Non-recruitment to and selection bias in studies using echocardiography in haemodialysis patients.

Background: It is unknown whether patients recruited to renal cardiac imaging studies are fully representative of the investigated population and whether there are differences in characteristics and survival between participants and non-participants (excluded or declined consent). Subjects and Methods Four hundred thirty-five maintenance haemodialysis patients were screened in an observational, prospective study. Baseline demographics, laboratory results, social deprivation scores and survival data were collected from patient records. All patients were followed-up until death, renal transplantation or 16 November 2015.

Results: Forty-four patients were excluded (16 language barrier, 10 mental incapacity, 9 severe co-morbid illness and 9 because of immobility), 172 patients declined consent (84% due to reluctance to attend for an extra visit) and 219 patients were recruited. Excluded patients had a lower mean haemoglobin (10.2 g/dL vs 10.7 g/dL), phosphate (4.15 mg/dL vs 4.74 mg/dL), albumin (3.6 g/dL vs 3.8 g/dL) and higher C-reactive protein (3.2 mg/dL vs 1.6 mg/dL) compared with recruited patients. No difference was identified between groups for Charleston comorbidity index (P = 0.115) or social deprivation scores. After a median follow-up of 29.7 (25th-75th percentile, 21.1-34.3) months, there were 141 deaths. In a multivariable Cox regression model adjusting for BMI, age, Charleston comorbidity index, haemoglobin, albumin, smoking status and diabetes mellitus, patients who declined consent had an adjusted HR of 1.70, 95% CI 1.10-2.52, and excluded patients had an adjusted HR of 1.30, 95% CI 0.75-2.25, for all-cause mortality compared with recruited patients.

Conclusions: Patients recruited to the study had longer survival compared with non-participants. Research studies should document phenotypes of non-participants to aid interpretation and generalizability of results.