2A: Describe the use of skin allergy tests

Question 1

Is patch testing used to determine the cause of irritant contact dermatitis, allergic contact dermatitis, or both?

Answer 1

Only allergic contact dermatitis

Question 2

What type of hypersensitivity does patch testing test for?

Answer 2

Type 4 reaction

delayed

Question 3

What patients are candidates for patch testing?

Answer 3

1) patients with persistent eczematous eruptions
2) patients with chronic leg ulcers

patients with chronic leg ulcers can become sensitized to the components of dressings, bandages, or topical medications

Question 4

How is a patch test carried out?

Answer 4

1) chambers are labelled and prepared with allergens
2) the chamber strips are placed on the patient’s back and secured with tape on day 0
3) the chamber strips are removed and read on day 2
4) the patch test is read again for a final time on day 4

Question 5

Interpret this patch test result

Answer 5

Weak positive

+

Question 6

Interpret this patch test result

Answer 6

Moderate reaction

++

Question 7

Interpret this patch test result

Answer 7

Strong reaction

+++

Question 8

What is this?

Answer 8

an ‘angry back’ reaction

multiple reactions due to eczema being too active

Question 9

What do positive patch tests produce?

Answer 9

A patch of dermatitis at the application site of the allergen

Question 10

What type of hypersensitivty does skin prick testing detect?

Answer 10

Type 1 hypersensitivity

immediate

eg. atopy

Question 11

True or false?

Skin prick testing can be helpful in the investigations of atopic eczema

Answer 11

False

Question 12

How is a skin prick test performed?

Answer 12

1) drops of allergens are placed on marked areas on the arm
2) intradermal punctures are made through each drop with a different needle
3) histamine is used as a positive control
4) saline is used as a negative control

Question 13

What do positive skin prick tests produce?

Answer 13

an urticarial wheal >4mm after 10-15 minutes

The larger the wheal, the more severe the hypersensitivity

Question 14

True or false?

Skin prick testing is not indicated for patients with dermatitis

Answer 14

True

as an exception, some patients with hand dermatitis may have a type 1 and type 4 hypersensitivity to natural rubber latex

Question 15

Who should skin prick testing be offered to?

Answer 15

Patients with acute urticarial reactions

skin prick testing unhelpful in chronic urticaria