Validation of string test for diagnosis of Helicobacter pylori infections

Abstract

The method of recovering Helicobacter pylori DNA or viable cells absorbed on a string that a person has swallowed and that is retrieved an hour later (string test) should be a useful alternative to traditional analysis of cells or DNA obtained by endoscopy, which is invasive, uncomfortable, relatively costly, and ill-suited for community-based and pediatric studies. Here we assayed the sensitivity and validity of the string test versus conventional endoscopic biopsy for detecting and analyzing H. pylori infection. Forty-four people with gastric complaints were studied using both H. pylori culture and urease gene (ureB) PCR. H. pylori organisms cultured from strings and biopsy specimens from the same patients were fingerprinted by the randomly amplified polymorphic DNA (RAPD) method. Biopsy sections were also hematoxylin and eosin and silver stained for H. pylori detection. H. pylori was cultured from 80% of strings and detected by PCR from 91% of strings from participants whose biopsies had been H. pylori positive by culture, PCR, and/or histology. Strains recovered from strings and biopsy specimens yielded identical or closely related RAPD profiles in each of the 24 cases tested. We conclude that the string test is a useful method for H. pylori recovery and analysis when relatively noninvasive procedures are needed.

FIG. 1.

Representative RAPD fingerprint patterns. The profiles shown here were generated with DNAs from five single colonies and a pool of colonies. Lane M, molecular size markers (sizes shown are 0.4, 0.5, 1.6, 2.0, and 3.1 kb); lanes 1 to 6, pool and five single colonies from biopsy samples; lanes 7 to 12, equivalent cultures from a string test of the same patients. The profile in panel A was generated with RAPD primer 1281 and was chosen to illustrate a case of subtle but real difference between predominant genotypes of strains cultured from biopsy specimen versus string test: the lower polymorphic band in the 0.9-kb doublet is identified by an arrow in lane 12. (The additional differences in yield of RAPD products ≥1.8 kb long may reflect differences in size of genomic DNAs used for RAPD analyses and not differences in strain genotype.) The profiles in panel B were generated with primer 1283, are from another subject, and were chosen to illustrate heterogeneity among isolates (in this case, presence/absence of 1.2-kb and 1.5-kb bands in isolates from string).