In clinical practice, where the ability to directly quantify body iron stores by bone marrow iron staining is limited by practical considerations, serum levels of ferritin and TSAT are commonly used to assess body iron stores. In general, serum ferritin is considered a marker of stored iron and TSAT an indicator of the amount of iron available for transportation to the bone marrow for erythropoiesis. While KDOQI recommendations specify target thresholds for these markers during ESA treatment in children with CKD, target levels for ESA-untreated CKD patients are lacking. The prevalence of iron deficiency, defined by TSAT <20 % and low serum ferritin levels, among children with early stage CKD has been reported to be high (42 % in a small cohort by Baracco et al.), but this too has been determined without a gold standard assessment of stored iron [22
]. If ferritin and TSAT values are accurate reflections of stored iron, and if in turn iron deficiency is a primary etiologic factor for lower HGB in children with early stage CKD, we hypothesized that subjects with lower HGB values would also demonstrate lower values for markers of iron status. However, we found that children with ferritin below the KDOQI threshold did not have lower HGB values. Likewise, the TSAT threshold provided only moderate discrimination, with marginally higher HGB z-scores in those with TSAT ≥20 %. The use of thresholds from ESA-treated CKD populations did not clearly identify untreated children with lower HGB z-scores. Thus, we thus sought to identify other, more consistent, clinical factors associated with HGB in this population.
Our analysis suggests that in children with moderate to advanced non-dialysis CKD, the distribution of HGB is determined by a much more complicated constellation of factors, with iron markers comprising only one component. It is well established that the etiology of the anemia of CKD is multifactorial, with decreased production of erythropoietin a primary factor as GFR declines, but iron-restricted erythropoiesis is also an important contributor [3
]. A single threshold for any iron marker cannot account for the myriad factors that influence iron marker levels. Moreover, serum ferritin is an unreliable indicator of stored iron in the setting of acute inflammation as it is an acute phase reactant [13
]. Thus, while low serum ferritin nearly always indicates iron deficiency, high serum ferritin cannot rule it out; rather than indicating adequate amounts of accessible stored iron, it may be more consistent with inflammation-mediated iron sequestration/impaired iron trafficking [23
]. In our analysis, ferritin was found to be inversely associated with both HGB and GFR. A recent study by Fishbane et al. in adults with non-dialysis CKD similarly found that serum ferritin level was not associated with risk for anemia [24
]. Very few subjects demonstrated ferritin values consistent with iron deficiency as defined in healthy children.
Using regression tree analysis, a methodology suited to exploring complex relationships between variables, we examined the relationship between age-, sex-, and race-standardized HGB levels and iron marker levels, as well as other clinical markers of CKD. We showed that low GFR, low serum iron, and high serum ferritin were the variables most strongly associated with lower HGB levels. While in the absence of gold standard bone marrow iron studies for defining iron deficiency anemia we are unable to provide specific new thresholds to define “iron-deficiency” in CKD, our findings do suggest that definitions may need to be constructed in the context of CKD stage. For example, while the combination of a GFR ≥60 ml/min/1.73 m2 and serum iron <50 μg/dl was a marker profile associated with a “middle” level HGB percentile, a GFR <30 ml/min/1.73 m2, independent of serum iron or ferritin level, served as a marker profile associated with “low” HGB percentile. Among children in the lower-middle range of GFR (30–45 ml/min/1.73 m2) with ferritin <100 ng/ml, serum iron <50 μg/dl was associated with a “low” HGB. However, among children with slightly higher GFR (45–60 ml/min/1.73 m2), a similar iron and ferritin profile was associated with “middle” level HGB. The regression tree analysis consistently demonstrated that markers of iron status became less strongly associated with HGB percentiles as GFR declined. This suggests that at decreasing levels of GFR, iron-restricted erythropoiesis due to iron sequestration may become more prevalent, such that the availability of accessible iron dwindles.
Among the iron markers evaluated, we found serum iron to be the most strongly associated with HGB percentile. Serum iron, a direct measurement of circulating iron available for utilization by erythroid precursors, may be a reliable indicator of iron availability in general [25
]. The KDOQI guidelines do not indicate goal values for serum iron, likely due to concerns including the lack of data supporting the use of serum iron as a marker of iron status in CKD, possible diurnal variation in iron levels, and the oxidative stress associated with excess circulating iron [25
]. Rather, guidelines suggest measuring ferritin to assess iron stores and TSAT to assess adequacy of iron for erythropoiesis; caution in using IV iron agents in those with significantly elevated serum ferritin is also recommended [12
]. As the molecular mechanisms of iron homeostasis, including the role of increased serum hepcidin in iron sequestration, have been clarified, our understanding of iron-restricted erythropoiesis has been advanced [23
]. In the presence of normal or elevated serum ferritin, low serum iron values may serve to identify patients in whom iron sequestration is a significant contributor to anemia. In a prospective observational study of more than 1,200 adults maintained on hemodialysis examining the role of low serum iron levels on clinical outcomes, patients with serum iron in the lowest quartile (<45.5 μg/dl) had a mortality rate twice as high as seen in other quartiles [25
]. Serum iron was also inversely associated with mortality and hospitalization, independent of demographics, markers of nutrition and inflammation such as albumin and ferritin, hemoglobin, and doses of ESA and IV iron [25
]. The results of our cross-sectional analysis suggest that children with more advanced CKD may need higher levels of serum iron to maintain HGB as GFR declines, likely due to increasing iron-sequestration. While it has been shown in healthy children that increased BMI is associated with lower serum iron levels, BMI percentile was not significantly associated with HGB percentile in this analysis [26
This analysis does have limitations, including its cross-sectional design, which limits the ability to interpret the relationships between clinical variables and HGB as causal. In addition, we restricted the analysis to children not receiving ESA or iron treatment so as not to confound the relationship between iron markers and HGB, but acknowledge that these untreated children are likely a select population; further analyses will need to be performed to assess these relationships among treated children. Endogenous erythropoietin levels are not available for subjects in CKiD, and we were thus unable to examine the association of this factor with HGB percentile, although erythropoietin levels are likely closely related to GFR, which we did include. We were not able to include markers of inflammation, including C-reactive protein, in our analysis. As such, we were unable to investigate whether higher serum ferritin levels were correlated with laboratory evidence of inflammation, a finding that would have offered confirmatory evidence to support the concept of higher serum ferritin as primarily an indicator of inflammation-associated iron-sequestration. Finally, we are unable to compare biomarkers of iron status to a gold standard measure due to the practical limitations of obtaining bone marrow iron staining in this population.
The main strength of our study is that it was conducted using data from the largest prospective cohort study of children with CKD. GFR was directly measured in the majority of subjects; in those with no measured GFR available, we used an estimating equation derived from the CKiD study and shown to be precise [16
]. Data were collected using standardized forms and protocols and biomarkers were measured at a single central laboratory.
In conclusion, we have demonstrated that while the majority of children in the CKiD cohort fail to meet the KDOQI recommended values for serum ferritin, very few of them meet the ferritin criteria for absolute iron deficiency. Our results suggest that, in a cohort of children with non-dialysis dependent CKD, ferritin may be more a marker of inflammation than a marker of stored iron. Serum iron was identified as strongly associated with HGB level, suggesting that it may be a more specific marker of iron-sequestration than either ferritin or TSAT. Low serum iron in the context of normal or high ferritin (>100) and anemia may be a useful indicator of iron-restricted erythropoiesis, guiding specific therapeutic interventions targeted toward improving iron delivery and utilization. Further study should confirm the clinical utility of serum iron as a target in anemia management, and determine if higher targets for serum iron as GFR declines are associated with higher hemoglobin levels, until iron sequestration can be directly targeted therapeutically.