Patients and Its Effect on Various Disease Markers Adel B. Korkor, MD; Christine M. Bretzmann, PharmD; Daniel C. Eastwood, MS Drs. Korkor and Bretzmann are with Purity Dialysis Centers, Delafi eld, Wisconsin; D. Eastwood is with the Division of Biostatistics, Medical College of Wisconsin, Milwaukee, Wisconsin. BACKGROUND: Epidemiologic data has linked vitamin D defi ciency with a variety of negative health outcomes. The purpose of this study was to determine the prevalence of 25-hydroxy-vitamin D defi ciency in our outpatient dialysis patients and to assess the association between vitamin D level and various markers of disease. METHODS: We performed a cross-sectional study evaluating 25-hydroxy-vitamin D levels in 327 chronic dialysis patients. Associations between vitamin D level and various disease markers were measured using Pearson correlation, Spearman correlation, and Kendall s Tau. RESULTS: Mean 25-hydroxy-vitamin D level was 19.7 12.7 ng/ml. Vitamin D defi ciency, defi ned as 25-hydroxyvitamin D <30 ng/ml, was present in 85% of individuals (277/327). Using a Spearman correlation, there was a statistically signifi cant negative association between vitamin D level and triglycerides and transferrin saturation while a positive association existed with albumin. No other signifi cant correlations were noted. CONCLUSIONS: Although vitamin D defi ciency was highly prevalent in our dialysis patients, we were unable to demonstrate a correlation between a 25-hydroxy-vitamin D level and the majority of disease markers evaluated in this study. Further research is necessary to fully elucidate the consequence of vitamin D defi ciency in patients with end-stage renal disease. Medical journals have recently been inundated with research regarding the potentially detrimental effects of 25-hydroxyvitamin D deficiency. Within the last year alone, studies have been published linking low vitamin D levels to an increased risk for all-cause mortality, cardiovascular disease and mortality, myocardial infarction, fatal stroke, hip fracture, chronic pain, anemia, and upper respiratory tract infection. 1 9 Prior to this, vitamin D deficiency has been implicated as a risk factor for a multitude of illnesses ranging anywhere in severity from influenza to life-threatening cancer. 10,11 These findings are important because they suggest that vitamin D is not only important in optimal skeletal health, but may also play a role in the health of other major organ systems. Although the optimal level of 25- hydroxy-vitamin D has yet to be established, the National Kidney Foundation (NKF) Kidney Disease Outcomes Quality Initiative (KDOQI) guidelines indicate that serum levels of <30 ng/ml are insufficient; 25-hydroxy-vitamin D levels tend to decline with worsening kidney function 1 with 80% to 90% of dialysis patients having serum levels below 30 ng/ml. 12-15 Contributing to this increased prevalence may be the lower exposure to sunlight and lower ingestion of vitamin D rich foods commonly seen in dialysis patients. Additionally, the endogenous synthesis of vitamin D 3 from sunlight has been shown to be altered in individuals with compromised renal function as well as those over 60 years of age, which represent a substantial portion of the dialysis population. 16,17 The majority of studies evaluating the deleterious effects of vitamin D deficiency have excluded patients with severe renal insufficiency. Although Wolf et al 13 showed an increased risk for early mortality in vitamin D deficit hemodialysis patients, much is to be learned regarding the role vitamin D deficiency may play in the many disease processes common to this patient population. The purpose of this study was to determine the prevalence of 25-hydroxyvitamin D deficiency in our outpatient dialysis patients and to assess the association between vitamin D level and various markers of disease. Methods We performed a cross-sectional study evaluating vitamin D levels in chronic dialysis patients from 9 outpatient clinics in southeastern Wisconsin. Serum 25-hydroxyvitamin D levels were measured in July of 2008 using the Diasorin Liaison chemiluminescence assay (Diasorin, Stillwater, œ DOI: 10.1002/dat.20367 November 2009 Dialysis & Transplantation 1
TABLE I. Descriptive statistics for vitamin D and various markers of disease. Variable n Mean (SD) 25-hydroxy-vitamin D (ng/ml) 327 19.7 (12.7) Age (yrs) 327 69.1 (13.6) Parathyroid hormone, intact (pg/ml) Minn.). All blood samples were evaluated at a single laboratory. For the purpose of this study, vitamin D deficiency was defined as a serum 25-hydroxy-vitamin D level of <30 ng/ml. To evaluate the association between 25-hydroxy-vitamin D level and various disease markers, patients charts were reviewed for select laboratory values and dose of epoetin alfa (Epogen; Amgen, Thousand Oaks, Calif.) during the same time period that vitamin D was measured. The variables recorded were primarily focused on 3 disease processes: metabolic 312 277.9 (218.8) Alkaline phosphatase (U/L) 327 108.7 (71.0) Calcium-phosphorus product (%) 327 44.1 (12.9) Hemoglobin (g/dl) 325 11.3 (1.4) Hematocrit (%) 325 34.6 (4.4) Transferrin saturation (%) 327 33.6 (18.4) Epogen dose (units/wk) 325 12,822 (15,125) Albumin (g/dl) 327 3.5 (0.4) Cholesterol (mg/dl) 311 136.2 (35.4) LDL-C (mg/dl) 305 64.7 (27.5) HDL-C (mg/dl) 311 42.4 (15.9) Triglycerides (mg/dl) 311 148.7 (86.0) Homocysteine ( mol/l) 310 28.4 (10.3) Median Minimum Maximum 15.7 6.9 * 92.7 72.0 21.0 94.0 222.7 4.0 1,600 90.0 27.0 769.0 42.0 14.0 88.0 11.3 7.2 17.0 34.1 24.0 51.8 29.0 6.0 101.0 9,000 0 66,000 3.6 2.0 4.5 133.0 52.0 272.0 60.0 8.0 185.0 40.0 8.0 133.0 125.0 39.0 629.0 27.3 8.9 118.0 * Values that were considered undetectable (<7 ng/ml) were denoted a value of 6.9. LCL-C, lowdensity lipoprotein cholesterol; HDL-C, high-density lipoprotein cholesterol. bone disease, which included measurements for intact parathyroid hormone, alkaline phosphatase, and calcium-phosphorus product; anemia, which included measurements for hemoglobin (Hgb), hematocrit, transferrin saturation, and weekly Epogen dose; and dyslipidemia, which included measurements for total cholesterol, lowdensity lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL- C), and triglycerides. Additional variables recorded included age, albumin, and homocysteine. Laboratory variables were typically measured on the first Monday or Tuesday of the month prior to the initiation of dialysis. With the exception of parathyroid hormone (PTH), which was measured between May and July of 2008, and markers of dyslipidemia (cholesterol, LDL-C, HDL-C, triglycerides) and homocysteine, which were measured in August of 2008, all other variables were measured in July of 2008. Weekly Epogen dose was determined by calculating the average dose per week during the month of July 2008. The association between 25-hydroxyvitamin D level and hospitalizations, infections, and fractures was also evaluated. A retrospective chart review was performed collecting data on the number of hospital admissions, total number of hospital days, hospital-related infections, and fractures requiring hospitalization. Hospital-related infections included infections requiring hospital admission, infections occurring at the time of hospital admission but not necessarily the primary admitting diagnosis, or infections developing during the course of the hospital stay. All data were collected for an approximate 2-year period prior to when 25-hydroxy-vitamin D levels were drawn (i.e., prior to July of 2008). Because data were collected retrospectively, an assumption was made that vitamin D level did not change during that time period. Statistical Analysis Descriptive statistics, presented as means standard deviation (SD), were used to describe 25-hydroxy-vitamin D level and all other covariates evaluated in this study. 25-hydroxy-vitamin D levels that were undetectable (<7.0 ng/ml) were recorded as 6.9 ng/ml in order to perform statistical analyses. Associations between vitamin D level and various markers of disease were measured using Pearson correlation, Spearman correlation, and Kendall s Tau as appropriate for the type of data that were examined. Because of the large number of associations that were examined, a significance level of 0.01 was used for the interpretation of results in an attempt to reduce the false discovery rate. All statistical analyses were performed by the Biostatistics Division of the Medical College of Wisconsin using SAS version 9.1 (SAS Institute, Inc., Cary, NC). 2 Dialysis & Transplantation November 2009
TABLE II. Relationship between 25-hydroxy-vitamin D level and various markers of disease. Results Spearman Correlation Data were obtained from a total of 327 chronic dialysis patients. The mean age of patients was 69.1 13.6 years and 56% (183/327) were male. The majority of patients were receiving hemodialysis (n 305) with a small subset of patients on peritoneal dialysis (n 22). Mean 25-hydroxy-vitamin D level was 19.7 12.7 ng/ml. Mean vitamin D levels were higher in men (21.5 13.3 ng/ml) than in women (17.4 11.4 ng/ml). The majority of individuals (85% [277/327]) were found to be vitamin D deficient (25- hydroxy-vitamin D <30 ng/ml). Mean values and other descriptive statistics for 25-hydroxy-vitamin D and the covariates evaluated in the study are presented in Table I. Using a Spearman correlation, there was a statistically significant negative association between vitamin D level and triglycerides and transferrin saturation while a positive association existed with albumin (Table II). However, only the association with triglycerides remained significant using a Pearson correlation. No other significant correlations were noted. Data relating to hospitalizations and hospital-related infections and fractures were available in a subset of patients (n 175). The mean number of hospital admissions was 2.7 2.7 and the mean total number of hospital days was 19.9 23.0. Hospital-related infections and fractures requiring hospitalization occurred relatively infrequently with means of 1.0 1.4 and 0.07 0.32, respectively. No significant associations with 25-hydroxy-vitamin D level were found using any of the statistical analyses. Conclusions Pearson Correlation Variable Coefficient p-value Coefficient p-value Age 0.03371.54 0.00008.9 Parathyroid 0.01948.73 0.04531.43 hormone, intact Alkaline phosphatase 0.07581.17 0.00173.9 Calcium-phosphorus 0.06885.21 0.10070.07 product Hemoglobin 0.05009.37 0.06351.25 Hematocrit 0.06244.26 0.04689.40 Transferrin 0.16271.003 * 0.12010.03 saturation Epogen dose 0.04569.41 0.03191.57 (weekly) Albumin 0.15010.007 * 0.08793.11 Cholesterol 0.10017.08 0.09036.11 LDL-C 0.01058.85 0.03607.53 HDL-C 0.02399.67 0.03074.59 Triglycerides 0.17540.002 * 0.18904 <.001 * Homocysteine 0.04664.41 0.08820.12 * Statistically signifi cant correlation (p <.01). A total of 85% of patients in our outpatient dialysis clinics were found to be vitamin D deficient, which compares well with previous reports on this topic. However, because 25-hydroxy-vitamin D measurements were all performed in the summer month of July, it is likely that true prevalence may have been underestimated. When evaluating the relationship between 25-hydroxy-vitamin D and various disease markers, we were only able to identify a weak association between vitamin D level and triglycerides, transferrin saturation, and albumin. Martins et al 18 recently examined the association between cardiovascular risk factors and 25-hydroxyvitamin D level using data from the Third National Health and Nutrition Examination Survey (NHANES III). Similar to findings from our analyses, a significant association was found between hypertriglyceridemia and low vitamin D level. The implication of these findings to the increased cardiovascular mortality seen in dialysis patients has yet to be established. In their evaluation of NHANES III data, Martins et al 18 also found a significant relationship between low albumin and low 25-hydroxy-vitamin D level. Conversely, Melamed et al 1 found no correlation in their analysis of the NHANES III data. Studies conducted in dialysis patients have also yielded conflicting results. 13,19 In our analyses of dialysis patients, we were able to detect a weak positive association between albumin and vitamin D level. Kendrick et al 8 recently examined the association between 25-hydroxy-vitamin D level and Hgb in patients with chronic kidney disease and found that risk for anemia was lowered by 29% for every 10 ng/ml increase in 25-hydroxyvitamin D. Monthly vitamin D (ergocalciferol) replacement therapy decreased the weekly Epogen dose requirements in over 60% of hemodialysis patients according to another study, although Hgb was not significantly changed. 15 We evaluated several markers of anemia including Hgb, hematocrit, transferrin saturation, and weekly Epogen dose and only found a weak negative association with transferrin saturation. Vitamin D deficiency can lead to secondary hyperparathyroidism in individuals with normal renal function. In the study by Dobnig et al 2 which excluded patients with chronic kidney disease, mean PTH levels were 36% higher in the group with the lowest 25-hydroxy-vitamin D levels. œ November 2009 Dialysis & Transplantation 3
However, the relationship between PTH and 25-hydroxy-vitamin D has been less clear in clinical trials evaluating dialysis patients. Only a poor correlation between the variables was noted in the large study by Wolf et al 13 evaluating more than 800 hemodialysis patients. Additional studies conducted in dialysis patients have shown a lack of correlation. 19,20 Similarly, no correlation was detected in our analyses evaluating PTH and vitamin D level. Vitamin D deficiency has been associated with low bone mineral density and an increased risk for fracture. 21,22 Vitamin D level did not correlate with any bone disease marker evaluated in our study. Incidence of hospital-related fracture also did not correlate with vitamin D level. However, hospital-related fractures occurred infrequently during the study period making it difficult to draw statistical inferences. Additionally, we examined hospitalizations over a 2-year period, which may not have been sufficient time to assess fracture risk. Cauley et al 6 showed an increased risk for hip fracture with low 25-hydroxy-vitamin D levels, but their observations were completed over a median period of 7.1 years. This was an observational study, and inherent to any study of this nature is the inability to identify causal relationships. For this study, a small subset of variables thought to be related to 25-hydroxyvitamin D was evaluated. However, other variables exist such as history of diabetes mellitus, smoking, hypertension, increased body mass index, and ethnicity that were not measured, but have been shown to be independently associated with vitamin D level. 1,18 Because the statistical analyses utilized for this study did not adjust for these unmeasured variables, it is possible they could have accounted for residual confounding and potentially may have influenced our final results. Laboratory variables evaluated in this study were typically drawn at the beginning of the week as compared with midweek, which may be standard in other dialysis centers. Unfortunately, there remains a lack There remains little evidence showing that treatment of vitamin D defi ciency with replacement therapy actually improves health outcomes. of clear guidelines as to the optimal time to collect laboratory samples in patients undergoing dialysis. Hgb levels, for example, are subject to a dilutional effect with higher interdialytic weight gain. Therefore, Hgb levels drawn on a Monday or Tuesday, when interdialytic weight gains tends to be greater, would be expected to be lower than levels drawn midweek. Hemoglobin levels have been shown to be 0.5 to 0.6 g/dl lower when drawn after a long interdialytic interval (3 d) versus a shorter interval (2 d). 23 However, because differences in patient outcomes relating to timing of Hgb blood sampling are lacking, KDOQI guidelines maintain that Hgb levels can be drawn without regard to dialysis day. Other laboratory variables, such as phosphorus, may be falsely elevated after a long interdialytic interval. Yokoyama et al evaluated hemodialysis patients with serum phosphorus levels >7.0 mg/dl and found that levels were significantly higher when drawn after a 3-day dialysis interval versus midweek (8.11 mg/dl versus 7.13 mg/dl, respectively). 24 Although the timing of blood samples should be considered when interpreting mean laboratory values identified in our study, it should have minimal influence on whether or not the variable correlated with vitamin D level. An additional consideration when evaluating our study findings is that lipid profiles were often drawn in a non-fasting state, particularly when patients were receiving dialysis later in the day. Because eating can potentially increase levels of triglycerides and cholesterol, KDOQI recommends an overnight fast prior to drawing lipid profiles if possible. Although it is possible that mean values for triglycerides and cholesterol were falsely elevated due to non-fasted sampling, this should not have had a significant effect on the interrelationship between these parameters and vitamin D level. Despite evidence showing a strong association between vitamin D deficiency and myriad illnesses, the majority of these findings have been based on epidemiologic data alone. There remains little evidence showing that treatment of vitamin D deficiency with replacement therapy (ergocalciferol or cholecalciferol) actually improves health outcomes. Dialysis patients are at a greater risk for vitamin D deficiency and many of the diseases thought to be associated with it. Randomized controlled trials are necessary to fully elucidate the consequence of vitamin D deficiency in this patient population. D&T References 1. Melamed ML, Michos ED, Post W, Astor B. 25- hydroxy-vitamin D levels and the risk of mortality in the general population. Arch Intern Med. 2008;168:1629-1637. 2. Dobnig H, Pilz S, Scharnagl H. Independent association of low serum 25-hydroxy-vitamin D and 1,25-dihydroxy-vitamin D levels with all-cause and cardiovascular mortality. Arch Intern Med. 2008;168:1340-1349. 3. Wang TJ, Pencina MJ, Booth SL, et al. Vitamin D defi ciency and risk of cardiovascular disease. Circulation. 2008;117:503-511. 4. Giovannucci E, Liu Y, Hollis BW, Rimm EB. 25- hydroxy-vitamin D and risk of myocardial infarction in men: a prospective study. Arch Intern Med. 2008;168:1174-1180. 5. Pilz S, Dobnig H, Fischer JE, et al. Low vitamin d levels predict stroke in patients referred to coronary angiography. Stroke. 2008;39:2611-2613. 6. Cauley JA, LaCroix AZ, Wu L, et al. Serum 25- hydroxy-vitamin D concentrations and risk for hip fracture. Ann Intern Med. 2008;149:242-250. 7. Atherton K, Berry DJ, Parsons T, et al. Vitamin D and chronic widespread pain in a white middle-aged British population: evidence from a cross-sectional population survey. Ann Rheum Dis. 2009;68:817-822. 8. Kendrick J, Smits G, Chonchol M. 25-hydroxyvitamin D and infl ammation and its association with hemoglobin levels and chronic kidney disease. National Kidney Foundation (NKF) 2008 Spring Clinical Meetings. Available at: www.kidney.org/ news/meetings/clinical08/cm08_abstracts. cfm. Accessed February 19, 2009. 9. Ginde AA, Mansbach JM, Camargo CA. Association between serum 25-hydroxy-vitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2009;169:384-390. 10. Cannell JJ, Vieth R, Umhau JC. Epidemic infl uenza and vitamin D. Epidemiol Infect. 2006;134:1129-1140. 11. Lappe JM, Travers-Gustafson D, Davies KM, et al. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr. 2007;85:1586-1591. 12. Levin A, Bakris GL, Molitch M, et al. Prevalence of abnormal serum vitamin D, PTH, calcium, and phosphorus in patients with chronic kidney disease: results of the study to evaluate early kidney disease. Kidney Int. 2007;71:31-38. 13. Wolf M, Shah A, Gutierrez O, et al. Vitamin D levels and early mortality among incident hemodialysis patients. Kidney Int. 2007;72:1004-1013. 14. Blair D, Byham-Gray L, Lewis E, McCaffrey S. Prevalence of vitamin D [25(OH)D] defi ciency and effects 4 Dialysis & Transplantation November 2009
of supplementation with ergocalciferol (vitamin D2) in stage 5 chronic kidney disease patients. J Ren Nutr. 2008;18:375-382. 15. Saab G, Young DO, Gincherman Y, et al. Prevalence of vitamin D defi ciency and the safety and effectiveness of monthly ergocalciferol in hemodialysis patients. Nephron Clin Pract. 2007;150: c132-138. 16. Holick MF. Vitamin D and the kidney. Kidney Int. 1987;32:912-929. 17. Holick MF, Matsuoka LY, Wortsman J. Age, vitamin D, and solar ultraviolet. Lancet. 1989;2:1104-1105. 18. Martins D, Wolf M, Pan D, et al. Prevalence of cardiovascular risk factors and the serum levels of 25-hydroxy-vitamin D in the United States: data from the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2007;167:1159-1165. 19. Taskapan H, Ersoy FF, Passadakis PK, et al. Severe vitamin D defi ciency in chronic renal failure patients on peritoneal dialysis. Clin Nephrol. 2006;66:247-255. 20. Del Valle E, Negri AL, Aguirre C. Prevalence of 25(OH) vitamin D insuffi ciency and defi ciency in chronic kidney disease stage 5 patients in hemodialysis. Hemodial Int. 2007;11:315-321. 21. Bischoff-Ferrari HA, Dietrich T, Orav EJ, Dawson-Hughes B. Positive association between 25-hydroxy-vitamin D levels and bone mineral density: a population-based study of younger and older adults. Am J Med. 2004:116:634-639. 22. Bischoff-Ferrari HA, Willett WC, Wong JB, et al. Fracture prevention with vitamin D supplementation: a meta-analysis of randomized controlled trials. JAMA. 2005;293:2257-2264. 23. Bellizzi V, Minutolo R, Terracciano V, et al. Infl u- ence of the cyclic variation of hydration status on hemoglobin levels in hemodialysis patients. Am J Kidney Dis. 2002;40:549-555. 24. Yokoyama K, Katoh N, Kasai K, et al. The infl uence of method of calcium correction and the timing of blood collection on application of the K/DOQI Clinical Practice Guidelines for Bone Metabolism and Disease in Japan. Ther Apher Dial. 2006;10:257-261. November 2009 Dialysis & Transplantation 5