first half
• Any lightening cream, peel or laser on a fold, an underarm or the groin before a scraping and a Wood lamp: we decline. Darkening in a fold in someone using a mixed cream is a steroid-modified fungus until proven otherwise, and lightening over a fungus spreads it and drives it deeper
• A cortisone cream, or an unlabeled mixed cream, applied "just to calm the itch" while the diagnosis is still open: declined. And when you stop the cream you are already using, expect a temporary flare over the following days. That is the skin returning to its true appearance, it is explained before it happens, and it is not the new treatment failing
• Repeating the same antifungal a third time after two courses changed nothing. At that point the diagnosis is revisited rather than the prescription repeated. More than one condition is treated for a long time as tinea versicolor and is not it
• Freezing or cautery for a wart on the face or hands in deeper skin, where the mark left behind outlasts the wart itself: discussed first, and the gentler option chosen, or none. And any painful destructive treatment on the foot of someone with diabetes, reduced circulation or reduced sensation: we stay conservative and refer rather than improvise, because a blister on that foot is not a small problem
• Any laser, peel or needling session over skin with an active infection in it, without exception; and any elective procedure that breaks the skin while diabetes is poorly controlled — deferred until control improves, which is care rather than refusal, with a route back into the clinic once it is better
• Genital warts and genital molluscum in an adult are handled on a separate pathway, with appropriate referral and with discretion. Warts or molluscum that are widespread or unusually stubborn in an adult also prompt a look at why, including a check for undiagnosed diabetes

Possible effects

Stated in full, because the decision is not sound without them. The page already says that no oral course begins before the laboratory confirms it, and that is half the decision: it establishes that a fungus is there. The other half is what the medicine itself carries, and that is what you are actually consenting to when you start.

• Terbinafine: a change in taste or smell, or the loss of it — this happens, it is more disruptive than it sounds on paper, and in some people it lasts after the tablet is stopped. Rarely, liver injury. Stop the tablet and contact us the same day if the whites of the eyes or the skin turn yellow, if your urine darkens, if nausea will not settle, if there is pain in the upper right side of the abdomen, or if you are unusually tired; and likewise if a widespread rash appears while you are taking it
• Itraconazole: its interactions are its main hazard, which is why your medicines are listed before the prescription rather than after it, and why every doctor and pharmacist who gives you anything during the course has to be told you are on it. It is not used where there is heart failure; if you become short of breath, your feet swell, or you notice palpitations while taking it, report it straight away. It also carries a possible effect on the liver, with the same warning signs listed above
• What both oral courses share: a blood test before starting and during the course as the prescribing doctor decides, which is part of the medicine rather than paperwork to be completed. And the nail takes a long time to grow out clean and may not return to exactly its former shape — said before it starts, not afterwards. No tablet is taken from someone else's box, and none is passed to a relative whose nails look like yours
• Antibiotics: topical antibiotic used long and without need builds resistance, and that is one reason impetigo comes back and stops responding. Repeated long oral courses have their own cost, which is why every course is given a defined end from its first day
• Cryotherapy and cautery: the session hurts, a blister may follow, and a pale or dark mark can be left at the site that lingers on Fitzpatrick IV-VI and may outlast the wart it removed. Recurrence is possible with any method chosen and is not evidence that the treatment was wrong. And a blister on the foot of someone with diabetes is not a small problem
• Curettage of molluscum: it leaves a mark or a pigment change on darker skin, weighed against the fact that many of these lesions clear on their own if left
• Incision and drainage: it relieves the pressure and it leaves a scar. Recurrent abscesses in the underarm, the groin or under the breast are treated as hidradenitis suppurativa until proven otherwise, and they need the medical pathway rather than a drainage repeated each time
• Scabies treatment: the cream can sting and irritate. The itch carries on for weeks after the treatment has worked, because your skin is still reacting to what the mite left behind — this is expected and it does not mean failure. Do not repeat the medicine on your own
• Shingles: pain can remain along the path of the rash after the skin has fully healed — post-herpetic neuralgia. It becomes more likely with age and where the pain was severe at the outset, and it can last months. It is not something to endure in silence: it is assessed and it is treated, and coming back is worth more than putting up with it
• Stopping the cortisone or the mixed cream: a temporary flare follows, explained in advance. Skin that has thinned underneath a mixed cream reacts unpredictably to any later peel or laser, which is why both wait until it has recovered

A note on darker skin

Two things change on Fitzpatrick III-VI, and they pull in opposite directions. The first is recognition: hot redness does not read as red on deeper skin, so cellulitis can look dusky, brown or violaceous, and on type VI there may be barely any visible color change at all — which makes warmth, swelling, the degree of pain and how you feel in yourself better guides than color is, and makes the pen line around the edge of the swollen area described above more important here rather than less. The Wood lamp likewise gives less the deeper the skin tone, because higher natural melanin blunts the contrast, and it never excludes fungus on its own in any case — a scraping is still taken. The second is what treatment leaves behind. Freezing and cautery leave a pale or dark mark on deeper skin that can outlast the wart it removed, which is why the gentler option is chosen on the face and hands, or none at all; curettage of molluscum carries the same trade-off against the fact that many clear on their own. Almost every infection on this page leaves color behind once it clears — darker in some people, paler in others — and in tinea versicolor the pale patches persist long after the yeast is gone, which is read as failure and is not. Treating the infection promptly is most of the pigment plan; what remains afterwards is assessed separately and in its own time on post-inflammatory pigmentation, never during the infection. And the local trap runs the other way: darkening in the groin, the underarm or under the breast is often erythrasma or a steroid-modified tinea rather than a pigment problem, and lightening over either one spreads it and drives it deeper — which is why the scraping and the Wood lamp come before any lightening plan.

Afterwards

• Finish the course as it was written and for the period you were given, including the days when the skin already looks normal. Stopping at the first clear week is the commonest reason a fungal infection comes back
• Feet, groin and nails are treated together because they are one source feeding itself. Dry between the toes after wudu and after showering, change your socks, do not walk barefoot in shared washrooms, and do not share towels or nail clippers
• In scabies: everyone in the household and anyone working in the home is treated on the same day, including those with no itch, and bedding, clothes and towels are hot-washed or sealed in a bag as you were shown
• Photograph the nail today, on your phone. Judging by memory is what makes slow but real improvement look like failure
• If you are on an oral antifungal: report a change in taste or smell, yellowing of the whites of the eyes or the skin, dark urine, nausea that will not settle, or unusual tiredness — the same day, and hold the tablet until you are told otherwise. And tell every doctor and pharmacist who dispenses anything to you that you are taking it
• After shingles: report any pain that carries on after the rash has healed, rather than waiting for it to go on its own
• Report as well: the rash widening while you are using the cream, the agreed review date arriving with nothing changed, a fever, a red streak running away from the area, or that you are pregnant or think you might be
• Come to the review with everything you have used, in its container, including the mixed cream if there is one

Honestly

The diagnosis is confirmed by scraping, clipping or culture before a long course is prescribed, and an antifungal already in use has to be stopped before sampling or the result comes back falsely negative. Cryotherapy and cautery for warts leave a pale or dark mark on Fitzpatrick IV–VI, so the gentler option is chosen on the face and hands; spreading hot redness with fever, and shingles near the eye, are same-day urgent care rather than a clinic appointment.

Common questions

See also

Nail disorders, Acne treatment, Rosacea treatment, Eczema treatment