Correlation between Clostridium difficile Bacterial Load, Commercial Real-Time PCR Cycle Thresholds, and Results of Diagnostic Tests Based on Enzyme Immunoassay and Cell Culture Cytotoxicity Assay

The impact of Clostridium difficile fecal loads on diagnostic test results is poorly understood, but it may have clinical importance. In this study, we investigated the relationship between C. difficile fecal load and the results of four assays: a glutamate dehydrogenase (GDH) enzyme immunoassay (EIA), a toxin A/B antigen EIA (ToxAB), a cell culture cytotoxicity assay (CCA), and PCR targeting the tcdB gene. We also compared the PCR cycle threshold (C-T) with the results of quantitative culture using Spearman's rank correlation coefficient. Finally, we sequenced the genomes of 24 strains with different detection profiles. A total of 203 clinical samples harboring toxigenic C. difficile were analyzed and sorted into one of four groups: 17 PCR+ (group 1), 37 PCR(+)GDH(+)(group 2), 24 PCR(+)GDH(+)CCA(+) (group 3), and 125 PCR+ GDH(+)ToxAB(+) (group 4). The overall median fecal load in log(10) CFU/g was 6.67 (interquartile range [IQR], 5.57 to 7.54). The median fecal bacterial load of groups 1, 2, 3, and 4 were 4.15 (IQR, 3.00 to 4.98), 5.74 (IQR, 4.75 to 6.16), 6.20 (IQR, 5.23 to 6.80), and 7.08 (IQR, 6.35 to 7.83), respectively. Group 1 samples had lower fecal loads than those from each of the other groups (P<0.001). Group 2 samples had lower fecal loads than those from groups 3 and 4 (P<0.001). There was a significant correlation between PCR C-T and fecal loads (rho=-0.697; P<0.001). NAP1 strains were associated with the detection of toxins by EIA or CCA (P=0.041). This study demonstrates an association between C. difficile fecal load and the results of routinely used diagnostic tests.