Positive & Negative Controls
Postive Control - Histamine
Since a major physiological component of the allergic response is histamine, it is important to use a histamine reagent to confirm that the patient’s skin has sufficient reactivity to properly interpret the skin test results. In line with this rationale, the histamine positive control can help determine whether a patient is on suppressive medications, such as antihistamines, that may interfere with the skin test response and proper diagnosis. If you see a negative response to histamine, then this raises questions about the validity of the skin test. If this occurs, you could repeat the skin test at a later date, in case there is a residual antihistamine effect. Some practices also order blood tests (IgE), which are not affected by antihistamines.
What is the proper concentration of histamine to use in skin testing?
A 1.0 mg/mL solution of histamine base (2.75mg/mL histamine phosphate) is the recommended concentration for use with skin test medical device. The effect of histamine concentration on skin test reactivity is still under investigation but it appears to be dependent on the choice of skin test device as well as patient variables such as natural reactivity and antihistamine use. It is important to consider how the histamine strength may impact the proper interpretation of skin test results. An overly potent histamine concentration may override a patient’s antihistamine repression and may lead to a false negative diagnosis for some weaker allergens. While a weaker histamine control is more likely to produce a negative response in potentially compromised patients, which is understandably inconvenient to the practice, it might be better to err on the side of caution to avoid false negatives. A positive control that never fails has little diagnostic value.
Negative Control - Diluent
A negative control should be performed to ensure that the patient does not have a condition, such as Dermographism, that could invalidate the skin test results. The appropriate negative control is made up of the diluent of the extracts being tested. If the extracts are glycinerated, then the negative control should be a solution without allergen of the same glycerin concentration.
If you see a positive response to a negative control, then this raises questions about the validity of the skin test.