Organisation of the DEGREE project
The DEGREE project is an international collaboration involving standardized protocols for estimating population patterns of eGFR. Individual centres can register with the DEGREE study by submitting a registration document (Additional file 1). A number of study sites have either commenced, or are in the planning stages of, i.e. awaiting the outcome of funding decisions, studies with protocols using the DEGREE methodology (with minor modifications in some cases). Centres will be required to follow the standardised core protocol in order to be part of the DEGREE collaboration, and to be included in international comparisons. However, investigators will be encouraged to enhance this minimum protocol with additional investigations as appropriate to the local situation. Possible examples include: additional questions focused on work exposures, renal ultrasound imaging, formal albumin:creatinine ratio measurements. Example proformas for some of these additional modules are available in the Additional file 2 or from the authors.
Core protocol - ethical and regulatory issues
Participants will need to provide informed consent before taking part in the study, and Investigators will be responsible for producing participant information sheets and gaining approval from the local institutional review board. It is envisaged that eGFR results will be communicated back to participants once available. The need for repeat testing in those subjects with raised serum creatinine levels, the impact of abnormal findings on those taking part (including newly diagnosed low eGFR, glycosuria or hypertension), as well as mechanisms for onward referral to local healthcare systems will constitute an important aspect of the ethical framework in which this protocol is undertaken. These issues should be formalised into the local version of the protocol before commencing a DEGREE study. Furthermore, the need for close collaboration with the local and national health system during the planning phase of a DEGREE study includes not only the system for individuals in need of follow up, but also agreements on how results from the DEGREE study at the population level should be communicated to different stakeholders for public health purposes.
Core protocol - questionnaires and study instruments
The aims of the core questionnaire and clinical measurements are to obtain a minimum core dataset to allow the estimation of the distribution of eGFR within and between populations, and to estimate the prevalence of reduced GFR that is not due to diabetes, glomerulonephritis or associated with hypertension. Investigators should add additional questions on exposures that are relevant to the local context or concerns of the population. The questionnaires and protocols (see Additional file 3) have been developed by modification of the STEPS instruments . Translation and validation of questionnaires into other languages so that all groups within a region can participate will be the responsibility of local coordinators. Questionnaires should be conducted in a language spoken by all participants or translators recruited where participants may not all speak the same language.
Core protocol - population and sampling strategy
For robust international comparisons a representative population is critical. The protocol should be undertaken in randomly–selected sample (or alternatively the entire population) of adults aged over 18 years in a specified geographical area (i.e. study participants must not be selected based on them presenting to healthcare facilities or advertised screening sessions). A region or district should be selected and clearly defined using GIS coordinates or map. Sampling units can be individuals or households; in the former case, a new census should be conducted unless an up-to-date and complete census is already available for the entire defined area. This protocol may of course also be applied in specific populations, for example occupational groups, but studies based on non-representative samples will not be used for international comparisons in the first phase of the DEGREE project.
The ideal sample size for a population-based study (see below) is 1000 participants per study centre, and this is required for inclusion in international comparisons, but it is recognised that studies smaller than this and in specific populations, particularly in occupational groups, may generate valuable information.
Core protocol - study visits
The study visit will involve the administration of the core questionnaire, basic clinical measurements, dipstick urinalysis and a blood draw for serum creatinine measurement (proformas for these are included as Additional file 3). Where possible participants should be asked to attend first thing in the morning and fasted overnight, i.e. before the first meal of the day and before starting work.
Core protocol - testing
Dipstick urinalysis should be performed using commercial testing sticks according to manufacturers instructions. Sticks should report urine blood, protein (at macroalbminuric level), leucocytes, glucose, pH and specific gravity as a minimum and ideally read using an optical reader. Serum creatinine should be measured locally using a method calibrated to an isotope dilution mass spectrometry standard with assay and calibration details reported to the DEGREE data centre. Height should be measured using a stadiometer and weight using digital scales. Body composition should be quantified using calibrated single frequency (50 kHz) bioimpedence analysis instrument (supine instrument where possible) as a direct impedance output, with machine-calculated values also reported (examples and suppliers of suitable instruments can be obtained from the DEGREE executive). Additional bio-samples (e.g. whole blood, serum and urine) should be collected and stored at -20C (or lower where possible) to allow for cross-centre validation of serum creatinine values and the testing of additional markers of renal function such as cystatin C as discussed above. The number and type of these samples will depend on local circumstances but we suggest a minimum of one additional serum sample (total 5 mL stored as three 1.5 mL aliquots) for each participant. These samples should be handled in accordance with local standard operating procedures with appropriate attention to labelling, storage, equipment maintenance and documentation .
Core protocol - data management and reporting
All DEGREE protocols are freely available to interested investigators. Each individual Centre will ‘own’ their own data, but will be asked to submit an anonymised copy of individual level data, together with contextual information on the investigated population to the DEGREE Data Centre for inclusion in international comparisons.
Paper based studies should use double data-entry to minimise human manual transcription error. Samples and questionnaires will ideally be identified using a barcode system. Centres can submit their cleaned data using sample database available from the DEGREE team based in London.
A number of ongoing projects are currently attempting to assess the performance of eGFR estimating equations in different regions and developing ethnicity specific modifications. Until these are available, data should be provided as serum creatinine estimates to enable standardised calculation at the analysis centre with a variety of formulae.
International comparisons will be conducted and coordinated by the DEGREE Steering Committee. Descriptive statistics of eGFR using both CKD-EPI and MDRD formulae (with and without adjustment for body composition and self-reported meat consumption) will be presented by country, region, age and sex. The distributions of eGFR will be estimated in the full sample, and then restricted to those without evidence of hypertension, diabetes, or urinary evidence of glomerulopathy. As secondary analyses we will investigate the prevalence of people with eGFR < 90 ml/min restricted to those without hypertension, diabetes, or urinary evidence of glomerulopathy, adjusted for age, sex and body composition.
Reporting to the DEGREE data centre should be in accordance with previous guidelines . This includes description of the sampling frame, the characteristics of the population, the sampling method, response rates and the methodology used for laboratory and clinical measurements. Example reporting forms are presented in the Additional file 3.
Study size and power
The required study size has been estimated based on two considerations: (i) obtaining reasonably accurate estimates of eGFR in each centre; (ii) having sufficient statistical power for comparisons between centres or between population subgroups. For both these considerations, the power calculations have been done two ways: (i) using eGFR as a continuous variable (mean, sd) in order to estimate the population prevalences of impaired kidney function and (ii) ‘cutoffs’ for eGFR. The overall aim is to have sufficient statistical power to estimate prevalence, and detect population differences of epidemiological and health service significance.
Overall, a recommended sample size of 1,000 participants has been chosen.
Estimates within a single population: Using a continuous measure if the ‘true’ mean eGFR is 110 mL/min (although it is recognised this value is likely to vary substantially between different regions), with a standard deviation (SD) of 30 mL/min it is more than 95% likely that the estimated mean will be between 108 mL/min and 112 mL/min. Assuming a ‘true’ prevalence of eGFR <90 mL/min of 5%, then with a sample size of 1,000, it is 95% likely that the estimated prevalence will be between 3.6% and 6.4%.
Comparisons between populations: Using a continuous measure if one population has a mean eGFR 5 mL/min lower than that in another population, the study will have more than 95% power to detect this difference; for comparisons between population subgroups (e.g. three subgroups of equal size), the study will have more than 80% power to detect a difference of 7 mL/min in mean eGFR. If one centre has a prevalence of impaired kidney function (eGFR <90 mL/min) of 5% and another centre has double the risk (i.e. prevalence of 10%), then a sample size of 1,000 will provide 99% power to detect this difference; for comparisons within centres, if the centre participants are divided into three equal groups, the study will have approximately 80% power to detect a doubling of risk when comparing any two of the three subgroups.
Prevalence studies are the basis on which the populations and study design for further investigations can be determined. Once variation in eGFR distribution and estimates of CKD and CKDu prevalence have been established, appropriately designed aetiological studies can be pursued. In collaboration with other investigators we will continue to develop protocols for follow-up studies (examining the roles of occupational and environmental factors) and intervention studies in affected populations.