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Chris C.
Chris C., Nurse
Category: Health
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Experience:  Nurse with with 10+ yrs. in wellness care, geriatrics, hospice and acute care.
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I have just broken out in an itchy rash behind my knees. The

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I have just broken out in an itchy rash behind my knees. The rash came up in a matter of minutes. What is the difference between contact dermatitis and hives? So basically how can I tell if it's something I've eaten or something I've worn?
Submitted: 1 year ago.
Category: Health
Expert:  Dr. Saha replied 1 year ago.

Thank you for your question.
I am Dr Saha, I shall try my best to help you.
It can be difficult to determine whether you are having contact dermatitis or hives.
If you remember anything or irritant coming in contact with skin, then it can help.
In contact dermatitis there can be itching, burning, rash.
Blister may form if the irritation is severe.
You can try benadryl or claritin for itching.
Hydrocortisone cream can be applied.
If you do not see improvement, I would suggest to take an examination by doctor.
I hope this information helps.
If there’s anything else I can do, please let me know.
If you are satisfied, please rate my answer as good or excellent.

Expert:  Chris C. replied 1 year ago.

With the sudden onset and not knowing of any exposure to new items the general rules are as follows since both show similar rash patterns, itching, etc.

Individuals with allergic contact dermatitis typically develop dermatitis, within a few days of exposure, in areas that were exposed directly to the allergen. Certain allergens (eg, neomycin) penetrate intact skin poorly, and the onset of dermatitis may be delayed up to a week following exposure.

A minimum of 10 days is required for individuals to develop specific sensitivity to a new contactant. For example, an individual who never has been sensitized to poison ivy may develop only a mild dermatitis 2 weeks following the initial exposure but typically develops severe dermatitis within 1-2 days of the second and subsequent exposures. The immediate onset of dermatitis following initial exposure to material suggests either a cross-sensitization reaction, prior forgotten exposure to the substance, or nonspecific irritant contact dermatitis provoked by the agent in question.

Immediate reactions, ie, visible lesions developing less than 30 minutes after exposure, indicate contact urticaria "hives" (not allergic contact dermatitis). This is particularly true if the lesions are urticarial in appearance and if the skin reaction is associated with other symptoms, such as distant urticaria, wheezing, ophthalmedema, rhinorrhea, or anaphylaxis.

So basically the easiest way to know with some certainty is that if it is a sudden onset without exposure to a new or know substance you are allergic too it is likely hives and if it does cause other symptoms in the body this reinforces it being hives.

If there has been no new or know exposures then it is likely a delayed reaction and in most cases contact dermatitis.