Evaluating iron status in hemodialysis patients
Holly M. Enders
Effective management of anemia in hemodialysis patients with end stage renal disease (ESRD) requires close monitoring of iron status and effective treatment of iron deficiency. Determining iron status is necessary for optimal iron therapy, particularly during treatment with recombinant erythropoietin (rEPO). Monitoring the iron available for erythropoiesis allows the clinician to administer iron supplementation efficiently and, therefore, to maximize the patient’s response to rEPO therapy. Because the effectiveness of rEPO therapy is often dependent on the availability of iron, accurately assessing the iron status of a patient is essential. The utilization of unreliable iron parameters may lead to false conclusions of iron repletion in iron-deficient patients, which may encourage inappropriate discontinuation of iron supplementation. A decline in responsiveness to rEPO could then ensue, prompting the administration of higher doses of rEPO without achieving satisfactory improvements in hemoglobin levels. Conversely, a false diagnosis of iron deficiency in patients with sufficient iron may also occur when inaccurate measures of iron are used. This may lead to an increase in the administration of supplemental iron, causing iron overload. Therefore, assessing parameters that accurately reflect iron status is central to achieving good patient outcome.
Indirect measures used for evaluating iron status include transferrin saturation, serum ferritin, serum iron, and total iron-binding capacity (TIBC) of transferrin (Hudson & Comstock, 2001). These parameters usually correlate with hemoglobin levels during iron therapy and are useful for determining iron deficiency and for monitoring the effectiveness of iron supplementation (Nissenson, Lindsay, Swan, Seligman, & Strobos, 1999; Park, Uhthoff, Tierney, & Nadler, 1998; Taylor, Peat, Porter, & Morgan, 1996).
Serum ferritin and TSAT are the standard indices used to determine iron status in hemodialysis patients but are sometimes inaccurate. This is because they are highly variable and may be influenced by conditions common in hemodialysis patients that are not associated with iron, such as inflammation. At present, no single available test is both practical to administer and accurate in determining iron status. However, newer methods of measuring iron, such as reticulocyte hemoglobin content are currently being investigated. Results from recent studies suggest that CHr may be more accurate at determining iron status than the current parameters. The goal of this article is to review the current parameters of iron status and CHr as a measure of iron in hemodialysis patients.
Laboratory Assessment of Iron Status
As mentioned above, the parameters most often analyzed to determine iron status (storage and functional iron) are TSAT and serum ferritin (see Table 1) (Hudson & Comstock, 2001). Most of the iron in the plasma is bound to transferrin, which plays a central role in iron distribution. Transferrin transports iron from reticuloendothelial cells, parenchymal tissues, and mucosal cells of the intestine to the erythroid cells in the bone marrow and other tissues that require iron (Harrison & Arosio, 1996). Consequently, the TSAT provides an estimate of iron that is immediately available for erythropoiesis. TSAT is reduced when iron levels are low (Hudson & Comstock, 2001). Normal TSAT levels range from 20% to 50%. The TSAT is calculated by the following equation:
TSAT= [serum iron ([micro]g/dl) / TIBC ([micro]g/dl)] x 100
TSAT reflects iron that is readily available for erythropoiesis, while serum ferritin level is correlated with storage iron found in the liver, spleen, and bone marrow reticuloendothelial cells (National Kidney Foundation, 2000). Normal levels of serum ferritin range from 30 to 200 ng/ml. Serum ferritin level is most reliable at predicting iron deficiency or iron overload when it is extremely low or extremely high, respectively. In addition, levels of serum ferritin can be elevated even if supplemental iron has not been administered (Kalantar-Zadeh, 1999). This may be because ferritin levels increase during inflammation or infection, which is common in hemodialysis patients. Under these conditions, serum ferritin level no longer represents iron stores, a limitation that is important to keep in mind.
As shown in Table 1, the normal ranges of the key iron parameters are quite broad. In fact, certain iron indices have high day-to-day variation. Serum ferritin is especially variable (Borel, Smith, Derr, & Beard, 1991; Cooper & Zlotkin, 1996). In a study by Cooper and Zlotkin (1996), the total day-to-day intraindividual variability of serum ferritin was determined during 10 nonconsecutive days over a 4-week period in a group of healthy men (n=10) and women (n=11). The intraindividual variability of serum ferritin was found to be high, with a coefficient of variance (CV) of 18.1% (Cooper & Zlotkin, 1996). To accurately assess serum ferritin levels, three to six independent measurements were required. Therefore, day-to-day variability and multiple measurements should be considered when using the common iron indices for determining the iron status of hemodialysis patients.
Assessing the iron status of hemodialysis patients requires recognition that there are two forms of iron deficiency, absolute and functional. Absolute iron deficiency occurs when iron stores and circulating iron are depleted. In contrast, functional iron deficiency results when not enough iron from iron stores is released for erythropoiesis, which frequently occurs during rEPO therapy. Correctly identifying the form of iron deficiency is essential for proper iron management during rEPO therapy.
The two most common iron parameters used to determine iron deficiency are serum ferritin and TSAT (National Kidney Foundation, 2000). According to the National Kidney Foundation (2000), absolute iron deficiency is defined as TSAT levels less than 20% and serum ferritin levels less than 100 ng/ml, while functional iron deficiency is defined as TSAT levels less than 20% and serum ferritin levels within the normal range or slightly elevated (see Table 1). Although patients with functional iron deficiency appear to have adequate levels of storage iron (serum ferritin), they may still produce more erythrocytes when administered supplemental iron (National Kidney Foundation, 2000). This suggests that TSAT and serum ferritin may sometimes be unreliable indices of iron status in hemodialysis patients.
Limitations of TSAT and Serum Ferritin as Indicators of Iron Status
Results from a recent study by Fernandez-Rodriguez et al. (1999) suggest that serum ferritin level is a reliable diagnostic parameter of iron deficiency in chronic renal failure patients who are undergoing dialysis, but not receiving iron therapy or rEPO. However, serum ferritin and TSAT are highly variable for detecting absolute iron deficiency in hemodialysis patients receiving rEPO therapy (Fishbane, Shapiro, Dutka, Valenzuela, & Faubert, 2001). These iron parameters are also not sensitive to or specific for diagnosing absolute iron deficiency; a baseline TSAT of less than 20% has been reported to have a sensitivity of 57.1% and a specificity of 800/0 (Fishbane, Galgano, Langley, Canfield, & Maesaka, 1997). Similarly, a baseline serum ferritin level of less than 100 ng/ml has been shown to have a sensitivity of 48% to 71.4% and a specificity of 60% to 75% (Fishbane et al., 1997; Fishbane, Kowalski, Imbriano, & Maesaka, 1996).
A study by Fudin, Jaichenki, Shostak, Bennett, and Gotloib (1998) suggests that serum ferritin may also be a poor indicator of functional iron deficiency. The patients in this study were on hemodialysis therapy and were anemic (hemoglobin <11 g/dl). At the beginning of the study, the mean serum ferritin levels were above 100 ng/ml, the wean TSAT was below 21%, and bone marrow smears were negative for stainable iron. After several patients received iron supplementation with sodium ferric gluconate, TSAT and hemoglobin increased to normal levels. Therefore, these patients had functional iron deficiency, and the normal serum ferritin levels observed did not correlate with the lack of iron stores.
Serum ferritin can be an unreliable indicator of iron status because its levels may be influenced by factors unrelated to iron stores. For example, a recent study has shown that serum ferritin levels progressively increase with alcohol intake (Whitfield, Zhu, Heath, Powell, & Martin, 2001). In addition, ferritin is an acute-phase protein, which means that its levels increase during inflammation and infection (Gabay & Kushner, 1999). This increased synthesis of ferritin is stimulated by inflammatory cytokines, such as tumor necrosis factor and interleukin-1 (Feelders et al., 1998; Rogers et al., 1990). Increasing ferritin levels during infection is a protective mechanism to deprive microorganisms of external iron (Jurado, 1997). Under these conditions, stored iron remains sequestered and is not reutilized. This causes anemia because iron availability for erythrocyte production is also reduced (functional iron deficiency caused by inflammation). Although serum ferritin levels are increased during inflammation, they do not reflect true iron stores and, therefore, do not indicate elevated iron.
The limited value of serum ferritin as a parameter of iron status during inflammation is especially problematic in hemodialysis patients because inflammatory conditions are common in this patient population. Approximately 35% to 53% of hemodialysis patients have above normal levels of serum C-reactive protein (CRP), which is an indicator of inflammation (McIntyre et al., 1997; Owen & Lowrie, 1998; Qureshi et al., 2002). Complicating matters is the fact that inflammatory and infectious conditions are often not apparent in hemodialysis patients, and may be caused by the hemodialysis procedure itself (Haubitz et al., 1996; Memoli et al., 2002; Schindler, Boenisch, Fischer, & Frei, 2000; Stevenson et al., 2002). Haubitz et al. (1996) found that hemodialysis patients had significantly higher CRP levels compared with peritoneal dialysis patients. This was attributed to factors encountered during hemodialysis that can elicit an inflammatory response, such as the dialyzer membrane, dialysate buffer, and bacterial fragments in the dialysate (Haubitz et al., 1996). Since inflammation may exist undetected, parameters of iron that are more reliable than serum ferritin may be needed to accurately assess iron status in hemodialysis patients. Results from several recent studies suggest that CHr may help to accurately determine iron status when added to the list of parameters assessed during an iron work-up.
Reticulocyte Hemoglobin Content
Reticulocytes are cells in the peripheral blood that are at the stage of erythroid differentiation immediately preceding the mature erythrocyte (Brugnara, 2000). Although reticulocytes and erythrocytes contain hemoglobin, the utilization of iron for hemoglobin synthesis occurs in immature erythrocytes in the bone marrow, not in the peripheral blood (Besarab, 2001). However, of the cells in the peripheral blood, reticulocytes are the closest relatives to the cells in the bone marrow that are using iron for hemoglobin production (Besarab, 2001). Therefore, the hemoglobin content of reticulocytes in the circulation reflects the amount of iron available for hemoglobin production in the bone marrow. Because of this characteristic, CHr has been investigated for use as an iron status marker.
The laboratory values of CHr in healthy adults range from 25.9 pg to 33.9 pg (see Table 1) (Brugnara, 2000). Values of less than 26 pg, 28 pg, and 29 pg have been successfully used to predict iron deficiency (Fishbane et al., 1997; Fishbane et al., 2001; Mittman et al., 1997). However, additional investigations are needed to definitively establish the most reliable cut-off value. Overall, results from several recent studies indicate that CHr is an accurate measure of iron status in hemodialysis patients receiving rEPO (Cullen et al., 1999; Fishbane et al., 1997; Fishbane et al., 2001; Mittman et al., 1997). In fact, less variability (%CV) has been reported with CHr compared with serum ferritin (4.3% vs 43.6%, respectively) (Fishbane et al., 2001). Reticulocyte hemoglobin content is also highly sensitive and specific for detecting iron deficiency; a CHr value of less than 26 pg has a sensitivity of 100% and a specificity of 73% to 80% (Cullen et al., 1999; Fishbane et al., 1997). In addition, CHr provides a real-time estimate of iron availability. In two independent studies, increases in reticulocyte hemoglobin were detected 48 hours after hemodialysis patients received intravenous iron supplementation, indicating that CHr is a rapid responsive marker of iron status (Fishbane et al., 1997; Mittman et al., 1997). Importantly, CHr has also been shown to be a reliable iron marker for monitoring the effectiveness of iron therapy in hemodialysis patients (Bhandari, Brownjohn, & Turney, 1998; Fishbane et al., 2001).
Unlike serum ferritin, CHr is not elevated during inflammation. In fact, CHr values are probably reduced during anemia of inflammation because of the inhibition in iron mobilization from iron stores. It is difficult to distinguish between iron deficiency during anemia of inflammation and functional iron deficiency when the common iron parameters are assessed. This is because during both conditions the TSAT is less than 20% and serum ferritin is normal or elevated, although serum ferritin levels may be well over the normal range during inflammation (see Table 2) (National Kidney Foundation, 2000). If it is not clear which of these conditions exists after measuring TSAT and serum ferritin, clinicians should consider evaluating CHr and CRP levels. During functional iron deficiency, CRP is undetectable or low (<10 mg/l) and CHr is less than 29 pg, whereas CRP above 15 mg/l and CHr less than 29 pg suggest inflammation-induced iron deficiency (see Table 2)(Besarab, 2001).
Conclusion
Iron deficiency has been shown to occur in as many as 31% of hemodialysis patients who are receiving rEPO (Park et al., 1998). Since iron is necessary for hemoglobin synthesis, the lack of iron causes an inadequate erythropoietic response to rEPO therapy. Hemodialysis patients who become iron deficient are treated with supplemental IV or oral iron. Therapy with IV iron, either sodium ferric gluconate, iron dextran, or iron sucrose, is particularly effective in increasing responsiveness to rEPO (Nissenson et al., 1999; Park et al., 1998; Richardson, Bartlett, & Will, 2001).
Good patient outcome during therapy with rEPO and/or supplemental iron requires frequent monitoring of iron status. Currently, serum ferritin and TSAT are the laboratory parameters of iron that are most often used to assess iron status. However, these parameters may not provide conclusive criteria for determining iron deficiency or iron overload in hemodialysis patients because they often have high variability, low sensitivity, and low specificity. In addition, inflammation, which is common in hemodialysis patients, can complicate the interpretation of serum ferritin levels. Reticulocyte hemoglobin content is a method that should be considered, since it is a reliable measure of functional iron. Considering CHr values in conjunction with serum ferritin and TSAT may help to optimize iron management and enhance the effectiveness of rEPO therapy, which will increase the patient’s quality of life. Given the high cost of rEPO, increasing the responsiveness to treatment by accurately monitoring the effectiveness of iron supplementation may also reduce the economic burden to the patient. An enhanced knowledge of the interpretation of laboratory parameters of iron will allow patients with ESRD to receive the greatest benefits from rEPO and iron therapies.
Table 1. Normal Values of Iron Indices *
Iron Iron
Iron Normal Deficiency Deficiency
ParameterIron Assessed Range (Absolute) (Functional)
TIBC, [micro]g/dl Functional 240-450 ([dagger]) ([dagger])
Serum iron,
[micro]g/dl Functional 50-150 ([dagger]) ([dagger])
TSAT, % Functional 20-50 <20 <20
Serum ferritin,
ng/ml Storage 30-200 100
CHr, pg Functional 25.9-33.9 <29 <29
TIBC indicates total iron-binding capacity; TSAT,
transferrin saturation; and CHr, reticulocyte hemoglobin content.
* National Kidney Foundation (2000); Hudson & Comstock (2001);
Brugnara (2000); Fishbane et al. (2001); Robbins,
Kerhulas, Senger, & Fishbane (1997).
([dagger]) Not usually considered when determining iron deficiency.
Table 2. Functional Iron Deficiency Versus Inflammation-induced
Iron Deficiency *
Iron Deficiency Iron Deficiency
Characteristic (Functional) (Inflammation)
Cause Stimulation of Inflammation,
erythropoiesis infection
Iron status Iron stores Both iron stores
adequate, and functional
functional iron iron may be low
low
Mechanism Not enough iron Iron “locked” in
released from stores by
stores increasing
ferritin
TSAT, % <20 <20
Serum ferritin, >100 >100 ([dagger])
ng/ml
CHr, pg <29 <29
CRP, mg/l 15
TSAT indicates transferrin saturation; CHr, reticulocyte
hemoglobin content; and CRP, C-reactive protein.
([dagger]) Serum ferritin may be increased several times
above the normal range during infection.
* National Kidney Foundation (2000); Jurado (1997);
Besarab (2001).
Note: This article is supported by a financial grant from Watson Pharma, Inc. This article has undergone peer review. The information in this article does not necessarily reflect the opinions of ANNA or the sponsor.
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Holly M. Enders, MS, CRNP, is Early Renal Insufficiency Clinic Coordinator, University of Maryland Medical System, Department of Medicine, Nephrology Division, Baltimore, MD.
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