When is a bruise not a bruise? What do you do with infected-looking skin lesions that don’t respond to antibiotics? Should you trust a negative fungal scraping? Dr Amy Kline (Dermatologist) presented a great talk at the SMA March Medical Update, including a collection of dermatological cases where the diagnosis was missed or delayed, as well as giving us some practical advice on skin investigations. A few of us were left feeling very itchy afterwards!
Tips and Techniques
Fungal skin scrapings can take time to get enough material. The more sample you can supply, the quicker the answer you can get – especially if there is enough fungal material to see it under the microscope straight away without having to wait weeks for the culture. Try using a number 10 blade and go for the hair-bearing areas (the fungus loves hair follicles). You may need to scrape multiple areas. Keep going until you can see clearly visible scrapings in the bottom of the sample jar. Drop the whole blade in to the jar at the end and send the lot off to the lab.
(Number 10 blade, image: SnowBink)
For some difficult skin lesions, you might need to get tissue sent to the lab for histology AND culture to exclude infective causes. This is especially relevant if they aren’t responding to empiric antibiotics. If you are doing this in General Practice, go for a punch biopsy from the edge, trying to get down to dermal tissue and not just the top layer of the lesion / abscess. You may need to send two different samples to get a diagnosis:
- A tissue sample in formalin, for histopathology.
- A fresh tissue sample for culture / infective causes. This can be done by putting the sample in a sterile jar on some sterile gauze damped with saline. The sample doesn’t need to be submerged as this might damage it. Caution: it must be sent straight to the lab as a fresh sample without delay. You might need to ring the lab to ensure they can receive and process it promptly.
- A bruise that doesn’t resolve (especially on the head/neck of the elderly) may not be just a bruise. Consider biopsy.
- Not all that is inflamed is infected. Biopsy and consider neutrophilic dermatoses if cultures are negative and not responding to antibiotics.
- Inflammatory conditions are typically symmetrical, whereas fungal infections are often asymmetrical (e.g. 2 feet 1 hand syndrome).
- Diagnose scabies by looking for the ‘Stealth Bomber’ sign: tiny black triangles on dermatoscopy that don’t wipe off because they are mites burrowed into the skin.