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The screening RAST: Is it a valid concept?
The screening RAST: Is it a valid concept? ANDREW J. LEHR,MD, RICHARD L. MABRY,MD, and CYNTHIA S. MABRY,RN, BSN,Dallas, Texas Dr. William King in 1982 advocated the use of a "miniscreen" panel of six antigens to costeffectively initiate allergy testing. In a study of 100 consecutive patients, we found that a "midiscreen" of nine antigens was more sensitive and efficient and more accurately identified negative responders. However, the miniscreen was also effective if adjusted for regional antigen differences. (©tolaryngol Head Neck Surg 1997;I17:54-5.)
| n 1982 Dr. William King 1 presented a retrospective analysis of almost 500 radioallergosorbent test (RAST) batteries and concluded that a cost-effective initial inhalant allergy evaluation could be obtained by the use of a screening panel of antigens consisting of one grass, one weed, one tree, two molds, and one dust mite (with epidermals added if indicated by history). Further testing for relevant antigens could be carried out depending on the results of this initial screen, but a negative screening RAST was stated to be 94% sensitive and 96% efficient. In the practice of one of the authors (R. L. M.), a clinical impression revealed that such a screen failed to indicate the presence of significant inhalant allergy in a number of patients. Therefore over the years the initial screen was expanded to an eventual panel of 15 relevant antigens (including epidermals). To assess the effectiveness of a limited initial screening RAST, we analyzed the results of a large, randomly selected group of RASTs performed in our practice. A comparison of diagnostic effectiveness was made among the following panels (excluding epidermals): a 6-antigen screen, a 9-antigen screen, and a 13-antigen screen. METHODS One hundred consecutive modified RASTs (using a radioisotope marker) performed in the practice of one author (R. L. M.) were reviewed. Patients undergoing
From the Department of Otorhinolaryngology,University of Texas Southwestern Medical Center. Presented at the Annual Meeting of the American Academy of OtolaryngicAllergy,New Orleans, La., Sept. 14, 1995. Reprint requests: Richard L. Mabry, MD, Professor, Department of Otorhinolaryngology, UTSMC, 5323 Harry Hines Blvd., Dallas, TX 75235-9035. Copyright © 1997 by the American Academy of OtolaryngologyHead and Neck SurgeryFoundation, Inc. 0194-5998/97/$5.00 + 0 23/1/72889 54
RAST testing had symptoms and/or signs suggestive of allergic rhinitis or inhalant allergy. Each RAST panel consisted of the following 13 antigens (excluding epidermals): two grasses (Bermuda and timothy), two weeds (ragweed and marsh elder), four trees (mountain cedar, oak, elm, and pecan), three molds (Cladosporiurn, Alternaria, and Helminthosporium) and two dust mites (Dermatophagoides pteronyssinus and Derma-
tophagoides farinae ). Standard F/N class scores were obtained for the above 13 antigens in all patients. Equivocal scores (class 0/I) were counted as positive based on the experience of one of the authors (R. L. M.) that many of these equivocal antigens will prove positive on subsequent skin end-point titration testing. 2 This is in contrast to King's I consideration of class II reactors and higher as positive and class I reactors as equivocal. The above 13-antigen screening RAST was then compared in sensitivity and efficiency to the following smaller screens: a 6-antigen "miniscreen" and a 9-antigen "midiscreen." The miniscreen consisted of one grass (timothy), one weed (common ragweed), one tree (oak), two molds (Alternaria and Helminthosporium), and one dust mite (D. farinae), whereas the midiscreen consisted of two grasses (Bermuda and timothy), one weed (common ragweed), two trees (oak and mountain cedar), three molds (Alternaria, Helminthosporium, and Cladosporium), and one dust mite (D. farinae). Using the full panel as standard, we calculated sensitivity by dividing the number of true-positive results by the sum of true-positive and false-negative results and calculated efficiency as the quotient of the sum of the true-positive and true-negative results divided by the total number of tests. RESULTS Twenty-one patients had negative results on the 13antigen screening RAST. Of the remaining 79 patients with positive results on the 13-antigen panel (i.e., F/N class 0/I or greater) 72 would have had positive results
OtolaryngologyHead and Neck Surgery
LEHR et al. 55
Volume 117 Number I
Table 1. Sensitivity, efficiency, a n d predictive value
of negative results for three screens (standard, 13antigen screen) Test
Sensitivity
Miniscreen 72/79 (91) (6 antigens) Midiscreen 78/79 (99) (9 antigens) Miniscreen 76/79 (96) (mountain cedar)
Efficiency(%)
Predictive value (%)
93/100 (93)
21/28 (75)
99t100 (99)
21/22 (95)
97/100 (97)
21/24 (88)
detected by the miniscreen panel, and 78 would have positive results detected by the midiscreen. The 6-antigen miniscreen thus demonstrated 91% sensitivity and 93% efficiency and a 75% predictive value of a negative result, whereas the 9-antigen midiscreen produced 99% sensitivity and 99% efficiency and a 95% predictive value of a negative result (Table 1). Of note, the only patient missed by the midiscreen demonstrated only an equivocal allergy (F/N class 0/I) to cat and had negative results to all other antigens. Analysis of the above data also revealed that if mountain cedar had been substituted for oak in the 6-antigen miniscreen, the sensitivity and efficiency of this screen would have increased to 96% and 97%, respectively (Table 1). Although epidermals were positive: in 42% of patients (cat, 38%; dog, 31%), the incidence of either of these being the lone positive antigen was quite small. As mentioned above, only I (2%) of 42 patients demonstrating epidermal allergy would have not been detected by the above 9-allergen midiscreen, and only 2 (5%) of 42 would have been missed by the 6-allergen miniscreen. DISCUSSION
The RAST has proved to be a convenient, safe allergy screening test providing quantitative data that can be used in the same fashion as skin end-point titration for the initiation of immunotherapy. In addition, RAST testing offers specificity superior to that of in vivo testing and is unaffected by ongoing pharmacologic therapy.3 The primary disadvantage of RAST is the cost.
Current prices in RAST testing regularly fall in the range of $15 to $20 per antigen. However, considering the fact that a negative RAST screen may obviate the need for skin testing and a positive RAST screen may be used to strategically guide any necessary additional RAST or skin testing, it can be argued that RAST testing may be quite cost-efficient in many circumstances. 4 Regardless, RAST testing adds an additional expense to the diagnosis and management of the atopic patient., and finding ways to limit this expense may make it more palatable to its critics. The most obvious means to reduce the cost of RAST testing is by limiting the number of antigens screened. This study confirmed King's 1 findings of the efficacy of a screening RAST. Although the 6-antigen screen in this study provided 91% sensitivity and 93% efficiency, the 9-antigen midiscreen essentially offered the same sensitivity and efficiency (nearly 100%) of the expanded 13-antigen screen. Furthermore, tailoring the antigens selected for a pa~icular screen to the most prevalent and likely offenders in a particular region may allow reduction of the number of antigens screened without concomitant loss of sensitivity or efficiency. For example, substitution of mountain cedar for oak in our 6-antigen miniscreen increased the sensitivity and efficiency of this screen to 96% and 97%, respectively. With an estimated cost of approximately $20 per antigen in RAST testing, use of a midiscreen or miniscreen could result in a $120 to $180 savings per patient, respectively. In conclusion, a screening RAST does appear to be a valid concept. Careful selection of a 6- or 9-antigen screening RAST can provide comparable sensitivity and efficiency to expanded antigen screens at a much reduced cost. REFERENCES
1. KingWP. Efficacyof a screeningradioatlergosorbenttest. Arch Otolaryngology1982;108:78l-6. 2. MabryRL, MabryCS. Significanceof borderlinelevelsof specific IgE obtainedby FAST-Plusassay. OtolaryngolHead Neck Surg 1992;106:250-2. 3. MabryRL. Blendingskin endpointtitrationand in vitromethods in clinicalpractice.OtolaryngolClin NorthAm 1992;25:61-70. 4. NalebuffDJ. Use of RAST screeningin clinical allergy: a costeffective approach to patient care. Ear Nose Throat J 1985;64:15-35.
| n 1982 Dr. William King 1 presented a retrospective analysis of almost 500 radioallergosorbent test (RAST) batteries and concluded that a cost-effective initial inhalant allergy evaluation could be obtained by the use of a screening panel of antigens consisting of one grass, one weed, one tree, two molds, and one dust mite (with epidermals added if indicated by history). Further testing for relevant antigens could be carried out depending on the results of this initial screen, but a negative screening RAST was stated to be 94% sensitive and 96% efficient. In the practice of one of the authors (R. L. M.), a clinical impression revealed that such a screen failed to indicate the presence of significant inhalant allergy in a number of patients. Therefore over the years the initial screen was expanded to an eventual panel of 15 relevant antigens (including epidermals). To assess the effectiveness of a limited initial screening RAST, we analyzed the results of a large, randomly selected group of RASTs performed in our practice. A comparison of diagnostic effectiveness was made among the following panels (excluding epidermals): a 6-antigen screen, a 9-antigen screen, and a 13-antigen screen. METHODS One hundred consecutive modified RASTs (using a radioisotope marker) performed in the practice of one author (R. L. M.) were reviewed. Patients undergoing
From the Department of Otorhinolaryngology,University of Texas Southwestern Medical Center. Presented at the Annual Meeting of the American Academy of OtolaryngicAllergy,New Orleans, La., Sept. 14, 1995. Reprint requests: Richard L. Mabry, MD, Professor, Department of Otorhinolaryngology, UTSMC, 5323 Harry Hines Blvd., Dallas, TX 75235-9035. Copyright © 1997 by the American Academy of OtolaryngologyHead and Neck SurgeryFoundation, Inc. 0194-5998/97/$5.00 + 0 23/1/72889 54
RAST testing had symptoms and/or signs suggestive of allergic rhinitis or inhalant allergy. Each RAST panel consisted of the following 13 antigens (excluding epidermals): two grasses (Bermuda and timothy), two weeds (ragweed and marsh elder), four trees (mountain cedar, oak, elm, and pecan), three molds (Cladosporiurn, Alternaria, and Helminthosporium) and two dust mites (Dermatophagoides pteronyssinus and Derma-
tophagoides farinae ). Standard F/N class scores were obtained for the above 13 antigens in all patients. Equivocal scores (class 0/I) were counted as positive based on the experience of one of the authors (R. L. M.) that many of these equivocal antigens will prove positive on subsequent skin end-point titration testing. 2 This is in contrast to King's I consideration of class II reactors and higher as positive and class I reactors as equivocal. The above 13-antigen screening RAST was then compared in sensitivity and efficiency to the following smaller screens: a 6-antigen "miniscreen" and a 9-antigen "midiscreen." The miniscreen consisted of one grass (timothy), one weed (common ragweed), one tree (oak), two molds (Alternaria and Helminthosporium), and one dust mite (D. farinae), whereas the midiscreen consisted of two grasses (Bermuda and timothy), one weed (common ragweed), two trees (oak and mountain cedar), three molds (Alternaria, Helminthosporium, and Cladosporium), and one dust mite (D. farinae). Using the full panel as standard, we calculated sensitivity by dividing the number of true-positive results by the sum of true-positive and false-negative results and calculated efficiency as the quotient of the sum of the true-positive and true-negative results divided by the total number of tests. RESULTS Twenty-one patients had negative results on the 13antigen screening RAST. Of the remaining 79 patients with positive results on the 13-antigen panel (i.e., F/N class 0/I or greater) 72 would have had positive results
OtolaryngologyHead and Neck Surgery
LEHR et al. 55
Volume 117 Number I
Table 1. Sensitivity, efficiency, a n d predictive value
of negative results for three screens (standard, 13antigen screen) Test
Sensitivity
Miniscreen 72/79 (91) (6 antigens) Midiscreen 78/79 (99) (9 antigens) Miniscreen 76/79 (96) (mountain cedar)
Efficiency(%)
Predictive value (%)
93/100 (93)
21/28 (75)
99t100 (99)
21/22 (95)
97/100 (97)
21/24 (88)
detected by the miniscreen panel, and 78 would have positive results detected by the midiscreen. The 6-antigen miniscreen thus demonstrated 91% sensitivity and 93% efficiency and a 75% predictive value of a negative result, whereas the 9-antigen midiscreen produced 99% sensitivity and 99% efficiency and a 95% predictive value of a negative result (Table 1). Of note, the only patient missed by the midiscreen demonstrated only an equivocal allergy (F/N class 0/I) to cat and had negative results to all other antigens. Analysis of the above data also revealed that if mountain cedar had been substituted for oak in the 6-antigen miniscreen, the sensitivity and efficiency of this screen would have increased to 96% and 97%, respectively (Table 1). Although epidermals were positive: in 42% of patients (cat, 38%; dog, 31%), the incidence of either of these being the lone positive antigen was quite small. As mentioned above, only I (2%) of 42 patients demonstrating epidermal allergy would have not been detected by the above 9-allergen midiscreen, and only 2 (5%) of 42 would have been missed by the 6-allergen miniscreen. DISCUSSION
The RAST has proved to be a convenient, safe allergy screening test providing quantitative data that can be used in the same fashion as skin end-point titration for the initiation of immunotherapy. In addition, RAST testing offers specificity superior to that of in vivo testing and is unaffected by ongoing pharmacologic therapy.3 The primary disadvantage of RAST is the cost.
Current prices in RAST testing regularly fall in the range of $15 to $20 per antigen. However, considering the fact that a negative RAST screen may obviate the need for skin testing and a positive RAST screen may be used to strategically guide any necessary additional RAST or skin testing, it can be argued that RAST testing may be quite cost-efficient in many circumstances. 4 Regardless, RAST testing adds an additional expense to the diagnosis and management of the atopic patient., and finding ways to limit this expense may make it more palatable to its critics. The most obvious means to reduce the cost of RAST testing is by limiting the number of antigens screened. This study confirmed King's 1 findings of the efficacy of a screening RAST. Although the 6-antigen screen in this study provided 91% sensitivity and 93% efficiency, the 9-antigen midiscreen essentially offered the same sensitivity and efficiency (nearly 100%) of the expanded 13-antigen screen. Furthermore, tailoring the antigens selected for a pa~icular screen to the most prevalent and likely offenders in a particular region may allow reduction of the number of antigens screened without concomitant loss of sensitivity or efficiency. For example, substitution of mountain cedar for oak in our 6-antigen miniscreen increased the sensitivity and efficiency of this screen to 96% and 97%, respectively. With an estimated cost of approximately $20 per antigen in RAST testing, use of a midiscreen or miniscreen could result in a $120 to $180 savings per patient, respectively. In conclusion, a screening RAST does appear to be a valid concept. Careful selection of a 6- or 9-antigen screening RAST can provide comparable sensitivity and efficiency to expanded antigen screens at a much reduced cost. REFERENCES
1. KingWP. Efficacyof a screeningradioatlergosorbenttest. Arch Otolaryngology1982;108:78l-6. 2. MabryRL, MabryCS. Significanceof borderlinelevelsof specific IgE obtainedby FAST-Plusassay. OtolaryngolHead Neck Surg 1992;106:250-2. 3. MabryRL. Blendingskin endpointtitrationand in vitromethods in clinicalpractice.OtolaryngolClin NorthAm 1992;25:61-70. 4. NalebuffDJ. Use of RAST screeningin clinical allergy: a costeffective approach to patient care. Ear Nose Throat J 1985;64:15-35.