Diurnal Variation of Serum Iron, Iron-Binding Capacity, Transferrin Saturation, and Ferritin Levels

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1 Clinical Chemistry / DIURNAL VARIATION OF SERUM IRON LEVELS Diurnal Variation of Serum Iron, Iron-Binding Capacity, Transferrin Saturation, and Ferritin Levels Jane C. Dale, MD, 1 Mary F. Burritt, PhD, 1 and Alan R. Zinsmeister, PhD 2 Key Words: Anemia, iron deficiency; Biological markers; Chemistry, clinical; Ferritin; Iron; Iron overload; Laboratories, hospital; Quality improvement; Transferrin Abstract Serum iron levels vary throughout the day. Morning levels are generally assumed to be higher than afternoon or evening levels. We studied whether our practice of restricting serum iron collections to the morning was necessary. Serum iron, iron-binding capacity, transferrin saturation, and ferritin levels were determined on blood specimens obtained from 2 healthy adult volunteers at, noon, and (day 1) and (day 2). Although statistically significant differences among mean values for the collection times were observed for iron, iron-binding capacity, and (log) ferritin, no consistent diurnal variation was seen. Morning iron levels were higher than afternoon levels for only half of the subjects. Between-day variation for all 4 analytes was similar to within-day variation. We conclude that the practice of restricting iron specimen collections to a specific time of day does not improve the reliability of the test result. Serum iron, total iron-binding capacity (TIBC), and calculated transferrin saturation (TS) tests are used to screen for and monitor conditions of iron deficiency and iron overload. Although the usefulness of these tests for diagnosing iron deficiency can be debated, 1 they continue to be ordered with some frequency. The tests have proven useful for screening for hereditary hemochromatosis, and, because hemochromatosis is the most common single-gene inherited disorder in whites, many have advocated population-based screening. 2-4 Because we anticipated increased requests for serum iron studies, we reviewed our collection requirements for these analytes. Although the diurnal variation of iron is well documented, published studies offer conflicting conclusions about the pattern of variation. The prevailing opinion is that iron levels are higher in the morning than in the afternoon or evening. 5,6 However, some studies have shown iron levels to be higher in the afternoon or evening than in the morning, 7,8 and other studies have found significant variation but no systematic trend. 9,1 Our laboratory s iron reference range is based on specimens obtained in the morning. When we receive a request for an afternoon or evening test of iron levels for the investigation of iron deficiency, we contact the ordering physician, explain that afternoon or evening levels may be misleading, and suggest a ferritin level as an alternative test. Striving to streamline our processes, we questioned whether this practice, which introduces delays in specimen collection and interrupts the physicians, is necessary. Because many published studies were conducted a number of years ago and did not use state-of-the-art methods, we investigated the effect of time of day on iron, TIBC, TS, and ferritin levels. 82 Am J Clin Pathol 22;117:82-88 American Society for Clinical Pathology Downloaded from on 18 August 218

2 Clinical Chemistry / ORIGINAL ARTICLE Materials and Methods Twenty healthy, white, adult volunteer subjects (18 women, 2 men), ranging in age from 29 to 45 years (mean, 38 years), participated in this study. Eighteen subjects worked the day shift, 1 worked from noon to 8 PM, and 1 worked the evening shift. No subjects were blood donors or had ingested iron-containing supplements within 48 hours of blood specimen collection; none of the women were menstruating during the study. All subjects were seated for 1 minutes before collection. Tourniquets were used and remained in place for the duration of the collection. All collections were completed in less than 2 minutes. A 7-mL plain evacuated tube (Monoject, Sherwood Medical, St Louis, MO) was used to collect blood at, noon, and on day 1 and at on day 2 for iron, TIBC, TS, and ferritin determinations. Both specimens were obtained after an overnight fast, and all subjects were fasting for at least 2 hours before collection of the noon and specimens. Specimens from day 1 were refrigerated before analysis on day 2. For each subject, specimens from the 4 collection times were analyzed on the same run for each analyte. Iron levels were determined by FerroZine-iron complex spectrophotometry on a Hitachi Chemistry Analyzer (Roche Diagnostics, Indianapolis, IN). In our laboratory, the iron reference range is 5 to 15 µg/dl ( µmol/l) for men and 35 to 145 µg/dl ( µmol/l) for women. TIBC was determined on the Hitachi Chemistry Analyzer by the automated determination of unbound ironbinding capacity using the FerroZine-iron complex method. TIBC equals unbound iron-binding capacity plus serum iron. The TIBC reference range is 25 to 4 µg/dl ( µmol/l). TS is calculated as follows: (iron TIBC) 1. The TS reference range is 14% to 5%. Ferritin was determined on a Bayer ACS: 18 Analyzer (Bayer, Tarrytown, NY) by a 2-site chemiluminometric sandwich immunoassay. The ferritin reference range is 2 to 3 ng/ml (2-3 µg/l) for men and 2 to 2 ng/ml (2-2 µg/l) for women. Within-run and between-run precision are shown in Table 1. The data from specimen collections at the 4 time points were analyzed separately for iron, TIBC, TS, and (log-transformed) ferritin values using a repeated measures analysis of variance with an unstructured variance-covariance specification. Log-transformed values were used for statistical analysis of ferritin results because of the skewed distribution of results. The results for ferritin, however, are summarized in the original units scale. Specific contrasts (eg, vs 4 PM) were tested at an adjusted alpha level (alpha =.17; ie, for 3 pairwise comparisons) if the overall repeated measures effect (collection time) was significant at an alpha level of.5. A descriptive assessment of the within-subject variation (within-day vs between-day) was summarized based on the within-subject pairwise deltas (ie, noon-, -, and day 1- day 2). The study was approved by the institutional review board of the Mayo Foundation. Results The means (SDs) for each test result for blood drawn at, noon, and on day 1 and at on day 2, respectively, were as follows: iron, 9 (37) µg/dl (16.1 [6.6] µmol/l), 98 (36) µg/dl (17.5 [6.4] µmol/l), 93 (43) µg/dl (16.6 [7.7] µmol/l), and 95 (32) µg/dl (17. [5.7] µmol/l); TIBC, 335 (43) µg/dl (6. [7.7] µmol/l), 348 (42) µg/dl (62.3 [7.5] µmol/l), 346 (41) µg/dl (61.9 [7.3] µmol/l), and 336 (45) µg/dl (6.1 [8.1] µmol/l); TS, 27.3% (11.8%), 28.4% (1.9%), 27.1% (12.3%), and 28.7% (1.7%); and ferritin, 38.1 (38.) ng/ml (38.1 [38.] µg/l), 38.8 (38.2) ng/ml (38.8 [38.2] µg/l), 36.8 (35.8) ng/ml (36.8 [35.8] µg/l), and 38.4 (37.7) ng/ml (38.4 [37.7] µg/l). Distributions for each of these analytes are shown in Figure 1. Table 2 shows the overall mean differences (and 95% confidence intervals) between on day 1 and noon and 4 PM on day 1 and on day 2. Overall, significant differences among mean values for the various collection times were observed for iron (P =.1), TIBC (P <.1), and (log) Table 1 Analytic Precision Within-Run Precision Between-Run Precision Analyte CV, % Pool Mean CV, % Pool Mean Iron, µg/dl (µmol/l) (21.7) (21.3) (45.6) (45.1) TIBC, µg/dl (µmol/l) (14.5) (19.3) (38.8) (22.9) Ferritin, ng/ml (µg/l) (46) (49) (4) (41) CV, coefficient of variation; TIBC, total iron-binding capacity. American Society for Clinical Pathology Am J Clin Pathol 22;117: Downloaded from on 18 August 218

3 Dale et al / DIURNAL VARIATION OF SERUM IRON LEVELS A B Serum Iron (µmol/l) Iron-Binding Capacity (µmol/l) C D 8 2 Transferrin Saturation (%) Ferritin (µg/l) Figure 1 Distribution of results for analyses of blood collected at 4 times. Box plots display minimal, 5th percentile, 5th percentile, 95th percentile, and maximal levels. A, Serum iron. B, Total iron-binding capacity. C, Transferrin saturation. D, Ferritin. Values are given as Système International units. To convert µmol/l to conventional units (µg/dl) for iron and iron-binding capacity, divide by.179. To determine conventional units for ferritin (ng/ml), divide by 1.. The percentage for transferrin saturation remains the same. ferritin (P =.3). Concordance among values at the 4 collection times is shown in Figure 2. Significant pairwise differences (eg, -) were noted for both noon vs and vs for TIBC. No significant pairwise differences were seen for iron, TS, or (log) ferritin. Accordingly, the within-day and between-day differences for iron, TS, and (log) ferritin were not statistically significant. The intraindividual differences observed are shown in Figure 3. For individual subjects, iron levels at noon on day 1, on day 1, and on day 2 ranged from 25%, 54%, and 42% lower to 63%, 11%, and 17% higher than levels on day 1, respectively. The between-day iron difference was greater than the within-day difference in 13 subjects (65%). No consistent pattern of change was seen. In 1 subjects, morning iron levels were higher than noon or 4 PM levels; in the remaining 1 subjects, noon or iron levels were higher than morning levels. One subject had subnormal iron levels; iron levels collected at each of the 4 time points were below our reference range. TIBC levels for samples collected at noon on day 1, 4 PM on day 1, and on day 2 ranged from 3%, 4%, and 12% lower to 2%, 13%, and 9% higher than levels on 84 Am J Clin Pathol 22;117:82-88 American Society for Clinical Pathology Downloaded from on 18 August 218

4 Clinical Chemistry / ORIGINAL ARTICLE Table 2 Mean Within-Day and Between-Day Differences (95% Confidence Intervals) Compared With, * Analyte/ Collection Time Conventional Units SI Units Iron, day ( 17.6 to 2.4) 1.4 ( 3.2 to.4), day ( 22.6 to 16.9).5 ( 4. to 3.), day ( 18.7 to 1.2).8 ( 3.3 to 1.8) TIBC, day ( 2.2 to 5.) 2.3 ( 3.6 to.9), day ( 17.1 to 4.5) 1.9 ( 3.1 to.8), day 2.8 ( 8. to 6.5).1 ( 1.4 to 1.2) TS, day ( 4. to 1.8) 1.1 ( 4. to 1.8), day 1.2 ( 5.3 to 5.7).2 ( 5.3 to 5.7), day ( 5.7 to 2.9) 1.4 ( 5.7 to 2.9) Ferritin, day 1.7 ( 1.7 to.3).7 ( 1.7 to.3), day (.6 to 3.2) 1.3 (.6 to 3.2), day 2.3 ( 1.7 to 1.1).3 ( 1.7 to 1.1) TIBC, total iron-binding capacity; TS, transferrin saturation. * Conventional units of measure are as follows: iron and TIBC, µg/dl; TS, %; ferritin, ng/ml. Système International (SI) units of measure are as follows: iron and TIBC, µmol/l; ferritin, µg/l (the percentage for transferrin saturation remains the same). P <.5. Confidence intervals for ferritin are based on observed data, not a log-transformed scale. Other confidence intervals are based on the results of repeated measures analysis of variance. day 1, respectively. The between-day TIBC difference was greater than the within-day difference in 7 subjects (35%). TS levels for samples collected at noon on day 1, on day 1, and on day 2 ranged from 27%, 56%, and 41% lower to 57%, 15%, and 135% higher than levels on day 1, respectively. The between-day TS difference was greater than the within-day difference in 11 subjects (55%). Ferritin levels for samples collected at noon on day 1, on day 1, and on day 2 ranged from 11%, 23%, and 22% lower to 14%, 13%, and 33% higher than levels on day 1, respectively. The between-day ferritin difference was greater than the within-day difference in 15 subjects (75%). Ferritin levels were subnormal in 1 subject and mildly elevated in another subject; for both subjects, ferritin levels collected at each of the 4 time points were abnormal. Discussion In the past at our institution, blood samples for routine laboratory testing typically were obtained in the morning and analyzed in batches, and the results were reported in the afternoon. Most reference values were determined based on an overnight fast and morning sample collection. Random access instrumentation now allows continuous analysis and reporting throughout the day. This improvement makes it possible for specimens to be obtained at times more convenient to patients and for physician appointments to be more frequent and efficient. However, interpretation of test results for analytes with significant diurnal variation may be problematic if collection times change. As a part of a quality improvement initiative in our laboratory, we critically evaluated our patient preparation and specimen collection practices. In this study of the effect of specimen collection time on results for various indicators (iron, TIBC, TS, and ferritin concentrations) used to screen for iron status, we found no consistent diurnal pattern of iron and TS diurnal variation for any of them. For more than half of the subjects, the between-day change was greater than the within-day change for both iron and TS. In some subjects, between-day changes greater than 1% were observed for both iron and TS. Therefore, we conclude that restricting iron and TS collections to a particular time of day does not improve the reliability of the result. Our study has several limitations. First, we included only healthy subjects in our evaluation. However, Heilmann 11 showed that the diurnal pattern seen in patients with iron deficiency was similar to the pattern seen in healthy subjects. Furthermore, earlier studies showed that transferrin saturation measurements remain diagnostically abnormal throughout the day in patients with hemochromatosis Second, because most of our subjects were women, differences between sexes were not assessed. However, earlier diurnal variation studies did not demonstrate differences between men and women. 5,8,9 Third, we did not study repeated 24-hour cycles to determine whether patterns demonstrated by each subject were consistently repeated over time. Bowie et al 9 found that the iron level ranged from 86 to 2 µg/dl ( µmol/l) for 1 subject during a 3-day fast. The subject s pattern seemed cyclic, but over a 48-hour period rather than a 24-hour period, with levels rising from the morning of day 1 to the morning of day 2 and then falling to the morning of day 3. Finally, our sample size was relatively small. Larger studies are needed to confirm the findings of these studies. The within-day and between-day variations in iron levels seen in our study are well documented and limit the diagnostic usefulness of iron measurements. 1,15 Although the diurnal variation seems unpredictable and may be outside the laboratory s control, control of dietary factors may reduce iron variability. Statland et al 16 reported a statistically significant increase in the iron level 2 hours after subjects had eaten. Lammi-Keefe et al 17 demonstrated a reduction in the between-day iron coefficient of variation from 26.1% to 16.6% after fasting. In our study, the variability seen in specimens obtained at noon and (at least a 2-hour fast) was not significantly greater than the variability seen in 2 fasting collections. When TS is used to screen for hemochromatosis, the fasting state has been shown to decrease the number of American Society for Clinical Pathology Am J Clin Pathol 22;117: Downloaded from on 18 August 218

5 Dale et al / DIURNAL VARIATION OF SERUM IRON LEVELS A B Serum Iron (µmol/l), Total Iron-Binding Capacity (µmol/l), C D Transferrin Saturation (%), Ferritin (µg/l), Figure 2 Concordance among values at 4 collection times. x, on day 1; y, noon on day 1 (closed circle), on day 1 (open circle), or on day 2 (square). A, Serum iron. B, Total iron-binding capacity. C, Transferrin saturation. D, Ferritin. To convert µmol/l to conventional units (µg/dl) for iron and iron-binding capacity, divide by.179. To determine conventional units for ferritin (ng/ml), divide by 1.. The percentage for transferrin saturation remains the same. false-positive TS screens. 18 Published screening algorithms recommend repeated testing on a fasting specimen if the initial screen is abnormal. 19,2 Others have reported that diet has little effect on iron levels. 8,14 Avoidance of iron-containing supplements before specimen collection also may reduce intraindividual variability of iron levels and yield a more accurate reflection of iron status. When a small dose of iron was ingested in an iron tolerance test, iron levels were increased over the baseline for at least 8 hours, with significantly greater changes observed in iron-deficient subjects compared with ironreplete subjects. 21,22 A similar study using iron-fortified foods found the effect to be lower and more delayed. 21 Currently, to minimize changes caused by the recent ingestion of iron, we recommend that iron-containing supplements be discontinued 24 hours before the collection of blood specimens for determination of iron levels. However, the minimal time needed to eliminate food and iron-supplementation effects is not yet established. When iron is used to screen for or monitor iron deficiency or overload, several measurements are needed. 9 Lammi-Keefe et al 17 found that accurate assessment of iron status required 3 testing cycles in healthy women and 8 cycles in elderly women with rheumatoid arthritis. Economic concerns and health care payer rules for frequency of testing make multiple cycles of testing for a single diagnosis undesirable. For iron deficiency, ferritin has been shown to be the best single test. 1,23,24 In our study, between-day variations of ferritin were smaller than those of iron and TS. After completing our study, despite finding that the time of day was not critical, we decided that the opportunity to convert orders from iron to ferritin justified the time spent contacting 86 Am J Clin Pathol 22;117:82-88 American Society for Clinical Pathology Downloaded from on 18 August 218

6 Clinical Chemistry / ORIGINAL ARTICLE A 12 1 B Serum Iron, % Change From (8.4) (3.2) (4.7) Iron-Binding Capacity, % Change From (3.8) (3.2) (.2) C 16 D 14 Transferrin Saturation, % Change From (4.) (.5) (5.2) Ferritin, % Change From (1.8) ( 3.4) (.8) Figure 3 Intraindividual differences between results of analysis on blood collected at on day 1 and those for blood collected at 3 other times. The mean percentage difference is shown in parentheses. A, Serum iron. B, Total iron-binding capacity. C, Transferrin saturation. D, Ferritin. the physician. A more direct approach such as that taken by laboratories in Ottawa (ie, determining only the ferritin level if iron, TIBC, and ferritin were ordered) resulted in an 8% reduction in iron and TIBC tests performed. 25 In our setting, we have chosen to maintain physician choice. In the future, we hope to include ordering and patient preparation recommendations in our electronic order system as a way to educate physicians and improve test-ordering practices. Conclusions Although overall statistically significant differences among the collection times were observed for iron, TIBC, and (log) ferritin, no consistent diurnal variation pattern was seen. For half of the subjects, morning iron levels were higher than afternoon iron levels; for the remaining half, the reverse was true. For all 4 analytes, the relative magnitude of the between-day variation was similar to the relative magnitude of the within-day variation. The practice of restricting blood collection for iron determination to a specific time of day does not improve the reliability of the result. From the 1 Division of Clinical Biochemistry and Immunology and the 2 Section of Biostatistics, Mayo Clinic, Rochester, MN. Address correspondence to Dr Dale: Mayo Clinic, 2 First St SW, Rochester, MN Acknowledgments: We thank Melissa M. Ward for participation in this study and Constance L. Center for efforts in manuscript preparation. American Society for Clinical Pathology Am J Clin Pathol 22;117: Downloaded from on 18 August 218

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