Comparison of Clostridioides difficile Stool Toxin Concentrations in Adults With Symptomatic Infection and Asymptomatic Carriage Using an Ultrasensitive Quantitative Immunoassay

Abstract

Background: We used an ultrasensitive, quantitative single molecule array (Simoa) immunoassay to test whether concentrations of Clostridioides (formerly Clostridium) difficile toxins A and/or B in the stool of adult inpatients with C. difficile infection (CDI) were higher than in asymptomatic carriers of toxinogenic C. difficile.

Methods: Patients enrolled as CDI-NAAT had clinically significant diarrhea and a positive nucleic acid amplification test (NAAT), per US guidelines, and received CDI treatment. Potential carriers had recently received antibiotics and did not have diarrhea; positive NAAT confirmed carriage. Baseline stool samples were tested by Simoa for toxin A and B.

Results: Stool toxin concentrations in both CDI-NAAT (n = 122) and carrier-NAAT (n = 44) cohorts spanned 5 logs (0 pg/mL to >100000 pg/mL). Seventy-nine of 122 (65%) CDI-NAAT and 34 of 44 (77%) carrier-NAAT had toxin A + B concentration ≥20 pg/mL (clinical cutoff). Median toxin A, toxin B, toxin A + B, and NAAT cycle threshold (Ct) values in CDI-NAAT and carrier-NAAT cohorts were similar (toxin A, 50.6 vs 60.0 pg/mL, P = .958; toxin B, 89.5 vs 42.3 pg/mL, P = .788; toxin A + B, 197.2 vs 137.3 pg/mL, P = .766; Ct, 28.1 vs 28.6, P = .354). However, when CDI/carrier cohorts were limited to those with detectable toxin, respective medians were significantly different (A: 874.0 vs 129.7, P = .021; B: 1317.0 vs 81.7, P = .003, A + B, 4180.7 vs 349.6, P = .004; Ct, 25.8 vs 27.7, P = .015).

Conclusions: Toxin concentration did not differentiate an individual with CDI from one with asymptomatic carriage. Median stool toxin concentrations in groups with CDI vs carriage differed, but only when groups were defined by detectable stool toxin (vs positive NAAT).

Figure 1.

Dot plots showing distribution of toxin concentrations (measured by Simoa) and Ct values (measured by Xpert NAAT) in symptomatic (CDI-NAAT) vs asymptomatic (carrier-NAAT) cohorts (defined by positive stool NAAT result). A, Simoa toxin A concentration. B, Simoa toxin B concentration. C, Simoa toxin A + B concentration. D, Xpert Ct value. The bottom and top edges of the boxes for each cohort indicate the interquartile range, the horizontal line bisecting the box indicates the median value, and the whiskers represent maximum and minimum values. P values for comparison of the respective medians are shown. Abbreviations: CDI, Clostridioides difficile infection; Ct, cycle threshold; NAAT, nucleic acid amplification test; Simoa, single molecule array.

Figure 2.

Dot plots showing distribution of toxin concentrations (measured by Simoa) and Ct values (measured by Xpert NAAT) in symptomatic (CDI-Tox20) vs asymptomatic (carrier-Tox20) cohorts (defined by positive stool NAAT result and stool toxin A + B ≥20 pg/mL by Simoa). A, Simoa toxin A concentration. B, Simoa toxin B concentration. C, Simoa toxin A + B concentration. D, Xpert Ct value. The bottom and top edges of the boxes for each cohort indicate the interquartile range, the horizontal line bisecting the box indicates the median value, and the whiskers represent maximum and minimum values. P values for comparison of the respective medians are shown. Abbreviations: CDI, Clostridioides difficile infection; Ct, cycle threshold; NAAT, nucleic acid amplification test; Simoa, single molecule array; Tox20, stool toxin A + B ≥20 pg/mL.

Figure 3.

Correlations between toxin concentrations (measured by Simoa) and Ct values (measured by Xpert NAAT) and between toxin A and toxin B in the symptomatic cohort defined by positive stool NAAT result (CDI-NAAT). A, Simoa toxin A concentration vs Xpert Ct value. B, Simoa toxin B concentration vs Xpert Ct value. C, Simoa toxin A + B concentration vs Xpert Ct value. D, Simoa toxin A concentration vs Simoa toxin B concentration. Spearman r values and P values for each correlation are shown. Abbreviations: CDI, Clostridioides difficile infection; Ct, cycle threshold; NAAT, nucleic acid amplification test; Simoa, single molecule array.

Figure 4.

Correlations between toxin concentrations (measured by Simoa) and Ct values (measured by Xpert NAAT) and between toxin A and toxin B in the symptomatic cohort defined by positive stool NAAT result and stool toxin A + B concentration ≥20 pg/mL by Simoa (CDI-Tox20). A, Simoa toxin A concentration vs Xpert Ct value. B, Simoa toxin B concentration vs Xpert Ct value. C, Simoa toxin A + B concentration vs Xpert Ct value. D, Simoa toxin A concentration vs Simoa toxin B concentration. Spearman r values and P values for each correlation are shown. Abbreviations: CDI, Clostridioides difficile infection; Ct, cycle threshold; NAAT, nucleic acid amplification test; Simoa, single molecule array; Tox20, stool toxin A + B ≥20 pg/mL.