A swab or a scraping collects material from the skin surface so a laboratory can establish what is present. It is quick, it does not require anaesthetic, and it exists because appearance alone is unreliable for distinguishing infection from inflammation.
Why take a surface sample?
Because two quite different things can look alike, and the difference changes what happens next.
Inflammation and infection frequently produce overlapping appearances, and a fungal infection whose appearance has been altered by treatment intended for an inflammatory condition is a well-recognised trap with a name of its own. Establishing what is actually present, rather than proceeding on appearance, is the whole purpose.
A surface sample is the least invasive way of doing that. Nothing is cut, no anaesthetic is required, and taking the sample takes a moment. Compared with a biopsy it answers a narrower question, which is exactly why it is used when that narrower question is the one at hand.
How does a swab differ from a scraping?
By what they collect, and therefore by what they can find.
A swab is a soft tip drawn across a moist area: a weeping patch, the base of a blister, the surface under a crust. It suits organisms living in that moisture, and it is the usual choice where bacterial infection is the question.
A scraping collects the outermost layer itself, using a blunt edge to gather flakes from the active edge of a patch. Where hair or nails are involved, clippings or plucked hairs are taken instead. This suits organisms that live in keratin, which is where common fungal infections make their home.
The active edge matters for a scraping, and it explains why a clinician scrapes the border of a patch rather than its centre. The centre is where activity has usually subsided.
What happens to it?
It goes to a laboratory, where two quite different things may be done with it.
Direct examination looks at the sample under a microscope, sometimes after treatment to clear the keratin, to see whether anything is visible. This can be relatively quick.
Culture attempts to grow whatever is present, which identifies it more precisely and, for bacteria, allows sensitivity to be established. Growth takes as long as the organism takes, and fungal cultures are slow enough that people frequently assume something has gone wrong when nothing has.
Molecular methods exist and are used in some services. Which approach is applied depends on the question and on the laboratory, and it is a reasonable thing to ask about.
How should a result be read?
A positive result identifies what was found, which usually settles the question the sample was taken to answer.
A negative result is more nuanced, and worth understanding before it arrives. It says that nothing was found in what was collected. That may be because nothing is there; it may also be because the sample missed the active area, because treatment already applied had suppressed the organism, or because the amount collected was too small.
This is why clinicians ask what has been applied to the area, and why they sometimes ask that something be paused before a sample is taken. Any such instruction comes from the clinic, is specific to the test, and takes precedence over anything written here.
A negative result is still informative. It points away from one explanation and towards others, which is progress even when it feels like an absence.
What is worth asking?
Ask what the sample is looking for. Swab and scraping are answering different questions, and knowing which one is being asked tells you what a result will and will not settle.
Establish whether anything should be paused before the sample is taken, and get that instruction from the clinic rather than assuming.
Ask when the result is expected and who will contact you. Cultures take longer than most people anticipate, and knowing the interval prevents an anxious fortnight of wondering whether anything has gone astray.
Ask what a negative result would mean in your case. The answer is frequently more interesting than the result itself.
What this page does not do
It names no organism, no test, no treatment and no timescale, and it does not interpret any result. It does not tell anyone to stop or start applying anything before a sample; that instruction comes from the service taking it.
Common questions
What is the difference between a swab and a scraping?
A swab collects moist material from the surface, from a weeping area, a blister or under a crust. A scraping collects flakes of the outermost layer itself, along with hair or nail clippings where relevant. They suit different questions and different organisms.
Does taking a sample hurt?
Generally not. A swab is a light touch and a scraping is a blunt scrape of the surface layer, which most people describe as mildly uncomfortable rather than painful. No anaesthetic is needed and nothing is cut.
Why do results take so long?
Because some tests require growing an organism, and growth takes as long as it takes. Fungal cultures are notably slow. The laboratory cannot report what has not yet grown, which is why an interval that seems excessive is often unavoidable.
Does a negative result mean nothing is there?
Not necessarily. It may mean nothing was found in what was collected, which is a different statement. Recent treatment applied to the area is a common reason a sample comes back negative, and clinicians take that into account when interpreting it.
Why was I asked to stop using a cream before the sample?
Because treatment already applied can suppress or remove what the test is looking for, producing a negative result that means very little. Any such instruction comes from the clinic and is specific to what they are testing for.
Sources
- Ringworm and other fungal infections — conditions — NHS (United Kingdom) · accessed
- Patient information on skin conditions and treatments — British Association of Dermatologists (United Kingdom) · accessed
Last updated . How this page was written.