Background: Chronic kidney disease (CKD) is progressive and characterised by the destruction of nephrons. The loss of kidney functions leads to the accumulation of metabolic waste products that have an impact on the patient’s body. There are five CKD stages according to the estimated glomerular filtration rate (eGFR), and the last stage is end-stage renal disease (ESRD) or kidney failure (eGFR < 15 ml/min/1.73 m). The treatment of CKD consists of conservative therapy aiming at slowing down the progression of the disease. However, when these measures are insufficient, the patient undergoes dialysis or a transplantation is performed. A kidney transplantation is the treatment of choice for improved survival of ESRD patients, but when a transplantation is not possible either due to the medical condition of the patient or the lack of available organs, dialysis is a viable treatment option. There are two main types of dialysis: haemodialysis (HD) and peritoneal dialysis (PD). The decision to initiate dialysis treatment using either haemodialysis or peritoneal dialysis is often complex and remains open to debate. Although PD has proven to provide similar or better survival rates and a better quality of life in addition to PD being more economical than HD, in 2008, there were only 196 000 PD patients comprising 11% of the global dialysis population. Periodontitis is a bacteria-driven chronic inflammatory disease that destroys the connective tissue and bone supporting the teeth. Periodontitis represents a potential source of episodes of bacteraemia, especially in immunocompromised patients. Its impact on the general health status is becoming increasingly apparent. A recent meta-analysis confirmed that there is a link between CKD and periodontal disease. Periodontitis thus represents an often overlooked problem in CKD patients. The lack of oral health management may contribute to systemic consequences, such as inflammation, infection, protein-energy wasting and atherosclerotic complications, which can contribute to increased morbidity and mortality. The aim of this study was to quantify the inflammatory burden that periodontitis poses in dialysis patients and to examine whether PD and HD patients differ according to their periodontal status, which can be helpful in selecting the most appropriate type of dialysis. The main hypothesis put forth in this study is that PD patients have a better periodontal status than HD patients. Methods: A cross-sectional study including 58 consecutive HD patients and 31 consecutive PD patients was conducted. In addition to the usual periodontal indices, the periodontal inflamed surface area (PISA) was calculated based on bleeding on probing (BOP), clinical attachment level (CAL) and gingival recession (REC) measurements that were performed at six sites on each tooth using a periodontal probe (PCP 15; Hu-Friedy, Chicago, IL, USA). All periodontal examinations were performed by the same calibrated examiner. Based on existing literature and previous studies, before conducting our analysis, 15 variables were selected with possible confounding effects: age, duration of dialysis in months, number of teeth, smoking habits, C-reactive protein, dialysis adequacy measured by the ratio between dialyser clearance (K) (mL/min) multiplied by time in minutes (t) and the volume of water a patient’s body contains (Kt/V), thrombocytes, urea, phosphorus, high-density lipoprotein (HDL) cholesterol and treatment with beta-blockers, angiotensin-converting enzyme inhibitors, central α-2 receptor agents, angiotensin II AT1-receptor blockers, and α₁-adrenoceptor antagonists. We also controlled five additional possible confounders: diabetes mellitus, duration of diabetes, glycated haemoglobin (HbA1c), leukocytes, and the last visit to the dentist. Information about age, gender, education, smoking habits and consumption of alcohol, xerostomia, self-observed bleeding of gums and oral hygiene habits was obtained through a questionnaire that was designed specifically for this study. Results: After screening 127 patients, 89 were enrolled, 58 of whom were undergoing HD and 31 of whom were undergoing PD. The two groups were different with regard to many sociodemographic, vital, lifestyle and clinical characteristics. HD patients were older, had been undergoing dialysis longer, had fewer teeth, and had less self-reported bleeding of the gums. According to blood count, the largest differences were in Kt/V and parathyroid hormone levels. Kt/V was higher in the PD group and the parathyroid hormone was higher in the HD group. There were also relevant differences in C-reactive protein values (higher in the HD group) and in thrombocyte values (higher in the PD group). All periodontal indices were significantly different according to the type of dialysis; haemodialysis patients had higher scores than peritoneal dialysis patients. In the introductory bivariable analysis, PISA was significantly different between the two dialysis groups. The mean PISA (SD) was 738 (520.4) mm2 in HD patients and 470 (277.8) mm2 in PD patients. After adjusting for 20 confounding factors, the type of dialysis was found to be significantly associated (FDR<5%) with PISA. PD patients had a significantly lower PISA. After adjusting for 20 confounding factors, the mean (95% CI) PISA was 798 (681-914) mm2 in the HD group and 52 (0-417) mm2 in the PD group. This 746 mm2 absolute difference represented a 93% relative difference. The adjusted median PISA was 732 mm2 in the HD group and 190 mm2 in the PD group. This 542 mm2 absolute difference represented a 74% relative difference. The type of dialysis showed a semipartial correlation with PISA (sr=-0.50, p<0.017; FDR<5%). The variation in HbA1c values imputed for patients with no diagnosed diabetes mellitus 4, 5 and 7 revealed identical results. A sensitivity analysis was performed by multiple imputation of the missing data of 20 confounding factors. A pooled analysis on 30 data sets with complete (imputed) data revealed very similar results to the result of the per-protocol and complete case (listwise deletion) analyses: PD patients had a mean (95% CI) PISA of -613 (-995 to -232); robust regression, ( p=0.002) and a median (95% CI) PISA of -448 (-887 to -9; quantile regression, p=0.046). PISA was significantly lower in the PD group regardless of the duration of dialysis. After adjusting for confounding factors, the interaction between the duration and type of dialysis was not significant (F (2,44)=0.01; p=0.994; η2=0.00). The differences in PISA between patients who had been dialysed for less than a year, 2-3 years or ≥3 years were not significantly different in any of the two dialysis groups. Conclusions: Patients undergoing dialysis have poor periodontal conditions and require periodontal treatment. PD is associated with a lower PISA and other periodontal indices regardless of many sociodemographic, lifestyle, laboratory and clinical factors. A prospective, randomised control study is needed to test for a causal relationship.