Investigating a rash

With so many different causes, it can be very difficult to diagnose the reasons behind a rash and offer the appropriate treatment. Diagnosing a rash takes quite a bit of detective work, and like Sherlock Holmes, we dermatologists use all of our skills and equipment to identify the culprit.

Your investigation will take three stages, just like crime detection. First we ask lots of questions to establish the background and possible causes, then we look at the scene (examine the rash), and finally we may do some scientific tests to confirm our theory.

Examining your rash

Lots of information can be gleaned from the appearance of your rash. Rashes may comprise small spots, raised red areas, skin lesions or plaques. The rash may be concentrated, with many small spots, or more widespread with lots of widely spaced spots over a large area. The surrounding skin may be blistered or cracks, or hard and scaly. Some causes produce very distinct and identifiable rashes.

Another telling sign is the distribution of the rash. A very localised rash may be an indication of contact dermatitis, whereas a rash that has spread all over your body is more likely to be an indicator of a different condition, such as urticaria. Some rashes will only appear in creases of the body, such as behind the knees, in the elbows and in the groin, while others can appear in the ears and even in the lining of the mouth.

Finally, and quite strangely, some rashes, such as shingles, will only appear on half of the body, and will not cross the imaginary midline of the torso.

Questions about your rash

A lot of information about the cause of your rash can be gained by discussing the circumstances in which it first appeared, and what you have done or come into contact with in the days and hours before.

We will ask you a number of questions about your rash, such as:

• When did the rash start?
• Where did it start and has it spread?
• Is the rash itchy?
• Is there anything that improves it or makes it worse?
• Have you had a rash like this before?
• Do you have any other symptoms?

We will also ask about what you have done recently that may have caused the rash, including questions such as:

• Are you taking any new medications?
• Are there any new products that have come into direct contact with your skin?
• Have you eaten anything unusual, especially shellfish or nuts?
• Have you travelled overseas, or been outdoors, especially in woodlands?
• Have you been bitten by an insect or tic?
• Have your parents or any other family members ever had a similar rash?

Other tests

Dermatologists are able to gain a lot of information from our questions and a direct examination, and we will often be able to make a diagnosis based on these elements alone. However, if there is room for doubt, we can make use of a whole bank of diagnostic tests:

• Allergy testing – testing small patches of skin with the suspected problem substances to see which of these cause a reaction
• Blood tests – these can be used to identify or exclude skin conditions
• Skin swabs for bacterial culture
• Skin scraping for fungal culture
• Hair samples for microscopy and fungal cuulture
• Skin biopsy – this is a detailed examination of a small section of your skin under a microscope to  assess your rash more accurately.