The present study demonstrated a J-shaped association between serum ferritin levels and kidney outcome in patients with CKD. Furthermore, the Fine-Gray model showed a similar association. In Japanese dialysis patients, those with the lowest (< 50 ng/mL) and those with the highest serum ferritin levels (≥ 200 ng/mL) had a significant increase in the risk of all-cause mortality compared with those with serum ferritin of 50–99.9 ng/mL 20. Another study conducted in Japanese dialysis patients showed a J-shaped association of the high (versus median) ferritin and the lowest ferritin levels with mortality 21. To date, however, no reports have demonstrated J- or U-shaped associations between serum ferritin levels and poor kidney outcome in CKD. Therefore, the present study was the first report to clarify a J-shaped association.
Bone marrow iron depletion is highly specific for the diagnosis of ID, is unaffected by inflammation, and remains the gold standard for ID diagnosis 22. A few studies investigated the association between bone marrow iron stores and serum iron status markers in CKD. In patients with CKD, serum ferritin levels < 60 ng/mL were associated with absent or reduced bone marrow iron. Using a critical value of ≤ 60 ng/mL, the diagnostic efficiency of serum ferritin in terms of diagnosing absent or reduced bone marrow iron was high 23. In another report by Kalantar-Zadeh et al., when bone marrow iron was graded 0 to + 4, ranging from absent to diffuse homogeneous iron staining, serum ferritin concentrations (mean value, 83 ng/mL) in CKD patients with a bone marrow iron score of 0 were significantly lower than in those with a bone marrow iron score of + 2 or greater. It was also noted that there was a significant correlation between serum ferritin and bone marrow iron scores 24. These findings suggested that the specificity of low serum ferritin is high for absolute ID 25. Therefore, in our study, it was hypothesized that participants with the lowest quartile of serum ferritin (range, 6.4–72.5 ng/mL; mean value, 39.6 ng/mL) might have low body iron storage.
The underlying mechanisms for the association between low serum ferritin and kidney disease progression observed in the present study remains unknown. Deleterious biological consequences of ID are considered multifactorial as follows: impaired erythropoiesis, abnormalities in the immune response, DNA and cell cycle abnormalities, and mitochondrial dysfunction 4–6. The kidney is a high energy-consuming tissue, and the mitochondria are the major source of cellular ATP 26. Furthermore, dysfunctional mitochondrial energy production due to ID is associated with kidney impairment 5, 6. In chronic experimental atherosclerotic renovascular disease, loss of mitochondrial inner membrane cardiolipin was reported to contribute to kidney injury and dysfunction 27. Another animal study showed that feeding rats an iron-deficient diet caused an increase in the kidney malondialdehyde, an indicator of increased lipid peroxidation 28. Additionally, it was suggested that malondialdehyde plays an important role in the pathogenesis of glomerulosclerosis 29. Therefore, it was proposed that ID might adversely affect the kidney through mitochondrial dysfunction and oxidative stress 6. As mentioned above, lower serum ferritin is associated with body iron depletion. Given these findings, it is possible that the mitochondrial dysfunction and oxidative stress caused by ID which is probably associated with low serum ferritin are raised as the mechanisms by which the lowest quartile of serum ferritin showed a significant increase in the risk for poor kidney outcome in the present study. However, the effects of iron supplementation for ID on kidney function are scarce 30. Therefore, further studies are warranted to elucidate the precise mechanisms by which lower ferritin levels affect kidney disease progression, and to investigate whether treatment of ID with iron supplementation improves kidney function in CKD.
In CKD, very few studies have shown a significant association between higher serum ferritin levels and kidney disease progression. Tsai et al. suggested that patients with CKD who displayed serum ferritin levels > 288 ng/mL were more likely to experience adverse kidney outcome compared with those with serum ferritin levels < 132 ng/mL 18. In the present study, compared with Q2 of serum ferritin (73.3–139.0 ng/mL), Q3 and Q4 (> 139.0 ng/mL) were significantly associated with poor kidney outcome. According to World Health Organization guidelines, in adult, non-healthy individuals, serum ferritin concentration > 500 ng/mL may indicate a risk of iron overload 31. Serum ferritin values in the range of 200–2,000 ng/mL may be increased due to non-iron associated factors, including elements of malnutrition–inflammation complex syndrome, while extremely high serum ferritin levels of > 2,000 ng/mL are normally indicative of iron overload in patients with CKD 8, 32. In the present study, the number of participants with serum ferritin > 500 ng/mL was very small (n = 42; 4.7%); therefore, most participants may have had non-iron related conditions, rather than iron-overload. TIBC is a negative acute-phase reactant, that is, its plasma concentration is suppressed by inflammation 33. In our cohort, serum ferritin levels positively and inversely correlated with CRP and TIBC, respectively, suggesting that higher serum ferritin levels reflect a greater inflammatory status. Ferritin synthesis is activated by some pro-inflammatory cytokines (e.g., interleukin [IL]-1β and tumor necrosis factor [TNF]- α) 34, 35. Furthermore, these cytokines such as IL-1β 36 and TNF-α 37 can play an important role in kidney disease progression. Although we did not measure these cytokines (e.g., IL-1β and TNF-α), it was proposed that those cytokines concomitant with elevated serum ferritin might be considered to contribute to kidney disease progression.
Very few studies have addressed the association between TSAT and kidney outcome in patients with CKD. A previous study conducted in 453 male patients with CKD reported that higher TSAT levels were associated with worsening kidney function 19. Serum iron levels fluctuate diurnally and can change acutely depending on dietary iron intake. In the present study, blood samples were obtained from all participants early in the morning following an overnight fast; therefore, the measured serum iron levels were unaffected by diurnal variation or dietary intake. Compared with Q3 of TSAT, Q2 displayed a significant increase in the risk of kidney outcome, but neither Q1 nor Q4 was associated with kidney outcome. In this context, it remains unclear to what extent TSAT is associated with kidney outcome. Therefore, larger cohort studies are needed to clarify the association of TSAT levels with kidney outcome.
In the present subgroup analyses, in addition to male participants, participants with lower serum albumin, hemoglobin, or eGFR levels were more likely to have significant associations of both the lowest and highest ferritin levels with poor kidney outcome. These findings suggested that participants with malnutrition, anemic conditions, or advanced kidney dysfunction were susceptible to the influence of serum ferritin levels on kidney disease progression.
The present study had several limitations. First, all study participants were recruited at a single regional hospital. Therefore, the sample was fairly homogeneous, and selection bias was possible. We only recruited consecutive patients who were admitted to our hospital, and these participants were relatively old, with the number of male participants being approximately 1.8-times higher than that of female participants. Second, the present study used single iron status marker measurements, which may not provide sufficient accuracy regarding the predictive values of those factors for kidney outcome. Third, we did not measure serum hepcidin, which is a major iron-regulatory hormone that binds to ferroportin and inhibits iron export from enterocytes, hepatocytes, and macrophages through the internalization and degradation of ferroportin, thereby regulating iron metabolism in various diseases, including CKD 38. Serum hepcidin is reported to correlate positively with serum ferritin and negatively with the eGFR in patients with CKD 39. Consequently, serum hepcidin may mediate the development of functional ID in CKD [16]. Although functional ID in previous large cohorts of CKD was reported to be associated with all-cause mortality 16, 17, whether this functional ID affects kidney health remains unclear 40. Finally, there was a high inter-method variability for serum ferritin, probably due to the wide variety of assay procedures, whereas inter-method variability was small for TSAT. Serum ferritin values were considered suited only for intra-hospital comparison 3, 41. Additionally, reference ranges of “normal” serum ferritin levels can vary, and the “normal” range is both assay- and laboratory-dependent 42. Accordingly, reported serum ferritin levels should be interpreted with caution.
In conclusion, the present study demonstrated that lower and higher serum ferritin levels were associated with poor kidney outcome independent of relevant confounding factors. This J-shaped association may provide a novel insight into the influence of serum ferritin levels on kidney disease progression in patients with CKD not receiving dialysis.