The relationship of Bristol Stool Scale to Clostridium difficile infection
The Bristol Stool scale is a graded visual scale that classifies stool density, with scores of 1 (most dense stool) through 7 (liquid stool). At our institution, we require a Bristol score of ≥5 for stool specimens to reduce inappropriate C. difficile testing. We determined the relationship between the Bristol scale and the rates, complications, severity of inpatient C. difficile infection (CDI), and whether Bristol score 5 stools should be excluded from testing due to a lower rate of toxin detection.
We recorded the Bristol score on all stool specimens from adult inpatients tested for C. difficile over a 10-month period. Two inter-rater reliability studies were performed. We tested using an algorithm of enzyme immunoassay (EIA) for glutamate dehydrogenase and toxins A/B followed by molecular amplification and detection (MAD) for indeterminate EIA results. Hospital-onset CDI was defined by National Healthcare Safety Network criteria for assays sent 48 hours after hospitalization. Severe hospital-onset CDI was defined using the criteria of Zar et al (Clin Infect Dis 2007; 45:302-7). Positive C. difficile assays, severe CDI, and complications of hospital-onset CDI (colectomy, ICU transfer, death) were compared between Bristol scores using the χ2.
There were 3005 specimens tested for C. difficile. The Fleiss Kappa score for inter-rater reliability was 0.675. C. difficile assays were positive in 43 (15.0%), 144 (13.6%), and 177 (10.7%) specimens of Bristol scores 5, 6, and 7, respectively (p=0.031). Semi-formed stools (Bristol score 5 or 6) were more likely to be positive by MAD compared to liquid stool (Bristol score 7) (RR=1.50, 95% CI 1.11-2.04). Rates of hospital-onset CDI, severe CDI, and complications of CDI did not differ by Bristol score.
C. difficile toxin was more likely to be found in semi-formed stool. Bristol 7 stools had the lowest rate of detection, suggesting a lower clinical threshold to test liquid stools. The rate of toxin detection by MAD was 50% higher for semi-formed versus liquid stools, indicating a lower concentration of C. difficile toxin in semi-formed stool. In conclusion, when MAD testing is used, semi-formed stools account for a meaningful proportion of C. difficile-positive specimens.
K. Hamilton, None
D. Pegues, None
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