Objectives: Diarrhea often occurs in pediatric kidney transplant recipients and can result from infectious or noninfectious causes. Identifying underlying issues can allow for proper management and prevent complications such as dehydration, graft dysfunction, and prolonged hospital stay.
Materials and Methods: We retrospectively evaluated stool samples of 204 pediatric kidney transplant recipients seen at the Başkent University Hospital with complaint of diarrhea. Direct microscoping evaluation, bacterial cultures, and viral antigen testing for rotavirus and adenovirus were performed. Presence of cytomegalovirus and Clostridium difficile were evaluated in selected patients. In 18 patients, a comprehensive viral panel was performed based on clinical suspicion.
Results: One patient each showed Shigella species and enteropathogenic Escherichia coli. Candida species was isolated in 1 sample. Rotavirus was detected in 4 patients, adenovirus in 2 patients, and Clostridium difficile toxin in 0 patients. Among the 18 patients who underwent detailed panel testing, norovirus was detected in 2 patients and sapovirus in 1 patient. Forty-seven stool culture samples showed reduced or absent normal intestinal flora, suggesting possible intestinal dysbiosis. Entamoeba was detected in 7 patients. Cytomegalovirus was positive in 24 of 204 patients (11%). On direct microscopic examination, 19 patients showed minimal findings, and 24 patients had moderate findings. Fourteen patients had high numbers of leukocytes and erythrocytes in their stool. Because amebic colitis and Clostridium difficile infection could not be excluded, patients with moderate to severe findings were treated with metronidazole.
Conclusions: Among pediatric kidney transplant patients with diarrhea, the rate of intestinal pathogens was slow but reduced normal flora in stool cultures was common, indicating microbiota-related etiologies. The high rate of cytomegalovirus infection among patients suggested that cytomegalovirus infection should be considered as a potential etiological factor in pediatric transplant patients presenting with diarrhea. Noninfectious causes should also be considered in differential diagnosis of diarrhea in solid-organ transplant recipients.

Key words : Intestinal dysbiosis, Posttransplant diarrhea, Stool microbiology

Introduction
Infections remain a major cause of morbidity in pediatric kidney transplant recipients. In addition to routine childhood infections, pediatric kidney transplant recipients are particularly vulnerable to opportunistic pathogens due to immunosuppression.¹ Gastrointestinal complications are frequently observed in kidney transplant recipients, with diarrhea affecting up to one-fifth of patients within the first 3 years posttransplant.² Diarrhea is a common and clinically important complication in pediatric kidney transplant recipients and is generally defined as the passage of 3 or more loose stools per day. Diarrhea can be classified as acute when lasting up to 2 weeks or chronic when persisting beyond that period. Diarrhea may lead to dehydration, electrolyte imbalance, weight loss, malabsorption of immunosuppressive drugs, acute kidney injury, graft dysfunction, and prolonged hospitalization in transplant patients. The underlying causes are diverse, encompassing bacterial, viral, and parasitic infections.³ In addition, medications (particularly immunosuppressive agents such as mycophenolate, especially when used in combination with tacrolimus) are recognized contributors.⁴ The etiological spectrum of diarrhea in solid-organ transplant recipients differs from that of the general pediatric population. Although bacterial, viral, and parasitic pathogens may be involved, opportunistic infections, particularly cytomegalovirus (CMV), are of special concern. In addition, alterations in intestinal microbiota due to immunosuppressive therapy and repeated antibiotic exposure may contribute to gastrointestinal symptoms.⁵ Despite the clinical importance of diarrhea, data on the microbiological causes of diarrhea in pediatric kidney transplant recipients remain limited. This study aimed to evaluate the microbiological findings in stool samples of pediatric kidney transplant recipients presenting with diarrhea and to assess the frequency of infectious agents in this vulnerable population.

Materials and Methods
This retrospective study included pediatric kidney transplant recipients who presented with diarrhea to Başkent University Hospital between 2012 and 2025. We reviewed medical records and microbiological test results of 204 patients. Diarrhea was defined as the passage of 3 or more loose or watery stools per day. Nearly all patients had analyses of stool samples with routine direct microscopic examination and bacterial stool cultures. Viral antigen testing for rotavirus and adenovirus was performed. Based on clinical suspicion, some patients were evaluated for Clostridium difficile (C difficile) toxin and CMV infection. Because this was a retrospective study, standardized CMV polymerase chain reaction testing was not performed. Review of medical records showed that CMV evaluation was undertaken in patients presenting with concomitant fever and diarrhea, persistent diarrhea, unexplained cytopenia, or lack of clinical improvement. Testing appeared to be guided by the treating physician’s clinical judgment rather than predefined study criteria. In 18 patients with persistent or severe symptoms, a comprehensive viral stool panel was performed, including testing for norovirus and sapovirus. Similarly, multiplex gastrointestinal viral panel testing was not uniformly performed in all patients. Chart review showed that viral panel studies were more frequently obtained in patients with moderate-to-severe or prolonged diarrhea, with hospitalization, or when initial bacterial investigations were negative. Direct microscopic examination assessed the presence of leukocytes, erythrocytes, parasites, and other inflammatory findings. Stool culture results were also evaluated for alterations in normal intestinal flora. Patients with moderate-to-severe inflammatory findings on stool microscopy were treated empirically with metronidazole, as amebic colitis and C difficile infection could not be definitively excluded at presentation. Because of the descriptive nature of the study, we analyzed results primarily using descriptive statistics. We expressed categorical variables as numbers and percentages.

Results
Among 204 patients, almost all had routine stool cultures and direct microscopic examinations (Table 1). Bacterial pathogens were detected at a very low frequency: Shigella species was isolated in one patient, and enteropathogenic Escherichia coli was detected in another. Candida species was isolated in one stool sample. However, the clinical significance of this finding remains uncertain, as gastrointestinal colonization could not be excluded. Among viral pathogens, rotavirus antigen was detected in 4 patients and adenovirus antigen in 2 patients. Clostridium difficile toxin was not detected in any of the tested patients. Of the 18 patients who underwent comprehensive viral panel testing, norovirus was detected in 2 patients and sapovirus in 1 patient. Stool cultures from 47 patients showed reduced or absent normal intestinal flora, suggesting possible intestinal dysbiosis. Parasitological examination revealed Entamoeba species in 7 patients. Among 204 patients, CMV infection was detected via polymerase chain reaction in 24 patients (11%), representing a notably high proportion among this cohort. Direct microscopic examination showed minimal findings in 19 patients and moderate inflammatory findings in 24 patients. Fourteen patients demonstrated a high number of leukocytes and erythrocytes in stool samples.

Discussion
Infectious causes of diarrhea were identified in a relatively small proportion of pediatric kidney transplant recipients analyzed in this study. This finding suggests that noninfectious etiologies, including intestinal dysbiosis, play an important role in the development of posttransplant diarrhea. Similar results have been reported in both adult and pediatric transplant populations, where routine stool cultures yielded low rates of bacterial pathogen isolation.^6,7 Common bacterial pathogens include nontyphoidal Salmonella, Campylobacter jejuni, Shigella, Yersinia, and E coli. Infection with C difficile, on the other hand, has been increasingly reported in renal transplant recipients and presents with a wide clinical spectrum, ranging from mild diarrhea to fulminant colitis.⁸ Prior antimicrobial exposure is a major risk factor. Although C difficile infection has been reported in kidney transplant recipients with diarrhea, microbiological confirmation was not achieved in our cases. However, based on the clinical presentation and risk profile, a presumptive infectious etiology was considered, empirical treatment was initiated due to the inability to exclude infectious causes at presentation, and metronidazole therapy was initiated early in the course of illness. We recognize that the initiation of empirical metronidazole therapy in cases without proven C difficile infection may be questioned in light of current guidelines, which favor microbiologically confirmed and targeted treatment strategies. In particular, the role of metronidazole as first-line therapy for suspected C difficile infection has become limited in recent years. In our patient group, antimicrobial therapy was initiated before microbiological confirmation, reflecting the diagnostic uncertainty frequently encountered in immunocompromised hosts. This decision was guided by concern for a potentially treatable infectious etiology. This represents a limitation of our management approach and highlights the need for rapid diagnostic tools to support more precise and guideline-concordant therapy in similar high-risk settings. Immunosuppressed pediatric patients are particularly susceptible to viral gastrointestinal infections, with CMV being a key pathogen. Community-acquired viruses such as norovirus and rotavirus are also encountered, and norovirus-associated diarrhea has been linked to acute renal failure in hospitalized renal transplant recipients.^1,9 The relatively high CMV positivity rate (11%) observed in our cohort indicates that CMV infection remains a major concern in pediatric kidney transplant recipients presenting with diarrhea. Cytomegalovirus is a well-recognized cause of gastrointestinal disease after solid-organ transplant and has consistently been reported among the most common infectious etiologies of posttransplant diarrhea, together with norovirus.¹⁰ Our findings support recommendations that CMV infection should be routinely considered and actively screened for in transplant recipients with persistent or unexplained diarrhea. In addition, patients with more severe or persistent symptoms may have been more likely to undergo viral testing, potentially influencing the observed frequency of viral etiologies. Therefore, the true incidence of CMV and other viral infections in this cohort may be under- or overestimated. Prospective studies with uniform diagnostic strategies would provide more accurate epidemiological data. The detection of Candida species in stool samples should be interpreted with caution, as gastrointestinal colonization is common, particularly in immunocompromised hosts, and does not necessarily indicate invasive disease. In the absence of supportive clinical, endoscopic, or histopathological findings, the pathogenic role of Candida in diarrheal illness remains uncertain. Another important observation in our study was the frequent detection of reduced or absent normal intestinal flora in stool cultures. This finding likely reflects the cumulative effects of immunosuppressive therapy, repeated antibiotic exposure, and altered host immune responses on gut microbiota composition. Increasing evidence suggests that intestinal dysbiosis plays a key role in chronic diarrhea after transplant and may contribute to gastrointestinal symptoms even in the absence of identifiable pathogens.⁵ Several studies have also suggested that mycophenolate mofetil–associated diarrhea may be mediated through alterations in gut microbiota rather than direct mucosal toxicity alone.¹¹ Our findings are consistent with this hypothesis and further underline the multifactorial nature of posttransplant diarrhea, in which infectious agents, microbiota imbalance, and medication-related effects may coexist. Given the low diagnostic yield of routine stool cultures in our cohort, a more targeted diagnostic approach may be appropriate in pediatric kidney transplant recipients. Viral testing, particularly for CMV and norovirus, should be prioritized based on clinical presentation and symptom duration.^12,13 Furthermore, recognition of intestinal dysbiosis may open the door to future therapeutic strategies such as probiotic or microbiota-restoring interventions, although current evidence in pediatric transplant populations remains limited.¹⁴ We documented Entamoeba species in 7 patients. Species differentiation was not consistently available; therefore, the pathogenic role of these findings could not be definitively established. The identification of Entamoeba species should be carefully evaluated, especially when species-level differentiation is unavailable. Nonpathogenic species such as Entamoeba dispar cannot be excluded in retrospective analyses, limiting definitive attribution of symptoms to this organism. Although parasitic infections are uncommon in solid-organ transplant recipients in developed countries, pathogens such as Cryptosporidium, Giardia lamblia, Strongyloides stercoralis, and Microsporidium should be considered in patients with relevant epidemiologic exposures. Severe watery diarrhea and acute kidney injury have been reported in pediatric renal transplant recipients with cryptosporidiosis.¹ Evaluation of diarrhea should include stool cultures or multiplex molecular panels, testing for ova and parasites, and viral polymerase chain reaction studies. Infectious causes should be excluded before attributing symptoms to medication-related toxicity. Management consists of supportive care, targeted antimicrobial therapy, and judicious modification of immunosuppressive regimens when appropriate.¹⁵ This study had several limitations, including its retrospective design, incomplete comprehensive viral panel testing in all patients, and lack of long-term outcome data. Prospective studies incorporating systematic viral diagnostics and microbiome-focused analyses are needed to better clarify the mechanisms underlying posttransplant diarrhea and to improve management strategies in pediatric kidney transplant recipients.

Conclusions
Diarrhea is a clinically important problem in pediatric kidney transplant recipients and may adversely affect graft function and overall outcomes. In this study, the detection rate of conventional intestinal pathogens was low; however, CMV infection and alterations in intestinal microbiota were common findings. These results highlight the importance of considering CMV infection and microbiota-related etiologies in pediatric transplant patients presenting with diarrhea, alongside noninfectious causes related to immunosuppressive therapy.