We will conduct a systematic review to determine the relationship between vitamin D concentration or supplementation and subsequent allograft function in kidney transplant recipients. The primary outcome will be graft function determined by the measurement of GFR (e.g. inulin clearance) or creatinine clearance (timed urine collection) as estimated from serum creatinine concentrations at different times post-transplantation. Secondary outcomes will include acute rejection, chronic allograft nephropathy, proteinuria and graft loss.
Types of studies
In this review, we will include two types of study: (1) designs that examine the association between serum 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D concentration and our stated primary and secondary outcomes and (2) designs that examine the effect of vitamin D supplementation on our stated outcome measures. Although this is the most inclusive approach, we anticipate several challenges. First, the thresholds used to define inadequate 25-hydroxyvitamin D levels in various studies may differ from recent guidelines . For the purposes of this review, we will use thresholds selected by the study authors. Second, we anticipate heterogeneity in vitamin D formulation with some studies providing nutritional vitamin D supplements (e.g. cholecalciferol) and others using active vitamin D compounds (e.g. calcitriol and paricalcitol). As these formulations may affect graft function differently, pooling these results may be inappropriate (see Discussion).
A comprehensive electronic search will be conducted using MEDLINE, EMBASE, AMED and CINAHL with the assistance of a librarian experienced in systematic reviews. A structured search strategy will be based on controlled vocabulary and relevant key terms and will be broad to prioritize sensitivity (see the Appendix). The references of included articles and existing reviews will be scanned for additional resources.
Study screening and inclusion
All titles and abstracts from our comprehensive search will be screened by two independent reviewers. The inclusion and exclusion criteria used for each screening step are outlined below. If no abstract is available, the full text will be obtained unless the article can be confidently excluded by its title alone. In general, if there is any doubt as to whether a study should be excluded, the study will proceed to the full text screen to reduce the likelihood of incorrectly excluding a relevant study. Full text copies of potentially relevant papers will be obtained for independent analysis by two reviewers. Any disagreements will be reconciled by a third party.
Our review will focus on adult male and female subjects who received either a living donor or deceased donor kidney transplant. We will include retrospective and prospective studies (cross-sectional, case–control and cohort) and interventional studies (randomized and non-randomized). For studies that measure serum vitamin D concentration, we have not predefined when a sample needs to be measured (e.g. pre-transplant, 3-months post-transplant, 1-year post-transplant, etc.). For studies that assess the impact of vitamin D supplementation, all doses and formulations of vitamin D will be included. Studies must report one or more of the primary or secondary outcomes listed above to be eligible. Non-English articles will be included when there is a translator available at our institution. Publication dates will be restricted to 1990 and later given the changes in immunosuppression, transplant outcomes and measurement techniques for vitamin D that have occurred since then.
We will exclude case reports, narrative reviews, letters, animal studies and those with a sample size <30. Studies involving only pediatric patients or combined adult/pediatric populations where the data are not reported separately will be excluded. Studies involving multi-organ transplantation (e.g. kidney-pancreas) will also be excluded. We will exclude studies that report only bone-specific outcomes (e.g. fracture rates and bone mineral density), as there is an existing Cochrane review on this topic .
Each study included in the review will undergo a standardized data extraction process using a pre-formatted spreadsheet. The extracted data will be verified by a second reviewer to reduce reviewer errors and bias. Information pertaining to study identification (first author, year of publication, number and location of centers), study design (type of study, sample size, inclusion and exclusion criteria, length of follow-up, type and dosage of vitamin D supplements) and patient population (age, gender, type of immunosuppression, donor type, duration of renal-replacement therapy, percentage of hemodialysis patients, time since transplant, number of prior transplants and cold ischemia time) will be included. These variables will be extracted for all types of studies.
The continuous variables extracted include GFR (as measured by inulin clearance), estimated GFR (eGFR; estimated from serum creatinine) and proteinuria. Proteinuria will be measured by 24-hour urine protein collection or the albumin-to-creatinine ratio, depending on the information that is available. For categorical variables, the extracted outcomes will be acute rejection, delayed graft function, graft loss and biopsy scores for interstitial fibrosis and tubular atrophy.
Time since transplantation will be recorded for each of the outcome variables. Time points will differ depending on which follow-up intervals were selected by the study author; this will be considered during data analysis. Outcomes will be recorded on separate spreadsheets for studies assessing serum vitamin D levels and studies assessing vitamin D supplementation (treatment group vs control).
If there are eligible randomized controlled trials, quality will be evaluated using the Cochrane Risk Assessment Tool. Studies will be assessed on randomization, generation of allocation sequence, allocation concealment, blinding and follow-up. We will evaluate observational studies with the Newcastle–Ottawa Scale. The quality of evidence across studies will be assessed for each outcome using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach. GRADE considers the risk of bias, consistency of results across studies, precision of the overall estimate across studies, magnitude of effect and importance of the outcome . The quality of evidence will be rated as high, moderate, low or very low for each outcome.
For all included studies, we will provide a detailed description of the results in both tables and text. We will include data regarding study identification, study design and patient population (as outlined above). For studies assessing the effect of vitamin D supplementation, we will compare outcomes (e.g. acute rejection) for patients receiving vitamin D supplements to those for patients not receiving supplements.
For studies that measure serum vitamin D concentration, we will compare outcomes in patients with vitamin D insufficiency to patients with adequate serum vitamin D levels. As these definitions were not developed for the renal transplant population, we will also examine serum vitamin D as a continuous variable. We will pool treatment effect estimates where possible using standard statistical techniques. In the event that combining data across studies is not feasible due to inadequate information or excessive heterogeneity, we will use descriptive methods to present data by outcome.
Our inclusion criteria do not specify time points at which outcomes are measured (e.g. GFR at 12 months) to avoid excluding potentially useful information; however, we will account for timing during data analysis. We intend to group all studies together initially and then perform sensitivity analyses for different time points (e.g. GFR at less than one year or greater than one year), if possible.