In this study histological evaluation and rapid urease test were considered as the gold standard and results of serological tests in H. pylori infected group were compared with the control group. IgA, IgM and IgG have a high negative predictive value that could be used (scale) for a screening and diagnostic test. For all three antibodies, the negative predictive value was about 80%. These findings show that negative results have a higher value compared to positive results. When the test is negative with 80% probability, the person is not infected by H. pylori, while when it is positive, IgG has the highest (60.6%) and IgM the lowest (42.2%) accuracy. IgA accuracy is about 59.3% and the most specific antibody is IgG and the most sensitive is IgM.
Since Iran is a developing country with high prevalence of H. pylori infection, it is ideal to evaluate all controls by endoscopy to see how many of them were infected by H. pylori and not suppose that all are free of H. pylori. Therefore, the number of false positive and negatives would decrease and sensitivity, specificity, accuracy and positive and negative values of antibodies would increase. However, because of ethical issues this was impossible. Predictive value of a test is dependent on sensitivity, specificity and prevalence. In this study we could not estimate the exact prevalence of H. pylori with the small sample size. Thus, a study with a large sample size should be performed in Shiraz city to estimate the prevalence and predictive values.
A great deal of studies has been conducted in different countries. In an adults study, serum IgG with cut-off point of 15.2 U/mL had 94.1% sensitivity and 97.9% specificity for diagnosis of H. pylori infection (12). In another adult study, IgG antibody titers higher than 10 U/mL were considered positive for H. pylori infection; cut-off point of 3 U/mL had 100% sensitivity and 99% specificity for eradication of H. pylori (13). A study in Spain reported sensitivity, specificity, positive and negative predictive values for IgG as 81, 97, 89, and 93% and for IgA as 90, 76, 36, and 96%, respectively, in children. However, IgM was not evaluated in this study (2). Another pediatric study in Italy showed sensitivity, specificity, positive and negative predictive values for IgG to be 86, 80, 72 and 90%, respectively (14).
In Brazil it was shown that sensitivity, specificity, positive and negative predictive values and accuracy for IgG was 64, 83.7, 82, 66.6 and 73.1% and for IgA this was 72, 65.9, 72, 67.4 and 69.8%, respectively (15). Different values for sensitivity, specificity, positive and negative predictive values have been reported by various studies. This could be due to differences in prevalence of H. pylori between populations because of the variety in socioeconomic status, health status, crowding and other factors. Differences in prevalence have an obvious impact on statistical indexes.
A serological assay for H. pylori infection may be useful in epidemiological survey of prevalence, transmission mode and spontaneous clearance of the H. pylori infection, while it also may be helpful in the development of preventive measures for H. pylori infection. In Asia, the strains of H. pylori are different from those that are prevalent in other continents. Thus, the specificity, sensitivity and positive and negative predictive values of one kind of serology kit may differ in various geographic or ethnic populations.
We concluded that these antibody tests have a relatively high negative predictive value and a low positive predictive value. Thus, serological tests are not useful on their own, for diagnosis of H. pylori infection because the sensitivity and specificity of these tests for detection of antibodies against H. pylori in children vary widely and they have not been recommended for clinical practice in pediatric patients. More researches with a larger sample size are needed in the future to confirm the findings of this study.
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