A comparison of skin endpoint titration and skin-prick testing in the diagnosis of allergic rhinitis.
Among the many methods of allergy diagnosis are intradermal testing (IDT) and skin-prick testing (SPT). The usefulness of IDT has been called into question by some authors, while others believe that studies demonstrating that SPT was superior might have been subject to bias. We conducted a study to compare the validity of SPT and IDT--specifically, the skin endpoint titration (SET) type of IDT--in diagnosing allergie rhinitis. We performed nasal provocation testing on 62 patients to establish an unbiased screening criterion for study entry. Acoustic rhinometric measurements of the nasal responses revealed that 34 patients tested positive and 28 negative. All patients were subsequently tested by SET and SPT. We found that SPT was more sensitive (85.3 vs 79.4%) and more specific (78.6 vs 67.9%) than SET as a screening procedure. The positive predictive value of SPT was greater than that of SET (82.9 vs 75.0%), as was the negative predictive value (81.5 vs 73.0%). None of these differences was statistically significant; because of the relatively small sample size, our study was powered to show only equivalency. The results of our study suggest that the information obtained by the SET method of IDT is comparable to that obtained by SPT in terms of sensitivity, specificity, and overall performance and that both SET and SPT correlate well with nasal provocation testing for ragweed. Therefore, the decision as to which to use can be based on other factors, such as the practitioner's training, the desire for quantitative results, the desire for rapid results, and the type of treatment (i.e., immunotherapy or pharmacotherapy) that is likely to be chosen on the basis of test results.
Allergic rhinitis is one of the most common diseases in the United States, affecting 30% or more of the general population. (1) It is a source of significant morbidity; symptoms as wide ranging as rhinorrhea, cough, sneezing, itching, runny eyes, sinusitis, and generalized fatigue considerably diminish quality of life. Its economic costs are also considerable--not only with respect to the amount of money spent on treatments, but also with regard to the amount of time lost from school and work and decreased productivity.
The choice of treatment for allergic rhinitis is usually based on the severity of symptoms. In milder cases, decongestants or simple avoidance of the offending allergen can provide relief. Patients who are unable to control their exposure or tolerate their symptoms may opt for a nonsedating antihistamine and/or other medications, such as topical or systemic corticosteroids. Unfortunately, some patients are unable to achieve relief even with maximal pharmacologic therapy, and it is they who are most often referred for further allergy evaluation. These patients may be candidates for immunotherapy, which usually entails systematic desensitization to the offending antigens. Immunotherapy has been shown to be effective for the vast majority of patients who choose it, provided that they are diagnosed correctly. For these patients, finding the offending antigens is critical. After a careful history and physical examination, skin tests or in vitro tests are frequently employed. (2)
Among the factors that must be considered in evaluating the appropriateness of any given test are its safety (minimal risk of anaphylaxis), sensitivity, specificity positive and negative predictive values, reproducibility correlation with symptoms, and ease of administration The allergy testing modalities that are available to the clinician include in vitro testing (IVT), skin-prick testing (SPT), intradermal testing (IDT) (including skin endpoint titration [SET]), and nasal provocation testing (NPT).
In vitro testing. Several types of IVT are available to measure a patient's serum total and allergen-specific immunoglobulin E (IgE) levels. One of the earliest was the radioallergosorbent test (RAST), which involved the placement of a radioactive label on an anti-IgE antibody. During this assay, the patient's specific IgE remains bound to antigen in a test tube, where its relative abundance tan be measured. An effort to improve the sensirivity of RAST led to the development of the modified RAST (mRAST). Further advances in IVT involved the development of fluorometric or colorimetric labels, which spare technicians from exposure to radiation.
Among the advantages of IVT are its safety and sensitivity. It is particularly useful in (1) patients who are taking a beta blocker, because skin testing in these patients tan be dangerous and (2) patients who are on an antihistamine, because skin testing in these patients is unreliable. (3) Patients with dermatographism can be better assessed by IVT, as can children who balk at skin testing. IVT is slightly more expensive and slightly less sensitive than SPT, but it is slightly more specific. However, results are not immediately available.
Skin-prick testing. SPT was first described in the 1920s and popularized by Pepys in the 1970s. (4) These tests are well regarded because they are specific, sensitive, convenient, and relatively safe. In addition, SFT correlates well with IDT. SPT is performed by introducing a droplet of antigen approximately 1 mm deep into the skin with a high-gauge needle. This technique allows the antigen to cause a superficial skin reaction without entering the dermis. SPT requires that a positive control (histamine) and a negative control (saline with or without glycerin) be introduced into the skin at distinctly separate sites. (5)
In recent years, SPT has been modified to allow for the introduction of larger quantities of antigen. Needles with longer bevels can be used to deposit antigen into the dermis, and multiprong devices such as the Multi-Test instrument (Lincoln Diagnostics; Decatur, Ill.) can be used to test as many as eight antigens at once. With their longer prongs, these devices penetrate the skin somewhat more deeply than do conventional skin-prick instruments, and they arc considered by some to more closely resemble intradermal tests.
Individual skin-prick tests have been shown to correlate well with mRAST and with SET (81 to 89% for the latter). (6,7) However, studies have shown that multiprong devices do not correlate as well. (8) For example, Levine et al round a disparity in results obtained by mRAST and the Multi-Test device. (9) Although there was good agreement with respect to negative results in that study, agreement occurred in only 56 to 67% of positive tests.
Other advantages of SPT are its relative ease of use, short hands-on time, and quick results. One of the draw-backs of SPT is patient discomfort; intertester reproducibility may also be a problem. Other variables--including the specific sites being tested, the distance between test sites, and the consistency of technique--may confound results, as well. (5,9) The use of nonstandardized allergen extracts leads to further difficulties. Finally, there are several different scales used to read results.
Intradermal testing. IDT has been in use since the early part of the 20th century. (5) Dilute antigen extract is injected into the dermis, and a superficial wheal forms. This technique causes minimal patient discomfort. IDT is very sensitive and more reproducible than SPT, primarily because a larger volume of a known antigen concentration can be introduced. Allergists disagree as to whether IDT correlates well with clinically significant allergens and with SPT. (10) Moreover, there are questions about its specificity; in patients with a negative SPT, it is hot clear whether a reaction to an IDT represents a true positive or a local skin reaction (false positive).
Disadvantages that are unique to standard IDT include the need for more rime, labor, and supplies. Also, standard IDT is potentially more dangerous than SPT if it is not performed judiciously. The greater volume of antigen that is delivered is more likely to precipitate an anaphylactic reaction. Therefore, some authors recommend that SPT be performed prior to IDT. (10,11)
Skin endpoint titration. SET is a modified method of IDT in which serially diluted antigen extracts are used to determine the minimum amount of antigen that is required to elicit a significant allergic response, quantified by comparing the size of wheal increase with that evoked by a negative control test. The endpoint response is reached upon the determination of the lowest concentration of antigen that produces a wheal (1) that is the first wheal more than 2 mm larger than the negative control wheal and (2) that is followed by a second wheal that is at least 2 mm larger than the preceding one. The endpoint signifies the degree of sensitivity and indicates a safe starting point for immaunotherapy.
As a form of IDT, SET carries the same advantages as nontitrated IDT, and it has the additional benefit of being more quantitative. However, SET is time-consuming and is somewhat uncomfortable for the patient. But these disadvantages are often outweighed by the fact that SET is the only skin test that provides a semiquantitative assessment of a patient's allergen sensitivity. A modified form of SET is currently used by the federal Food and Drug Administration to validate the potency of allergenic extracts. (12)
Tandy et al reported that the correlation between SET and mRAST was 82.2%. (13) At dilutions of 1:12,500 (dilution #4) and 1:312,500 (dilution #6), SET is roughly equivalent to SPT in terms of the volume of antigen delivered. By comparison, standard IDT uses dilutions of 1:100 (equivalent to SET dilution #1) to 1:1,000 (between SET dilutions #1 and #2). In common practice, SET is started at the #6 dilution for seasonal allergens and at the #4 dilution for out-of-season allergens; both are comparable to SPT dilutions.
Nasal provocation testing. NPT is a direct method of assessing whether a given antigen produces symptoms of allergie rhinitis when it is introduced into the nasal mucosa. Passali and Bellussi characterized NPT as a "third-level" method of investigation, to be used only when nasal allergy i s suspected. (14) They asserted that skin testing and serum assays can establish a diagnosis in most patients, but provocation can localize previously primed toast cells in the "organ of shock"--that is, the nose. Their protocol involves three steps. First, anterior rhinomanometry is performed to rule out nasal stenosis. Second, lactose is insufflated into the nasal fossa and rhinomanometry is repeated. Third, Iyophilized antigen is introduced into the fossa. Rhinomanometric measurements are taken 10 to 15 minutes following the introduction of the antigen.
Another method of NPT is the disk method, which involves the administration of oxymetazoline to reduce mucosal edema. Afterward, an allergen-impregnated filter-paper disk is placed on the nasal septum. The disk releases antigen, and nasal secretions are collected at predetermined intervals. The disks are weighed before and after insertion into the nose, and mediators are eluted from the disk and quantified. The disk method is very labor-intensive and time-consuming. (15,16)
A third way of performing NPT is to introduce allergen into the nares via either a suspension spray or a filter-paper disk. After 10 minutes of reaction rime, the degree of mucosal swelling is assessed by either acoustic rhinometry or anterior rhinomanometry. (10)
Nasal provocation can be eschewed in favor of bronchial provocation, but the upper airway test has several distinct advantages. First, the nasal mucosa is readily available. Second, controlled provocations with various stimuli are easy to perform. Third, collecting specimens and measuring mucosal responses can be easily accomplished without the risk of inducing bronchospasm. (17)
Although SET has become the testing modality of choice for many otolaryngologists, some advocates of SPT continue to dismiss it without acknowledging its inherent benefits. As a result, SET has not always been accurately represented in recent comparative studies between it and SPT, standard IDT, IVT, and/or NPT. In some studies, SPT has been used as the gold standard. (10,18-20) However, allocation of subjects into different groups on the basis of SPT findings introduces a preselection bias into the investigation. Another factor that can hinder interpretation of results is introduced when only patients with negative SPT results are evaluated by IDT, despite the fact that one of the goals of the study is to compare SPT with IDT. Selecting a qualitative IDT technique other than the semiquantitative SET also introduces prejudice. Finally, NPT--which is the gold standard for determining the clinical relevance of a particular allergen has been used to assess differences between preselected groups on the basis of SPT results. Using NPT as the gold standard for determining the presence or absence of nasal allergy and for determining skin responses would yield more relevant information. Indeed, positive NPT results have been reported in as many as 44% of SPT-negative groups. (10) The results of previous studies of the value of IDT in the assessment of clinically relevant inhalant allergy are misleading and might have been biased in favor of SPT. (10,18-20)
To show that a group of patients has a clinically relevant sensitivity to any pollen, researchers can use one of two methods. One is to initiate nasal provocation with pollen grains, and the other is to correlate each patient's daily symptom scores and medication intake with daily pollen counts during the pollen season. The underlying assumption is that a patient with a clinically significant allergy to a pollen should report increased symptom severity following exposure to the pollen, either in the laboratory or in the environment.
We hypothesized that both SET and SPT correlate well with NPT. By recruiting study patients prospectively and performing all three types of test on every patient, we were able to use NPT as the gold standard without introducing selection bias into our study. Our objective was to determine if SPT should precede SET because of SPT's greater degree of sensitivity and specificity or if SPT and SET are equally reliable, in which case either could be used as a first-line assay.
Patients and methods
We recruited 62 patients aged 18 years or older who had a history of allergy to ragweed--specifically, seasonal symptoms that were temporally associated with ragweed pollination. We eliminated the chance that unpredictable priming would occur as a result of natural exposure to ragweed pollen by recruiting patients who were currently asymptomatic and by conducting out study outside the ragweed season (although we could hot eliminate the possibility of priming caused by exposure to allergens other than ragweed). We did not accept patients who had a history of asthma or nasal polyps, or those who had received immunotherapy for ragweed at any time during the preceding 10 years. For study purposes, allergy medications were discontinued as follows: oral decongestants for 3 days, most antihistamines for 3 days (except for astenfizole [3 months] and cetirizine [7 days]), topical and systemic steroids for 2 weeks, and nasal cromolyn for 1 week. All patients were free of upper respiratory infections during the week prior to testing. Out research protocol was approved by the Institutional Review Board at the University of Chicago. All study patients signed a document of informed consent prior to participating in the study.
We first obtained baseline measurements of nasal volume by acoustic rhinometry. Next, we performed a diluent provocation (0.9% saline with 0.4% phenol) to assess each patient's response. An abnormal response would have cast doubt on that particular patient' s future findings and would have disqualified him or her from the study; fortunately no response was abnormal. Patients were then assessed by NPT, SET, and SPT. NPT was always performed first to allocate patients into study groups. Patients were then randomly assigned to receive either SET first and SPT second or vice versa.
Immediately following provocation and before acoustic rhinometry, each patient completed a questionnaire to indicate the number of sneezes, and each made a mark on a visual analog scale to indicate the severity of his or her symptoms, including runny nose, stuffy nose, sneezing, and nose or throat itching (figure 1). All patients under went NPT to determine their clinical sensitivity to ragweed.
Clinical sensitivity was assessed with increasing amounts of ragweed pollen; dilutions progressed from 1:10,000 to 1:3,000 to 1:1,000 and finally to 1:300. Dilutions of the standardized skin-test extract were prepared in a 1/20 weight per volume (w/v) ragweed concentrate in phenolated saline. The solution was sprayed twice into each nostril with a spray pump identical to those used for aqueous nasal sprays. The clinical response to the provocation was monitored by acoustic rhinometry measurements taken 10 and 25 minutes after each antigen dose. Thirty minutes were allowed to elapse between provocations. We measured the volume of the nasal cavity after each administration of a different dilution. After each challenge, we recorded the number of sneezes that occurred during the first 5 minutes.
We assumed that an NPT result was positive if we detected a 15% reduction in volume after 10 minutes by acoustic rhinometry. Although the 15% threshold was arbitrary, it was based on the fact that variability between acoustic rhinometry readings approaches 10%; therefore a 15% change would represent a real change in volume and would be a sale distance from values that might have represented artifactual changes resulting from operator-dependent factors. (21) The amount of decrease in volume is reflective of the degree of congestion and inflammation caused by the allergie response, and thus is an indicator of response.
Both NPT-positive and NPT-negative patients under went SET and SPT to determine their sensitivity to ragweed, white ash, timothy grass, meadow fescue, and either marsh elder weed/white oak or dust mite/cat. Our allergen panel changed slightly over time as we modified it to identify patients who were "out of season" for this testing.
All skin tests included positive (histamine) and negative (glycerinated saline) controls. SPT was performed with a standard Multi-Test applicator and 1/20 w/v standardized extracts. The wheal response was read in 20 minutes. Results were scored according to the grading system described by Van Metre et al. (22)
SET was performed with techniques approved by the American Academy of Otolaryngic Allergy and Foundation. (23) Each intradermal injection of five-fold dilution of the 1/20 w/v ragweed concentrate was delivered by a 27-gauge, 1/2-inch, beveled hypodermic needle, starting with 0.01 ml of the #6 dilution (1:312,500). Wheal sizes were read 10 minutes later and compared with those raised by the negative control injections. Then the next strongest dilution was administered and read. We stopped SET when we reached the endpoint--that is, when successive injections raised wheals whose diameters were 2 mm or more larger than the preceding wheal, including the control.
We analyzed our data by using sensitivity and specificity calculations with NPT as the gold standard. We performed a receiver-operator characteristics (ROC) curve analysis according to a method described by DeLong et al (24) with STATA statistical software. ROC analysis can help assess the utility of a diagnostic tool in clinical practice. In a ROC graph, they axis represents the true positives (sensitivity) and the x axis represents the false positives (1-specificity). ROC analysis allows one to determine a threshold value or critical point from which results can be considered positive or negative. For our analysis, variable cut-off points for SPT and SET were based on the respective grading system of each test. For SET we used variable cut-off points of 0, 1, 2, 3, 4, 5, and 6, and for SPT we used cut-off points of 0, 1, 2, 3, and 4. At each cut-off point, the accuracy rate was calculated independently and used in the construction of the ROC graph.
After we obtained data on all patients, we assigned them to one of eight groups based on their NPT, SET, and SPT results (table 1). Of the 62 patients in our study, 34 had positive NPT results and 28 had a negative test. Of the 34 NPT-positive patients, 27 had positive SET results and 29 had positive SPT results (table 2).
We then calculated the sensitivity, specificity, positive predictive value, and negative predictive value of the SET and SPT results. In addition, we calculated the probability that a test would be negative when disease was present and the probability that a test would be positive when disease was not present. Although both tests were accurate, we found that there was a trend favoring SPT; however our sample was not large enough to show statistical significance (table 3).
In the ROC analysis, the area under the curve for SET was 0.7773 and the area under the curve for SPT was 0.8361 (figure 2). Although the difference was not statistically significant (p = 0.1276), our sample size was not large enough to allow for statistically significant comparisons. Nevertheless, the two sets of area under the curve were consistent and similar. In a ROC analysis, the similarity of the shape of the curves and their closeness reflects the concordance of the two tests in terms of negative and positive diagnoses.
In some previous articles on this subject, authors have claimed that IDT is unnecessary or even dangerous if SPT is used. (10,11) Although these authors claimed that they used NPT as the gold standard, they actually used SPT; they performed NPT or IDT only when SPT was negative. This might make sense clinically because this is how most physicians practice; most clinicians believe that IDT is the more sensitive modality. As a result, in 1993, Nelson et al recommended that IDT not be used. (10) However, they did hot take into account the many variables inherent in skin-testing methods, especially in IDT. Our data suggest that the information obtained by the SET method of IDT is comparable to that obtained by SPT in terms of sensitivity, specificity, and overall performance.
Compared with the other tests, treatment based on SET is believed to provide symptom relief much more rapidly because the endpoint is the symptom-relieving dose. (5,6) The safety of immunotherapy based on SET has been proven by the fact that the rate of associated adverse reactions is extremely low. (6) Treatment based on RAST is mathematically equivalent to SET-based treatment, and relief is rapid and safe. The results of our study suggest that bath SET and SPT correlate well with NPT for ragweed. Therefore, the decision as to which to use can be based on other factors, such as the practitioner's training, the desire for quantitative results, or the desire for rapid diagnosis, and the type of treatment (i.e., immunotherapy or pharmacotherapy) that is likely to be chosen on the basis of test results.
Table 1. Assignment of patients based on the results of nasal provocation testing (NPT), skin endpoint titration (SET), and skin-prick testing (SPT) results Group n NPT SET SPT 1 26 Positive Positive Positive 2 1 Positive Positive Negative 3 3 Positive Negative Positive 4 4 Positive Negative Negative 5 19 Negative Negative Negative 6 6 Negative Positive Positive 7 3 Negative Positive Negative 8 0 Negative Negative Positive Table 2. Results of skin endpoint titration (SET) and skin-prick testing (SPT) in the 62 patients, using nasal provocation testing (NPT) as the gold standard NPT results (no. patients) Positive Negative Total Positive SET 27 9 36 Negative SET 7 19 26 Total 34 28 62 Positive SPT 29 6 35 Negative SPT 5 22 27 Total 34 28 62 Table 3. Comparison of selected reliability factors with skin endpoint titration (SET) and skin-prick testing (SPT) SET SPT (%) (%) Sensitivity 79.4 85.3 Specificity 67.9 78.6 PPV * 75.0 82.9 NPV * 73.0 81.5 PNP * 21.0 15.0 PPN * 32.0 21.0 * PPV = positive predictive value; NPV = negative predictive value; PNP = probability that a test would be negative when disease was present: PPN = probability that a test would be positive when disease was not present.
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Anil Gungor, MD Rizwan Moinuddin, MD Steven M. Houser, MD Bulent Mamikoglu, MD Benjamin F. Aquino, MD Jacquelynne P. Corey, MD, FAAOA, FACS Imran Akbar, BA
From the Section of Otolaryngology, Department of Surgery, Pritzker School of Medicine, University of Chicago.
Reprint requests: Jacquelynne P. Corey, MD, Associate Professor, Section of Otolaryngology, Department of Surgery, Pritzker School of Medicine, University of Chicago, MC 1035, 5841 S. Maryland Ave., Chicago IL 60637. Phone: (773) 702-0382; fax: (773) 702-6809; e-mail: email@example.com
The study described in this article was funded by a combined grant from the American Academy of Otolaryngic Allergy and Foundation and the American Academy of Otolaryngology-Head and Neck Surgery. Dr. Gungor, Dr. Houser, and Dr. Mamikoglu received partial support via a grant from Hoechst Marion Roissel (now Aventis Pharmaceuticals).
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|Author:||Corey, Jacquelynne P.|
|Publication:||Ear, Nose and Throat Journal|
|Date:||Jan 1, 2004|
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