Verteporfin photodynamic therapy for neovascular age-related ...

14 MethodsThe strategy was based on electronic data entry taking place at each site, in real time, duringthe course of each patient visit. Thus, records clerks and reception staff would enter initialdemographic data, optometrists the BCVA measurements and ophthalmologists the data relatingto disease assessment and treatment. The use of paper case record forms would be avoided, and atthe end of each session data would be transmitted electronically to the data management centreat the LSHTM. As the number of data entered at each clinic grew, so it was hoped that the datasources would become administrative tools in their own right, and be used routinely to accessand review patient information. We expected regular review and appraisal of data to help toimprove the accuracy of data collected for the study.Data transmitted electronically to the LSHTM were placed into a secure central database andwere ‘queried’ extensively with respect to data ranges and consistency. The data queries thatresulted from these checks were e-mailed as a report to the relevant centres, which were askedto make corrections to the data held in their own local database or confirm that the original datawere indeed correct. Corrections were part of the next data transmission, and queries which hadbeen successfully resolved were labelled as such in the central database and removed from thesubsequent data monitoring report.The majority of data being collected was information that we expected to be collected in thecourse of usual care, although investigations were required to be carried out according tothe manual of operations. Additional data were required about potential predictors of visualfunction outcome – possible ARs and AEs that might otherwise have been considered ‘expectedoccurrences’ (e.g. back pain). Additional data collection (CS, HRQoL and resource use) wascarried out in the subset of centres.Although the data collection strategy strove to avoid using paper case record forms, therewere two elements of the study in which these were to be used. The first was the recordingof visual acuity onto paper data sheets by optometrists at each clinic. This paper record wasintended to act as a validation for the BCVA data (the primary outcome), which were also beingcollected electronically, and the paper forms were sent directly to the LSHTM for data entryand comparison with the electronic records. Paper forms were also used to collect HRQoL andresource use by the subset of 18 participating centres.Risk of biasesRisk of bias is described below for the main bias domains identified in the Cochrane Handbookfor Systematic Reviews of Interventions 41 when reviewing primary studies of the effectivenessof interventions.Selection biasThe VPDT cohort study focused on patients who were treated with VPDT. Although weattempted to collect baseline data for all patients referred for VPDT, including those subsequentlyfound to be ineligible, ineligible patients were clearly different from treated ones and we neverplanned to compare outcomes in these groups.The potential remained for selection bias when comparing outcome between subgroups in thetreated cohort, for example according to patients’ classification with respect to the inclusioncriteria for the TAP trials or baseline measurements of classic and occult CNV. Therefore, weattempted to characterise a range of potential confounding factors in order to minimise the riskof confounding (see Other predictors of visual function).