Impact of Clinical Symptoms on the Sensitivity and Specificity of Clostridium difficile Diagnostic Assays

Background: Clostridium difficile infection (CDI) is a clinical diagnosis with laboratory confirmation. Current assay comparisons seldom include patient symptoms.

Objective: Measure the sensitivity and specificity of assays for diagnosing CDI with and without the inclusion of patient symptoms.

Methods: Cross-sectional study of 150 stool specimens submitted for C. difficile toxin testing at Barnes-Jewish Hospital that met inclusion criteria. Inclusion criteria were:  minimum 10 ml stool, first specimen sent for C. difficile testing, patient able to communicate. Patients were interviewed within 48 hours of specimen collection on bowel movement consistency and frequency and an abdominal exam was conducted. The first 100 specimens were selected regardless of test results. The second 50 specimens were selected if positive by Remel ProSpecT Toxin A/B (PT). All specimens were tested and results reported per micro lab standard protocol with TechLab Tox A/B II (fresh TOX) and remaining specimen was frozen at -70˚C until the assay comparison. The following assays were compared: BD GeneOhm Cdiff (BD), TechLab Tox A/B II (frozen TOX), TechLab Chek 60 (GDH), and PT.  Clinically significant diarrhea was defined as ≥3 diarrheal bowel movement (diarrhea ≥6 on Bristol Stool Chart) per day or diarrhea plus abdominal pain. Gold standard is at least two positive assays. Sensitivity and specificity were calculated based on test results alone and test result plus presence of clinically significant diarrhea. CDI prevalence for positive predictive value (PPV) and negative predictive value (NPV) was based on the first 100 specimens collected (11%). Charts of patients negative by fresh TOX but positive by gold standard were reviewed to assess patient outcomes.

Results: 52 patients were positive by the gold standard and 39 patients were positive by the gold standard and had clinically significant diarrhea. The performance of the assays is displayed in table 1. 17 patients were negative by fresh TOX but positive by the gold standard. None of the 7 with additional specimens sent had a positive test on repeat testing. 11 patients either did not meet the clinical criteria or were on a laxative. Only two patients were treated for CDI empirically, and neither met criteria for clinically significant diarrhea. Two patients died from causes unrelated to CDI.

Conclusions: Including clinical symptoms reduces the specificity and PPV of CDI diagnostics with minimal impact on sensitivity and NPV. Patients whose original tests were reported negative but were positive by the gold standard did not suffer CDI-related adverse events.  

Table 1. Assay comparison results with and without clinical criteria

Assay	Sensitivity (%)		Specificity (%)		PPV (%)		NPV (%)
	without	With	Without	With	Without	With	Without	With
fresh TOX	67	74	100	95	100	65	96	97
frozen TOX	81	87	100	93	100	60	98	98
PT	92	90	77	68	33	25	99	98
GDH	98	100	94	84	67	43	100	100
BD	98	100	98	87	86	50	100	100