A patients with an itchy rash has suspected contact dermatitis. The standard patch is negative. An expanded patch test by an expert is positive for dust mites, but is this the cause of contact dermatitis?
The short answer is, we don’t know. The diagnostic meaningfulness of patch tests with house dust mite allergens is still questionable.
There long has been debate over the role of the dust mite antigen Dermatophagoides in provoking or perpetuating atopic and other forms of dermatitis, particularly airborne contact dermatitis. Reactivity to Dermatophagoides has been measured by specific IgE and by immediate-type responses to prick test challenge. This is well-established as a diagnostic tool in allergic rhinitis.
More recently, patch testing has been advocated as a means of determining reactivity to Dermatophagoides.
A commercially available mix of 2 species of Dermatophagoides, Dermatophagoides pteronyssinus and Dermatophagoides farinae, recently became available. The authors chose the lower available concentration of this mix (20%) and added it to their standard patch testing tray between December 1999 and January 2001. A total of 662 patients received patch testing to Dermatophagoides (men 32%, women, 68%). Mean age was 53.4 years (range, 37 to 94). The Dermatophagoides mix was the most common allergen in the standard tray to evoke a reaction.
At 96 hours, positive reactions were noted in 55% of patients. The reactions were interpreted as irritant in 16.2%, relevant in 6.7%, of questionable relevancy in 30.5%, and of no relevancy in 32.6%. The majority of reactions were of the "crescendo" pattern.
The authors concluded that the currently available formulation of Dermatophagoides mix, even at its lower concentration, led to a high rate of reactivity with a low rate of relevance.
Contact Dermatitis - An Approach to Treatment (click to enlarge the image).
Rate of patch test reactions to a Dermatophagoides mix currently on the market: a mite too sensitive? Davis MD1, Richardson DM, Ahmed DD. Am J Contact Dermat. 2002 Jun;13(2):71-3.
Atopic Dermatitis, Patch Testing, and House Dust Mites - Medscape, 2008 http://buff.ly/1uDLrB4
Positive patch tests with a dermatophagoides mix relate to an increased responsiveness to standard patch test allergens. Brasch J1, Uter W, Dibo M, Stockfleth E, Swensson O, Christophers E.
Contact Dermatitis. 2002 May;46(5):253-7.