Antibody screening for celiac disease
CMAJ 1998;158:172
Re: Effectiveness of antigliadin antibodies as a screening
test for celiac disease in children, Lucie J. Chartrand and
associates, CMAJ 1997;157(5):527-33 [full text / résumé]
In response to: J. Massé
We appreciate the opportunity to clarify a poorly worded sentence in our methods section. The optical density obtained with goat anti-human IgG or IgA antibodies without serum from a patient does indeed serve as a blank, not a negative control. Both positive and negative controls were routinely performed for each ELISA assay, with sera from patients with and without biopsy-proven celiac disease respectively. Furthermore, serum from a borderline serologically positive patient was also used, to verify inter-assay variability.
The optimal cutoff points were established previously by pilot studies in a similar pediatric population at our institution. Our paper describes a cohort of 176 children studied prospectively between 1992 and 1995 to test the discriminative ability of those cutoff limits.
Clearly if another laboratory used our assay and altered the conditions in any way (e.g., use of another manufacturer's ELISA plates, antibodies or substrate), new cutoff points would have to be established and validated for the population under study. We routinely check the variation in results obtained whenever the lot of immunoglobulins is changed and even when a new batch of buffers is prepared. Moreover, we stress that our results pertain to children. The cutoff points we established might not apply to adults.
The sensitivity of our ELISA (as presented in Table 2) was 93%, not 70%. Furthermore, clinicians normally consider the negative predictive value of a screening test when deciding to terminate investigations in a particular patient. In our study, the negative predictive value for IgA and IgG antigliadin antibodies was 98%. Nevertheless, duodenal biopsy should always be performed when there is a strong suspicion of celiac disease. No laboratory test is 100% accurate.
Ernest G. Seidman, MD
Chief, Division of Gastroenterology
Hôpital Sainte-Justine
Professor
Department of Pediatrics
Université de Montréal
Montreal, Que.
Lucie J. Chartrand, RD, MSc
Research Associate
Centre de recherche
Hôpital Sainte-Justine
Montreal, Que.
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