BOSTON -- Intradermal allergen testing adds no useful information when evaluating patients for cat allergies, since the results are usually false when they contradict those of skin-prick testing, a researcher said here.
In a study of 55 individuals whose actual cat allergy status was known conclusively, adding intradermal testing as a follow-up in patients with negative results on skin-prick testing actually decreased the diagnostic specificity without improving the sensitivity, which was already 100% on the basis of skin-prick results alone, said Jennifer Cole, DO, of Walter Reed National Military Medical Center in Bethesda, Md.
Action Points
- Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.
- Note that in this small study, skin prick testing was an extremely sensitive tool for the diagnosis of cat allergy with rhinoconjunctivitis when evaluating patients with a high pre-test probability of cat allergy.
- Note also that in contrast, intradermal skin testing was associated with a high rate of false positives and did not improve the rate of detection of true positives.
Because the study was relatively small -- only 13 of the 55 participants were found to be true positives for cat allergy -- Cole said she, "hesitate[d] to draw firm conclusions."
But, she told attendees at the American College of Allergy, Asthma, and Immunology, the results suggest that intradermal testing "has no diagnostic utility" in patients with suspected cat allergy, beyond what can be gleaned from cheaper and simpler skin-prick tests.
In many clinics, Cole explained, intradermal testing is routinely performed in patients with possible cat allergy who are negative with skin-prick testing. But the actual value of the intradermal follow-up had not been rigorously evaluated.
Her study involved 22 individuals with a clinical history of likely cat allergy and 33 controls with no such history. Participants then underwent challenge testing with cat allergen (the Fel d1 protein) placed in the conjunctiva of the eye, an exquisitely sensitive area for cat-allergic individuals. Cole described the conjunctival test as the "gold standard" for cat allergy diagnosis.
Among the 22 with initially suspected allergy, the challenge testing confirmed it in 13. Two of the control group had reactions during the challenge, leaving 31 confirmed to be negative for cat allergy.
Participants then underwent skin-prick testing, with intradermal testing performed on those with negative results.
All 13 participants with genuine cat allergy had positive skin-prick results, meaning that skin-prick testing was 100% sensitive. Among the 31 genuinely negative controls, two had positive skin-prick results, for a specificity of 93.5%.
Among the 29 controls with negative skin-prick results, 24 also had negative intradermal results. The other five had to be considered false since the controls all were confirmed negative with the challenge testing.
These false positives meant that the combined testing had a specificity of only 77.4%, Cole said.
The positive predictive value was also lower with the sequential testing than with skin-prick testing alone (65.0% versus 86.7%, respectively). The sensitivity and the negative predictive value of skin-prick testing alone was 100% and therefore could not be improved with intradermal testing, Cole noted.
She emphasized that, in addition to the small sample size, a limitation of the study is that conjunctival testing or other highly accurate diagnostic procedures such as "cat rooms" are seldom used in routine practice.
"Skin testing is usually interpreted in the context of clinical history," she said, which has substantially lower sensitivity. Thus it's possible that intradermal testing could be more useful than it appeared from her study results.
Session co-moderator Mark Dykewicz, MD, of Wake Forest Baptist Medical Center in Winston-Salem, N.C., commented that a member of his own lab had reviewed intradermal testing accuracy for other forms of allergy and found similar results.
In a study of 56 patients, he said, "16 [allergic] to all allergens had the profile of having a negative prick but a positive intradermal, and none of those patients, when we actually did nasal challenge, had a positive response."
He added that there was now a "considerable literature" suggesting that intradermal testing is unnecessary as a follow-up to skin-prick testing.
Disclosures
The study had no external funding. Ora Inc. allowed the researchers to use its assessment method for conjunctival allergy testing.
Cole declared she had no relevant conflicts of interest. She also indicated that her report did not necessarily reflect official positions of the U.S. Army or Department of Defense.
Dykewicz reported relationships with Merck and Shire.
Primary Source
Annals of Allergy, Asthma, and Immunology
Source Reference: Cole J, et al "Intradermal skin testing does not improve the diagnosis of cat allergy" Ann Allergy Asthma Immunol 2011; 107: A11-12.