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The Measurement and Interpretation of Serum Ferritin*

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The Measurement and Interpretation of Serum Ferritin* ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 10, No. 4 Copyright © 1980, Institute for Clinical Science, Inc. The Measurement and Interpretation of Serum Ferritin* DONALD T. FORMAN, P h .D. and SHARON L. PARKER, P h .D .! Departments of Pathology b- Biochemistry, and \Medical Allied Health Professions, University of North Carolina, School of Medicine Chapel Hill, NC 27514 ABSTRACT Determination of serum ferritin is an important means of assessing body iron stores. Trace amounts of ferritin normally present in serum are detecta­ ble by sensitive radioimmunoassay techniques or an enzyme immunoassay procedure. Ferritin normally accounts for no more than a very small fraction of the total iron in serum, but generally maintains a stable concentration that is proportional to the much larger pool of storage iron in tissues. The serum ferritin assay, in contrast to other measurements of iron status such as hemo­ globin, serum iron and iron-binding capacity, can distinguish differences in iron stores within the physiological range. In iron deficiency anemia, the concentration is below 10 ngper ml. Increased concentrations (above 200 ng per ml) are found in conditions with increased iron stores. The information it provides is similar to that obtained from bone-marrow aspirates stained for iron. In contrast to the percent transferrin-saturation and concentration of erythrocyte protoporphyrin, ferritin concentrations become abnormal be­ fore exhaustion of mobilizable iron stores and before the onset of anemia. Serum ferritin also provides a practical means of assessing new programs of iron supplementation, since it reflects various degrees of iron deficiency and overload. Introduction and the remainder as hemosiderin and iron containing enzymes.12 Ferritin is F erritin is the major iron storage protein found predominantly in the liver, spleen, of mammalian tissues.4 Of the four to five and bone marrow where it functions in grams of total body iron in the normal recycling iron for hematopoiesis.7 It is adult, approximately 65 percent is bound ubiquitously distributed in many other as hemoglobin, 15 to 20 percent as ferritin, tissues where its highly specialized pro­ three to five percent in bound myoglobin tein structure enables it to sequester and store iron intracellularly. Increases in Please send reprint requests to Dr. Forman at the Clinical Chemistry Laboratory, North Carolina cellular iron result in the rapid synthesis Memorial Hospital, Chapel Hill, NC 27514. of apoferritin, whereas iron depletion 345 0091-7370/80/0700-0345 $00.90 © Institute for Clinical Science, Inc. 346 FORMAN AND PARKER syndromes result in the mobilization of anemia from the various anemias of iron from its storage proteins with a sub­ chronic diseases. sequent decrease in tissue ferritin. Ferritin consists of a large spherical shell Methodological Aspects of of 24 single protein subunits surrounding Serum Ferritin Assays an inner core of insoluble ferric phos­ Several procedures are available for the phate. The iron-free protein, apoferritin, immunological quantitation of serum has a reported molecular mass of about ferritin. These include a two-site im- 450,000, which increases about twofold munoradiometric assay,* radioimmuno­ when the protein is fully saturated with assay—competitive binding assay,f and iron. The iron appears to pass to and from an enzyme immunoassay procedure in the inner core through small channels in developm ent.2,19 the protein shell.11 Recent data indicate The principle of the modified two-site that the ferritin can exist in multiple immunoradiometric assay originally in­ molecular forms (isoferritins) in different troduced by Addison1 is a two-stage reac­ tissues.5 The various isoferritin molecules tion. Stage 1 involves the binding of may represent varying percentage com­ human serum ferritin to a solid phase positions of two types of subunits in the 24 (plastic beads) and antihuman ferritin. subunit structure of the protein. The more Stage 2 involves the binding of radio­ basic isoferritins (L subunit) predominate labelled (125I) antihuman ferritin with the in liver and spleen while the acidic forms insoluble antihuman ferritin complex (H subunit) are present in highest concen­ generated in Stage 1. The solid phase is tration in heart, kidney and certain malig­ then washed, counted in a gamma nant tissues. The physiological signifi­ counter, and the ferritin concentration is cance of these tissue isoferritins is not calculated by comparison to a standard clear; however, the analytical significance curve. may be very important because the isofer­ The radioimmunoassay technique is a ritins appear immunologically distinct,8 competitive binding assay which utilizes and these immunological differences in a precipitating antiserum reagent to sepa­ isoferritin populations may have impor­ rate antibody-bound tracer from unbound tant consequences. tracer. The procedure is based on the Small, but clinically important concen­ competitive binding principles of radio­ trations of ferritin are detectable in serum immunoassay as described by Yalow and in both normal and pathological condi­ Berson.20 Non-radioactive ferritin from tions,13 with the concentration of serum patient samples, ferritin standards, and ferritin being directly proportional to the controls compete with a constant amount level of body iron stores and the stainable of (125I) ferritin tracer for binding sites on iron in bone marrow.6 Consequently, the the ferritin antibody, which is held at a principal use of serum ferritin is in the limiting concentration. The amount of diagnosis and management of iron defi­ labelled ferritin tracer which will bind to ciency and iron overload. This non- the antibody is inversely proportional to invasive technique, which is relatively the amount of non-radioactive ferritin in simple and sensitive, can also provide the the assay tube. A precipitating reagent so­ clinician and patient more information lution containing a second antibody in a than serum iron levels, transferrin satura­ tion, hemoglobin, hematocrit, red-cell in­ * Fer-Iron, Ramco Laboratories, Inc., Houston, TX dicies, and erythrocyte protoporphyrin 77098. levels. The assay is particularly useful in t GammaDab-Ferritin RIA, Clinical Assays, differentiating true iron deficiency Cambridge, MA 02139. MEASUREMENT AND INTERPRETATION OF SERUM FERRITIN 3 4 7 polymer solution is used to separate the antibody with radioactive iodine. Intra­ antibody bound (125I) ferritin from un­ assay variation showed a reproducibility bound labelled ferritin by immuno­ of less than 11 percent.19 The sensitivity of précipitation. The antibody-bound and this technique appears sufficient for clini­ labelled ferritin is counted. A standard cal purposes. curve is prepared in a like manner and the Each type of assay has its own attributes concentrations of the patients samples are and problems with as large and complex determined from the standard curve. an antigen as ferritin.14 In addition, there The enzyme immunoassay tech­ is a specificity problem owing to the im­ nique2,19 for human serum ferritin munological differences in the tissue employs an antibody adsorbed on a solid ferritins used as immunizing agents or phase. Adsorbed antibody against human standards in the serum ferritin procedure. ferritin is first allowed to react with Since ferritin occurs in multiple molecu­ ferritin. A second antiferritin antibody, lar forms (isoferritins), and these can which is labelled with an enzyme, is change markedly during development, added. At least three enzymes, alkaline iron status, and in malignancy, these dif­ phosphatase, B-galactosidase, and horse­ ferences can have important conse­ radish peroxidase, have been used. The quences for the standardization and in­ binding of the enzyme labelled antibody terpretation of serum ferritin assays and to the ferritin alters the activity of the en­ for attempts to quantitate selectively dis­ zyme. The resultant enzyme activity is tinct isoferritin populations (figure 1). correlated to ferritin concentration by means of a standard curve. D iscussion This technique has the advantage of using stable reagents that can be stored for Ferritin in serum was first reported17 in many months at 4°C, and it eliminates the patients with liver damage in 1956. Its need for constant labelling of antigen or presence as a component of normal serum 100 K. \ \ 75 \ l\ \ \ \ \ \ \ F i g u r e 1. C r o s s ­ « \ reactivity of ferritin an- S \ \ tiserum with human <a \ \ \ serum, spleen, heart, and a* 50 \ \ horse spleen. \ Farritin Cross Reactivity \ Source at 50% Inhibition \ C n istalline Human Livi9T N & 25 O Human Serum 100% ---------- Human Spleen 33.0 ---------- Human Heart 18.0 \ ---------- Horse Spleen <1.0 I I I I I 2 5 10 20 50 100 200 500 1000 2000 FERRITIN CONCENTRATION ng/ml 348 FORMAN AND PARKER has been established after the develop­ In normal individuals, the levels of ment of sensitive radioimmunological serum ferritin depend on age and sex. At techniques. birth, serum ferritin levels are approxi­ The positive correlation of serum mately 100 ng per ml.18 This level falls ferritin levels with body iron stores, as during the first few months of life, but assessed by marrow or liver biopsy Prus­ then returns to the 100 ng per ml range in sian Blue staining,6 has resulted in its in­ young adults.3 Serum ferritin levels can creased use as a diagnostic aid in asses­ also vary in females during the menstrual sing the physiological status of iron stores. cycle and pregnancy.1 The reference The mechanism of serum ferritin release range in norm al adult m ales is 15 to 200 ng is unknown. It may result from an active per ml with a mean of 90 ng per ml. secretory process or from normal cell Females have a lower range of 10 to 200 ng turnover or lysis. Several factors suggest per ml and a mean of 50 ng per ml.3 Values that serum ferritin differs from tissue in post-menopausal women and men tend ferritin8: (1) serum ferritin binds to Con- to converge and continue to increase conavalin A, whereas tissue ferritins have slightly with age.
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