Our results suggest that comorbid status assessed using CCI and for study purposes defined as CCI ≥ 7 is positively associated with the single failure of FMT in the treatment of recurrent CDI. It indicates that comorbidity is a potent risk factor for FMT failure. CDI is the most common cause of health care-associated diarrhoea in Europe [
4]. Tendency for recurrent infections makes treatment of CDI very challenging. More than 20% of patients with CDI experience the recurrence of disease within 2 months, and if CDI recurs, the chance for a subsequent recurrence increases to 60% [
7]. FMT is a highly effective method to prevent recurrent CDI. However, it fails in about 5 to 20% of patients [
15]. The mechanisms that lead to FMT failure and its risk factors are poorly understood. Risk factors of developing recurrent CDI after a first CDI episode treated with antibiotic therapy only are well-known. There is large body of evidence supporting the role of comorbid status as important risk factor of developing recurrent CDI after standard vancomycin therapy [
8‐
12]. On the other hand, risk factors for the development of further recurrences after FMT are more or less unknown. To our knowledge, only 3 studies have tried to identify the risk factors of FMT in the treatment of recurrent CDI, and none of these studies conclusively linked comorbidity to the risk of FMT failure [
17‐
19]. A relatively large retrospective study by Meighani et al. compared the risk of FMT failure between groups of patients with recurrent CDI stratified by the CCI. They defined comorbid status as CCI ≥ 3. However, they found no association of CCI ≥ 3 and risk of FMT failure [
18]. Our patients were more comorbid than in the study by Meighani et al. In their study, less than 40% of all patients had CCI ≥ 3. In our study, more than 40% had CCI ≥ 7. It is possible that Meighani et al. were unable to find an association between CCI and FMT failure because their population was less comorbid or and/or they chose too low of a CCI cut-off to stratify their patients. Another 2 retrospective studies did not assess the association of FMT failure with CCI or any other direct surrogate marker of comorbidity. A study by Fischer at al. found that inpatient status during FMT is associated with the risk of FMT failure. They suggested that inpatient status during FMT is an indicator for worse overall health condition, which might be affected by the presence of serious comorbidities [
19]. A study by Patron et al. did not look for any comorbidity markers of FMT failure [
17]. In our study, patients that experienced FMT failure were older. However the difference was not statistically significant. Patron et al. and Fischer at al. found that age is associated with FMT failure [
17,
19]. Advancing age has also been identified as a risk factor for developing recurrent CDI in patients treated with standard antibiotic therapy [
24]. Age is also one of the variables included in the CCI equation [
21,
22]. Naturally, age might be considered a possible confounder for association of FMT failure and comorbidity status. Therefore, we included age in the multivariate model and CCI ≥ 7 was nevertheless significantly associated with FMT failure. To objectively assess this association of comorbidity to the risk of FMT failure, we used CCI as a well-established surrogate marker of comorbidity. It was developed and validated by numerous studies as a measure of 1-year mortality risk and marker of disease burden. The CCI is used extensively in clinical research to address the influence of comorbidities on prognosis and outcome. Variables that affect CCI are age, presence of arterial hypertension, diabetes mellitus, solid tumors, leukaemia, lymphoma, heart failure, liver cirrhosis, hepatopathy, stroke or TIA, dementia, peptic ulcer disease, peripheral artery obliterating disease, myocardial infarction, connective tissue disease, COPD and AIDS [
20,
21].