data were collected electronically on a retrospective cohort of patients admitted to barnes - jewish hospital in saint louis from january 1 , 2003 , through december 31 , 2003 . toxin assay or the icd-9 code indicating c. difficile  associated disease ( 008.45 ) ( appendix ) . a case of cdad toxin assay ( tech laboratory c. difficile tox a / b ii toxin assay [ tech laboratory , blacksburg , va , usa ] ) or pseudomembranes seen on colonoscopy . because the hospital laboratory performs a c. difficile toxin assay only on unformed stool samples and stool toxin testing is ordered based on clinical suspicion of cdad , all patients with a positive toxin assay were considered cdad case - patients . a total of 45,486 admissions among 28,417 unique patients were included in the analysis ( figure 1 ) . a c. difficile the breakdown of admissions , according to toxin assay and icd-9 status , was as follows : 506 had both a positive toxin assay and received the icd-9 code ( concordant ; cdta+/icd9 + ) , 156 had a positive toxin assay but no icd-9 code ( cdta+/icd9 ) , and 239 received the icd-9 code but did not have a positive toxin assay ( cdta/icd9 + ) ( figure 1 ) . the concordance between toxin assays and icd-9 codes was good (  = 0.72 , p<0.01 ) . the overall mean cdad rate by icd-9 codes ( 16.4/1,000 admissions ) was significantly higher then the mean rate by toxin assays ( 14.6/1,000 admissions ) ( figure 2 ; rate ratio [ rr ] 1.13 , 95% confidence interval [ ci ] 1.011.25 ) . monthly rates of clostridium difficile  associated disease by diagnosis type . the median number of days from admission to stool collection was greater in admissions with a positive toxin assay but no icd-9 code ( cdta+/icd9 ) than in concordant ( cdta+/icd9 + ) admissions ( 6.0 days vs 3.0 days , p<0.01 ) ( table 1 ) . the first positive stool sample was collected within 48 hours of discharge for 68 ( 44% ) of admissions with a positive toxin assay only ( cdta+/icd9 ) admissions , compared with 72 ( 14% ) of concordant ( cdta+/icd9 + ) admissions ( p<0.01 ) . upon chart review , documentation of a previous history of cdad was evident in 142 ( 59% ) of the icd-9 code only ( cdta/icd9 + ) admissions . a c. difficile toxin assay had been ordered in 137 ( 57% ) of all icd-9 code only ( cdta-/icd9 + ) admissions . one - hundred thirty ( 54% ) had at least 1 stool negative for c. difficile toxin . overall , 92% of patients with positive toxin assay ( cdta+ ) and 90% of patients with icd-9 code only ( cdta/icd9 metronidazole was prescribed in 187 ( 78% ) of the icd-9 code only ( cdta/icd9 + ) admissions , compared with 591 ( 89% ) of patients with a positive toxin assay ( cdta+ ) ( p<0.01 ) . for 75 ( 31% ) of the icd-9 code only ( cdta/icd9 + ) admissions , oral vancomycin was prescribed , compared with 130 ( 20% ) of patients with a positive toxin assay ( cdta+ ) ( p<0.01 ) . toxin assay results and subsequently identified through chart review ( 3 missed positive toxins , 26 cdad patients transferred from other facilities , 3 positive outpatient toxin assays , and 2 diagnosed by colonoscopy ) , which brought the total number of cases with positive cdad diagnostics to 696 . icd-9 codes correctly identified 540 of these cases and correctly classified 44,741 admissions as non - cdad admissions ( sensitivity 78% , specificity 99.7% ) . when the cdad rate by toxin assays was adjusted for the additional cases , the adjusted cdad rate was 15.3/1,000 admissions . this rate was not significantly different from the unadjusted cdad rate by toxin assay results ( rr 0.95 , 95% ci 0.861.06 ) or the rate by icd-9 codes alone ( rr 1.07 , 95% ci 0.971.19 ) . overall , there was good correlation between c. difficile  toxin assay results and icd-9 codes . initially , the cdad rate by icd-9 codes appeared higher than the rate by toxin assays . however , once the additional cdad cases identified through chart review were added , this difference was not significant . admissions with only a positive c. difficile  toxin assay and no c. difficile icd-9 code ( cdta+/icd9 ) were more likely than concordant ( cdta+/icd9 + ) admissions to have their first positive toxin assay within 48 hours of discharge . for these admissions , toxin assay results may not yet have been back at the time of discharge or cdad may not have been considered a primary diagnosis by the physician and therefore not captured by the medical coders . antimicrobial drug treatment patterns suggest icd-9 code only ( cdta/icd9 + ) admissions were patients who were more likely to have a history of cdad . the observation that more icd-9 code only ( cdta/icd9 + ) patients received oral vancomycin indicates that recurrent cdad may have been suspected in these patients . in these patients , cdad appears to have been diagnosed on the basis of the patient 's history and symptoms instead of by a positive c. difficile true cdad cases may have been misclassified among the controls . a patient who did not have a positive c. difficile  toxin assay , who was not assigned the cdad icd-9 code , and whose diagnosis was made by colonoscopy would have been missed . however , misclassification is unlikely for two reasons . first , after charts were reviewed , only 2 additional patients were identified whose diagnosis was made by colonoscopy alone . second , the detection of cdad cases transferred from other institutions indicates that cdad cases diagnosed by methods other than the toxin assays are being captured by icd-9 codes . icd-9 codes are readily available from billing databases . in the absence of a national surveillance system for cdad , icd-9 codes provide a standard method for determining cdad rates that can be used at all types of hospitals . however , because icd-9 codes are assigned at discharge and not on the date of diagnosis , determining which cases are hospital acquired and which are community acquired is difficult . also , icd-9 codes are assigned by medical coders , who may not be able to accurately identify a patient 's principal diagnoses as well as a physician or medical professional . despite these limitations , icd-9 codes can likely be used to identify cdad cases and track cdad rates when c. difficile the international classification of diseases , 9th revision , clinical modification ( icd-9 ) code used in this study was 008.45 , " intestinal infection due to clostridium difficile , " and is the only icd-9 code related to cdad . to apply this code , medical coders must have documentation in a patient 's medical record by the treating medical providers that a patient 's gastroenteritis or colitis is due to c. difficile . positive laboratory tests alone are not sufficient to warrant application of the code . at our institution , icd-9 coding occurs , on average , 57 days after a patient is discharged from the hospital . the icd-9 system of classifying hospital discharge diagnoses is used throughout the united states . the definition for the code 008.45 is consistent between hospitals , although individual coding practices may vary . although icd-9 codes have limitations , they are readily available from administrative databases and have been used frequently to identify diagnoses and classify comorbidities ( 1 ) . a move to the international classification of diseases , 10 revision , clinical modification ( icd-10 ) system is anticipated for us hospitals but the exact time of this transition is not yet known . the icd-10 system does include a code for cdad ( a04.7 , enterocolitis due to c. difficile ) , so the icd - based system presented here could be modified to be used with the updated coding system . klabunde cn , warren jl , legler jm . assessing comorbidity using claims data : an overview . the international classification of diseases , 9th revision , clinical modification ( icd-9 ) code used in this study was 008.45 , " intestinal infection due to clostridium difficile , " and is the only icd-9 code related to cdad . to apply this code , medical coders must have documentation in a patient 's medical record by the treating medical providers that a patient 's gastroenteritis or colitis is due to c. difficile . positive laboratory tests alone are not sufficient to warrant application of the code . at our institution , icd-9 coding occurs , on average , 57 days after a patient is discharged from the hospital . the icd-9 system of classifying hospital discharge diagnoses is used throughout the united states . the definition for the code 008.45 is consistent between hospitals , although individual coding practices may vary . although icd-9 codes have limitations , they are readily available from administrative databases and have been used frequently to identify diagnoses and classify comorbidities ( 1 ) . a move to the international classification of diseases , 10 revision , clinical modification ( icd-10 ) system is anticipated for us hospitals but the exact time of this transition is not yet known . the icd-10 system does include a code for cdad ( a04.7 , enterocolitis due to c. difficile ) , so the icd - based system presented here could be modified to be used with the updated coding system . klabunde cn , warren jl , legler jm . assessing comorbidity using claims data : an overview .
we conducted a retrospective cohort study to compare clostridium difficile  associated disease rates determined by c. difficile  toxin assays and international classification of diseases , 9th revision ( icd-9 ) codes . 
 the correlation between toxin assay results and icd-9 codes was good (  = 0.72 , p<0.01 ) . 
 the sensitivity of the icd-9 codes was 78% and the specificity was 99.7% .