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Serum iron concentration and transferrin saturation in the diagnosis of iron deficiency in children: Normal developmental changes
December 1977
870
The Journal o f P E D I A T R I C S
Serum iron concentration and transferrin saturation in the diagnosis of iron deficiency in children: Normal developmental changes A group of 359 healthy children and 49 adults were studied for the purpose of estimating the normal limits for serum iron concentration and transferrin saturation. The 144 children and seven adults who had any other laboratory evidence of iron deficiency (abnormal values of serum ferritin, free erythrocyte protoporphyrin, hemoglobin concentration, or mean corpuscular volume) were excluded. In evaluating the 215 children and 42 adults who met the criteria to be considered normal we found that serum iron concentration and transferrin saturation were significantly lower in children between the ages of 0.5 and 12 years than in adults. We conclude that in children between the ages of O.5 and 12 years, a transferrin saturation of < 16% constitutes good evidence of iron deficiency only in conjunction with anemia and low mean corpuscular volume.
Marion A. Koerper, M.D.,* and Peter R. D a l l m a n , M . D . , S a n F r a n c i s c o , Calif.
SINCE LOW SERUM IRON concentration or low transferrin saturation is commonly a basis for the diagnosis of iron deficiency,' it is necessary to establish reliable ranges of normal values in children in order to minimize diagnostic error. Previous studies have suggested that healthy infants have lower serum iron values than adults, ~ even when they are supplemented with iron? We have found that the median serum iron and transferrin saturation in children are significantly below the respective normal adult values; it is necessary to take these differences into account in making the diagnosis of iron deficiency in infants and children. METHODS We studied 359 healthy children brought to the Pediatric Clinic of the University of California Medical Center for well-child care. This group included 121 between the ages of 0.5 and 2 years, 84 between 2 and 6 years, 80 between 6 and 12 years, and 74 between 12 and 18 years. From the Department of Pediatrics, University o f California, San Francisco. Supported by National Institutes of Health Grant No. HD 00828 and A M 13897. *Reprint address: Universityof California, Department of Pediatrics 650 M, San Francisco, CA 94143.
Vol. 91, No. 6, pp. 870-874
In order t o compare our results with published adult values, 4 we also studied 49 healthy men (primarily young adults) aged 18 to 66 years. All of the children and 40 of the adults were not fasting when blood samples were obtained; nine additional adults had samples drawn both While fasting and again after a meal. There was no significant difference between the matched fasting and nonfasting samples; only the latter were used for comparison with values in the children. All subjects had venous blood drawn between 9 AM and 2 PM for determination of
See related articles, pp. 8~/5 and 1027. hemoglobin, hematocrit, red blood cell count, mean corpuscular volume, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration with a Model S Coulter Counter. Free erythrocyte protoporphyrin was determined fluorometrically after extraction with ethyl acetate? Serum iron, total iron-binding capacity, and serum ferritin were measured as previously described? Percent transferrin saturation was calculated from the ratio (serum iron/total iron-binding capacity) x 100. Only those subjects were considered normal who met all of the following criteria: hemoglobin concentration _ 11.0 gm/dl between 0.5 and 6 years, _> 11.5 gm/dl
Volume 91 Number 6
Serum iron and transferrin in iron deficiency
(52)
(58)
(58)
(47)
150
(42)
&
9
871
-
30
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20
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6-12
12-18
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18-66
AGE, YEARS Fig. 1. Normal values for serum iron and transferrin saturation in subjects who met the criteria for normal values of hemoglobin, mean corpuscular volume, free erythrocyte protoporphyrin, and serum ferritin. The number of subjects in each group is indicated at the top in parentheses. Serum iron values, in the top portion, are shown in microgram/ deciliter on the left and micromole/liter on the right; transferrin saturation is shown below. The heavy horizontal lines indicate the median values; the dark gray area is that below the 95% range. The light gray stippled area represents an intermediate zone in which values of iron-deficient subjects are likely to overlap into the normal range.
between 6 and 12 years, _>. 12.0 g m / d l in girls between I2 and 18 years, _> 13.0 g m / d l in boys between 12 and 18 years, and _> 14.0 g m / d l in adult men; m e a n corpuscular volume _> 70 between 0.5 and 2 years, ~> 74 between 2 and 6 years, _> 76 between 6 and 12 years, _> 78 between 12 and 18 years, and _> 80 above 18 years; free erythrocyte protoporphyrin _< 3.0 /~g/gm hemoglobin; serum ferritin _> 12 n g / m l . We chose the stringent criterion for serum ferritin o f > 12 n g / m l in order to exclude subjects who might have borderline iron deficiency. The usual lower limit for serum ferritin is 7 or 10 n g / m l . O f the total of 359 children and 49 adults, the n u m b e r that met the criteria to be designated normal included 52 children
between the ages o f 0.5 and 2 years, 58 between 2 and 6 years, 58 between 6 and 12 years, 47 between 12 and 18 years, and 42 adults between 18 and 66 years. Ranges of normal values for laboratory tests are most commonly based on the m e a n _+2 SD, but the use of standard deviations to estimate confidence limits is only appropriate when values conform to a Gaussian distribution. Because serum iron and transferrin saturation did not appear to follow a strict Gaussian distribution, we calculated the median and the lower limit of the 95% range according to the percentile method o f Herrera, 7 which does not require the assumption o f a normal distribution.
872
Koerper and Dallman
The Journal of Pediatrics December 1977
Table I. Median values for serum iron concentration and transferrin saturation in normal children aged 0.5 to 12 years and in children judged abnormal by one or more criteria
All values normal Low serum ferritin Low mean corpuscular volume Low serum ferritin and mean corpuscular volume Low hemoglobin, mean corpuscular volume, and serum ferritin Low hemoglobin, mean corpuscular volume, and serum ferritin and elevated free erythrocyte pfotoporphyrin All other combinations
Published adult values for normal serum iron and transferrin saturation vary widely. Mean values for serum iron, derived from a large number of studies, average about l l 0 /~g/dl in women and 120/~g/dl in men with lower limits of normal of 62 and 72/~g/dl, respectively.4 In the case of transferrin saturation, a lower limit of 20% is listed in a recent tabulation respresenting a composite of values from several laboratories, 4 but a lower limit of 16% is widely used based largely on the studies of Bainton and Finch? Which lower limit is used often depends on the clinical circumstance. Because of the variability of the serum iron there is great overlap in transferrin saturation values between iron-deficient and normal individuals. Thus a lower limit of 20% rather than 16% can be expected to result in detection of a larger percentage of irondeficient individuals but at a cost of including more individuals with normal iron stores. Because of this uncertainty we have calculated two lower limits, one below which the probability of iron deficiency is high and an intermediate one below which there is less certainty about the diagnosis (Fig. 1). RESULTS Fig. 1 depicts the median values and lower limits of the 95% range for serum iron and transferrin saturation in subjects who met the criteria to be considered normal in each age group. The median values in the first three age groups were serum iron 68, 70, and 70/~g/dl and transferrin saturation 22, 25, and 25%; all were significantly below the adult medians of 90/~g/dl and 30% (p < 0.001 for serum iron and p < 0.01 for transferrin saturation). The median values for the 12-to 18-year-old group, serum iron 80/~g/dl a n d transferrin saturation 27%, were intermediate between childhood and adult levels, with wide scatter in individual values reflecting this stage of transition. For purposes of calculating lower limits of normal we
No. of children
% of total
Serum iron OLg/ dl)
Transferrin saturation (%)
168 67 11 11 8 3
59 23 4 4 3
70 70 62 51 35
24 22 23 16 9
1
23
5
17
6
61
18
combined the youngest three groups because their median values and ranges did not differ significantly. Lower limits of the 95% range were serum iron, 20/~g/dl, and transferrin saturation, 7%, between 0.5 and 12 years; the values were identical for the 12- to 18-year-old group. In the adult males the lower limits were serum iron 50/~g/dl and transferrin saturation 16%. The zone below these minimum values is shaded dark gray (Fig. l) to indicate a high probability of iron deficiency. In Fig. 1 the lower portion of the normal range for transferrin saturation is shown in stippled gray to indicate an estimate o f the extent to which values of iron-deficient individuals are likely to overlap. The limits of this intermediate zone were estimated by analyzing the data of 285 children in the pooled 0.5- to 12-year group who had one or more laboratory abnormalities (see Methods). Table I shows that the serum iron and transferrin saturation decreased as the certainty of iron deficiency increased or the stage of iron deficiency became more advanced. The screening tests for iron deficiency become abnormal in three definite but overlapping stages. The first stage is a decrease in iron stores that is reflected by a falling serum ferritin value. The second stage is characterized by a decrease in serum iron and an elevation in total iron-binding capacity that result in a low transferrin saturation. The first two stages are sometimes termed latent iron deficiency and are most reliably detected by serum ferritin. In the third stage there is a gradual decrease in production of hemoglobin. Early in the development of anemia, changes in free erythrocyte protoporphyrin and mean corpuscular volume are usually of greater magnitude than those of hemoglobin. Table I shows that most children were excluded on the basis of low or borderline ( < 12 ng/ml) serum ferritin alone. Their values for serum iron and transferrin saturation were similar to the normal group, indicating solely a decrease in iron stores with little overlap into the second
Volume 91 Number 6
stage of iron deficiency. A smaller group was excluded on the basis of mean corpuscular volume alone and included the individuals who proved to have thalassemia minor; their serum iron and transferrin saturation values were also equivalent to those of the normal group. In contrast, the two groups whose laboratory values indicated the third stage of iron deficiency (low serum ferritin, mean corpuscular volume, and hemoglobin with or without high free erythrocyte protoporphyrin) had much lower median values for serum iron and transferrin saturation with some individual values overlapping into the normal range. Of the 11 subjects comprising these latter two groups, six had a transferrin saturation below 7%, four had saturations between 7 and 16%, and one had a saturation _> 16%. Thus a transferrin saturation of < 16% in combination with anemia and a low mean corpuscular volume constituted strong evidence of iron deficiency. However, a transferrin saturation < 16% as the sole criterion would have indicated iron deficiency in fully 15% of children who were normal by all other laboratory measurements. Another basis for considering a transferrin saturation of 7 to 16% to be an intermediate zone was the higher incidence of low serum ferritin values ( < 10 ng/ml) in these children compared to children with transferrin saturation _> 16%. Of the children with transferrin saturations ~ 16%, 25% had low serum ferritin; 45% of those with saturations between 7 and 16% had subnormal serum ferritin values; and 70% of those with saturations < 7% had low serum ferritin values. It could be argued that serum iron and transferrin saturation values were found to be lower in children than in adults, owing to the choice of laboratory criteria that did not adequately exclude iron deficiency. For this reason we compared the mean serum iron in the screened children in each age group whose hemoglobin, mean corpuscular volume, and serum ferritin, respectively, were below the median for each test with those whose values were above the median. Using Student's t test, we found no significant difference in serum iron between the high and low hemoglobin, mean corpuscular volume, and serum ferritin groups (p > 0.5 for all three variables), providing additional assurance that the developmental difference did not simply result from a high incidence of iron deficiency. DISCUSSION Determinations of hemoglobin concentration and mean corpuscular volume are among the most widely available methods of screening for iron deficiency. When subnormal values are found, the choice of confirmatory tests includes the transferrin saturation, serum ferritin,
Serum iron and transferrin in iron deficiency
873
free erythrocyte protoporphyrin, or a therapeutic trial of iron. Determinations of serum ferritin and free erythrocyte protoporphyrin are not yet in extensive use. The therapeutic trial of iron in mild iron deficiency is often the simplest approach, but it has the disadvantage of a threemonth delay before the diagnosis can be confirmed by correction of hemoglobin concentration and mean corpuscular volume. Consequently, in many clinical settings it is most practical to determine the transferrin saturation when a patient is found to have low hemoglobin concentration and mean corpuscular volume for age. Our results indicate that serum iron and transferrin saturation, similar to the values of hemoglobin and mean corpuscular volume,~ are significantly lower in a screened population of healthy children than in adults. The values for serum iron and transferrin saturation may partly reflect the marginal iron reserves that are characteristic of childhood. There is no basis, however, for considering these values abnormal by the application of all other laboratory criteria, including the more sensitive serum ferritin assay. Although the transferrin saturation for a group of individuals should become abnormal at an earlier stage of iron deficiency than do the hemoglobin and mean corpuscular volume, the usefulness of the transferrin saturation as the sole criterion of iron deficiency in individual patients is decreased by its poor reproducibility. The wide variations that have been noted in serum iron and transferrin saturation9, TM are partly due to diurnal, dietary, and other factors that are independent of age. This helps to explain the unexpectedly low value of 7% for the lower limit of normal in children when the transferrin saturation is used alone and the validity of regarding 7 to 16% as an intermediate range. The use of 7% as the lower limit of normal for transferrin saturation will result in missing many cases of iron deficiency with values that overlap into the normal range. On the other hand, the use of 16% transferrin saturation by itself as a lower limit produces an unacceptably high number of false positive diagnoses. As discussed in a recent article in THE JOtrRNAL,11 the hemoglobin and mean corpuscular volume are convenient starting points in the diagnosis of iron deficiency. When both values are low for age, the major differential diagnosis is between iron deficiency and thalassemia minor. A transferrin saturation < 16% in conjunction with anemia and a low mean corpuscular volume constitutes good evidence for iron deficiency. We acknowledge the skillful technical assistance of Evelyn Manies, Robert Spirito, and Alice Nishioka.
874
Koerper and Dallman
The Journal of Pediatrics December 1977
REFERENCES
1. Select Committee on Nutrition and Human Needs, United States Senate: Medical evaluation of the special supplemental food program for women, infants and children, Washington, DC, 1976, United States Government Printing Office. 2. Sturgeon P: Studies of iron requirements in infants and children, Pediatrics 13:107, 1954. 3. Andelman MB, and Sered BR: Utilization of dietary iron by term infants, Am J Dis Child 111:45, 1966. 4. Wintrobe MM: Clinical hematology, ed 7, Philadelphia, 1974, Lea & Febiger, Publishers, p 1802. 5. Piomelli S: A micromethod for free erythrocyte porphyrins: The FEP test, J Lab Clin Med 81:932, 1973. 6. Koerper MA, Mentzer WC, Brecher G, and Dallman PR:
7. 8. 9.
10. 11.
Developmental change in red blood cell volume: Implication in screening infants and children for iron deficiency and thalassemia trait, J PEDIATR 89:580, 1976. Herrera L: The precision of percentiles in establishing normal limits in medicine, J Lab Clin Med 52:34, 1958. Bainton DF, and Finch CA: The diagnosis of iron deficiency anemia, Am J Med 37:62, 1964. Cook JD, Lipschitz DA, Miles LEM, and Finch CA: Serum ferritin as a measure of iron stores in normal subjects, Am J Clin Nutr 27:681, 1974. Cook JD, Finch CA, and Smith NJ: Evaluation of the iron status of a population, Blood 48:449, 1976. Dallman PR: New approaches to screening for iron deficiency, J PEDIATR 90:678, 1977.
870
The Journal o f P E D I A T R I C S
Serum iron concentration and transferrin saturation in the diagnosis of iron deficiency in children: Normal developmental changes A group of 359 healthy children and 49 adults were studied for the purpose of estimating the normal limits for serum iron concentration and transferrin saturation. The 144 children and seven adults who had any other laboratory evidence of iron deficiency (abnormal values of serum ferritin, free erythrocyte protoporphyrin, hemoglobin concentration, or mean corpuscular volume) were excluded. In evaluating the 215 children and 42 adults who met the criteria to be considered normal we found that serum iron concentration and transferrin saturation were significantly lower in children between the ages of 0.5 and 12 years than in adults. We conclude that in children between the ages of O.5 and 12 years, a transferrin saturation of < 16% constitutes good evidence of iron deficiency only in conjunction with anemia and low mean corpuscular volume.
Marion A. Koerper, M.D.,* and Peter R. D a l l m a n , M . D . , S a n F r a n c i s c o , Calif.
SINCE LOW SERUM IRON concentration or low transferrin saturation is commonly a basis for the diagnosis of iron deficiency,' it is necessary to establish reliable ranges of normal values in children in order to minimize diagnostic error. Previous studies have suggested that healthy infants have lower serum iron values than adults, ~ even when they are supplemented with iron? We have found that the median serum iron and transferrin saturation in children are significantly below the respective normal adult values; it is necessary to take these differences into account in making the diagnosis of iron deficiency in infants and children. METHODS We studied 359 healthy children brought to the Pediatric Clinic of the University of California Medical Center for well-child care. This group included 121 between the ages of 0.5 and 2 years, 84 between 2 and 6 years, 80 between 6 and 12 years, and 74 between 12 and 18 years. From the Department of Pediatrics, University o f California, San Francisco. Supported by National Institutes of Health Grant No. HD 00828 and A M 13897. *Reprint address: Universityof California, Department of Pediatrics 650 M, San Francisco, CA 94143.
Vol. 91, No. 6, pp. 870-874
In order t o compare our results with published adult values, 4 we also studied 49 healthy men (primarily young adults) aged 18 to 66 years. All of the children and 40 of the adults were not fasting when blood samples were obtained; nine additional adults had samples drawn both While fasting and again after a meal. There was no significant difference between the matched fasting and nonfasting samples; only the latter were used for comparison with values in the children. All subjects had venous blood drawn between 9 AM and 2 PM for determination of
See related articles, pp. 8~/5 and 1027. hemoglobin, hematocrit, red blood cell count, mean corpuscular volume, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration with a Model S Coulter Counter. Free erythrocyte protoporphyrin was determined fluorometrically after extraction with ethyl acetate? Serum iron, total iron-binding capacity, and serum ferritin were measured as previously described? Percent transferrin saturation was calculated from the ratio (serum iron/total iron-binding capacity) x 100. Only those subjects were considered normal who met all of the following criteria: hemoglobin concentration _ 11.0 gm/dl between 0.5 and 6 years, _> 11.5 gm/dl
Volume 91 Number 6
Serum iron and transferrin in iron deficiency
(52)
(58)
(58)
(47)
150
(42)
&
9
871
-
30
-
20
w Y
Y
v
T
N
0 lO0
"lz 3 o_.
LU 00 e~
50
60
t
t
99 ,,
Z
_o
2
9
6-12
12-18
40
2O
0.5-2
2-6
18-66
AGE, YEARS Fig. 1. Normal values for serum iron and transferrin saturation in subjects who met the criteria for normal values of hemoglobin, mean corpuscular volume, free erythrocyte protoporphyrin, and serum ferritin. The number of subjects in each group is indicated at the top in parentheses. Serum iron values, in the top portion, are shown in microgram/ deciliter on the left and micromole/liter on the right; transferrin saturation is shown below. The heavy horizontal lines indicate the median values; the dark gray area is that below the 95% range. The light gray stippled area represents an intermediate zone in which values of iron-deficient subjects are likely to overlap into the normal range.
between 6 and 12 years, _>. 12.0 g m / d l in girls between I2 and 18 years, _> 13.0 g m / d l in boys between 12 and 18 years, and _> 14.0 g m / d l in adult men; m e a n corpuscular volume _> 70 between 0.5 and 2 years, ~> 74 between 2 and 6 years, _> 76 between 6 and 12 years, _> 78 between 12 and 18 years, and _> 80 above 18 years; free erythrocyte protoporphyrin _< 3.0 /~g/gm hemoglobin; serum ferritin _> 12 n g / m l . We chose the stringent criterion for serum ferritin o f > 12 n g / m l in order to exclude subjects who might have borderline iron deficiency. The usual lower limit for serum ferritin is 7 or 10 n g / m l . O f the total of 359 children and 49 adults, the n u m b e r that met the criteria to be designated normal included 52 children
between the ages o f 0.5 and 2 years, 58 between 2 and 6 years, 58 between 6 and 12 years, 47 between 12 and 18 years, and 42 adults between 18 and 66 years. Ranges of normal values for laboratory tests are most commonly based on the m e a n _+2 SD, but the use of standard deviations to estimate confidence limits is only appropriate when values conform to a Gaussian distribution. Because serum iron and transferrin saturation did not appear to follow a strict Gaussian distribution, we calculated the median and the lower limit of the 95% range according to the percentile method o f Herrera, 7 which does not require the assumption o f a normal distribution.
872
Koerper and Dallman
The Journal of Pediatrics December 1977
Table I. Median values for serum iron concentration and transferrin saturation in normal children aged 0.5 to 12 years and in children judged abnormal by one or more criteria
All values normal Low serum ferritin Low mean corpuscular volume Low serum ferritin and mean corpuscular volume Low hemoglobin, mean corpuscular volume, and serum ferritin Low hemoglobin, mean corpuscular volume, and serum ferritin and elevated free erythrocyte pfotoporphyrin All other combinations
Published adult values for normal serum iron and transferrin saturation vary widely. Mean values for serum iron, derived from a large number of studies, average about l l 0 /~g/dl in women and 120/~g/dl in men with lower limits of normal of 62 and 72/~g/dl, respectively.4 In the case of transferrin saturation, a lower limit of 20% is listed in a recent tabulation respresenting a composite of values from several laboratories, 4 but a lower limit of 16% is widely used based largely on the studies of Bainton and Finch? Which lower limit is used often depends on the clinical circumstance. Because of the variability of the serum iron there is great overlap in transferrin saturation values between iron-deficient and normal individuals. Thus a lower limit of 20% rather than 16% can be expected to result in detection of a larger percentage of irondeficient individuals but at a cost of including more individuals with normal iron stores. Because of this uncertainty we have calculated two lower limits, one below which the probability of iron deficiency is high and an intermediate one below which there is less certainty about the diagnosis (Fig. 1). RESULTS Fig. 1 depicts the median values and lower limits of the 95% range for serum iron and transferrin saturation in subjects who met the criteria to be considered normal in each age group. The median values in the first three age groups were serum iron 68, 70, and 70/~g/dl and transferrin saturation 22, 25, and 25%; all were significantly below the adult medians of 90/~g/dl and 30% (p < 0.001 for serum iron and p < 0.01 for transferrin saturation). The median values for the 12-to 18-year-old group, serum iron 80/~g/dl a n d transferrin saturation 27%, were intermediate between childhood and adult levels, with wide scatter in individual values reflecting this stage of transition. For purposes of calculating lower limits of normal we
No. of children
% of total
Serum iron OLg/ dl)
Transferrin saturation (%)
168 67 11 11 8 3
59 23 4 4 3
70 70 62 51 35
24 22 23 16 9
1
23
5
17
6
61
18
combined the youngest three groups because their median values and ranges did not differ significantly. Lower limits of the 95% range were serum iron, 20/~g/dl, and transferrin saturation, 7%, between 0.5 and 12 years; the values were identical for the 12- to 18-year-old group. In the adult males the lower limits were serum iron 50/~g/dl and transferrin saturation 16%. The zone below these minimum values is shaded dark gray (Fig. l) to indicate a high probability of iron deficiency. In Fig. 1 the lower portion of the normal range for transferrin saturation is shown in stippled gray to indicate an estimate o f the extent to which values of iron-deficient individuals are likely to overlap. The limits of this intermediate zone were estimated by analyzing the data of 285 children in the pooled 0.5- to 12-year group who had one or more laboratory abnormalities (see Methods). Table I shows that the serum iron and transferrin saturation decreased as the certainty of iron deficiency increased or the stage of iron deficiency became more advanced. The screening tests for iron deficiency become abnormal in three definite but overlapping stages. The first stage is a decrease in iron stores that is reflected by a falling serum ferritin value. The second stage is characterized by a decrease in serum iron and an elevation in total iron-binding capacity that result in a low transferrin saturation. The first two stages are sometimes termed latent iron deficiency and are most reliably detected by serum ferritin. In the third stage there is a gradual decrease in production of hemoglobin. Early in the development of anemia, changes in free erythrocyte protoporphyrin and mean corpuscular volume are usually of greater magnitude than those of hemoglobin. Table I shows that most children were excluded on the basis of low or borderline ( < 12 ng/ml) serum ferritin alone. Their values for serum iron and transferrin saturation were similar to the normal group, indicating solely a decrease in iron stores with little overlap into the second
Volume 91 Number 6
stage of iron deficiency. A smaller group was excluded on the basis of mean corpuscular volume alone and included the individuals who proved to have thalassemia minor; their serum iron and transferrin saturation values were also equivalent to those of the normal group. In contrast, the two groups whose laboratory values indicated the third stage of iron deficiency (low serum ferritin, mean corpuscular volume, and hemoglobin with or without high free erythrocyte protoporphyrin) had much lower median values for serum iron and transferrin saturation with some individual values overlapping into the normal range. Of the 11 subjects comprising these latter two groups, six had a transferrin saturation below 7%, four had saturations between 7 and 16%, and one had a saturation _> 16%. Thus a transferrin saturation of < 16% in combination with anemia and a low mean corpuscular volume constituted strong evidence of iron deficiency. However, a transferrin saturation < 16% as the sole criterion would have indicated iron deficiency in fully 15% of children who were normal by all other laboratory measurements. Another basis for considering a transferrin saturation of 7 to 16% to be an intermediate zone was the higher incidence of low serum ferritin values ( < 10 ng/ml) in these children compared to children with transferrin saturation _> 16%. Of the children with transferrin saturations ~ 16%, 25% had low serum ferritin; 45% of those with saturations between 7 and 16% had subnormal serum ferritin values; and 70% of those with saturations < 7% had low serum ferritin values. It could be argued that serum iron and transferrin saturation values were found to be lower in children than in adults, owing to the choice of laboratory criteria that did not adequately exclude iron deficiency. For this reason we compared the mean serum iron in the screened children in each age group whose hemoglobin, mean corpuscular volume, and serum ferritin, respectively, were below the median for each test with those whose values were above the median. Using Student's t test, we found no significant difference in serum iron between the high and low hemoglobin, mean corpuscular volume, and serum ferritin groups (p > 0.5 for all three variables), providing additional assurance that the developmental difference did not simply result from a high incidence of iron deficiency. DISCUSSION Determinations of hemoglobin concentration and mean corpuscular volume are among the most widely available methods of screening for iron deficiency. When subnormal values are found, the choice of confirmatory tests includes the transferrin saturation, serum ferritin,
Serum iron and transferrin in iron deficiency
873
free erythrocyte protoporphyrin, or a therapeutic trial of iron. Determinations of serum ferritin and free erythrocyte protoporphyrin are not yet in extensive use. The therapeutic trial of iron in mild iron deficiency is often the simplest approach, but it has the disadvantage of a threemonth delay before the diagnosis can be confirmed by correction of hemoglobin concentration and mean corpuscular volume. Consequently, in many clinical settings it is most practical to determine the transferrin saturation when a patient is found to have low hemoglobin concentration and mean corpuscular volume for age. Our results indicate that serum iron and transferrin saturation, similar to the values of hemoglobin and mean corpuscular volume,~ are significantly lower in a screened population of healthy children than in adults. The values for serum iron and transferrin saturation may partly reflect the marginal iron reserves that are characteristic of childhood. There is no basis, however, for considering these values abnormal by the application of all other laboratory criteria, including the more sensitive serum ferritin assay. Although the transferrin saturation for a group of individuals should become abnormal at an earlier stage of iron deficiency than do the hemoglobin and mean corpuscular volume, the usefulness of the transferrin saturation as the sole criterion of iron deficiency in individual patients is decreased by its poor reproducibility. The wide variations that have been noted in serum iron and transferrin saturation9, TM are partly due to diurnal, dietary, and other factors that are independent of age. This helps to explain the unexpectedly low value of 7% for the lower limit of normal in children when the transferrin saturation is used alone and the validity of regarding 7 to 16% as an intermediate range. The use of 7% as the lower limit of normal for transferrin saturation will result in missing many cases of iron deficiency with values that overlap into the normal range. On the other hand, the use of 16% transferrin saturation by itself as a lower limit produces an unacceptably high number of false positive diagnoses. As discussed in a recent article in THE JOtrRNAL,11 the hemoglobin and mean corpuscular volume are convenient starting points in the diagnosis of iron deficiency. When both values are low for age, the major differential diagnosis is between iron deficiency and thalassemia minor. A transferrin saturation < 16% in conjunction with anemia and a low mean corpuscular volume constitutes good evidence for iron deficiency. We acknowledge the skillful technical assistance of Evelyn Manies, Robert Spirito, and Alice Nishioka.
874
Koerper and Dallman
The Journal of Pediatrics December 1977
REFERENCES
1. Select Committee on Nutrition and Human Needs, United States Senate: Medical evaluation of the special supplemental food program for women, infants and children, Washington, DC, 1976, United States Government Printing Office. 2. Sturgeon P: Studies of iron requirements in infants and children, Pediatrics 13:107, 1954. 3. Andelman MB, and Sered BR: Utilization of dietary iron by term infants, Am J Dis Child 111:45, 1966. 4. Wintrobe MM: Clinical hematology, ed 7, Philadelphia, 1974, Lea & Febiger, Publishers, p 1802. 5. Piomelli S: A micromethod for free erythrocyte porphyrins: The FEP test, J Lab Clin Med 81:932, 1973. 6. Koerper MA, Mentzer WC, Brecher G, and Dallman PR:
7. 8. 9.
10. 11.
Developmental change in red blood cell volume: Implication in screening infants and children for iron deficiency and thalassemia trait, J PEDIATR 89:580, 1976. Herrera L: The precision of percentiles in establishing normal limits in medicine, J Lab Clin Med 52:34, 1958. Bainton DF, and Finch CA: The diagnosis of iron deficiency anemia, Am J Med 37:62, 1964. Cook JD, Lipschitz DA, Miles LEM, and Finch CA: Serum ferritin as a measure of iron stores in normal subjects, Am J Clin Nutr 27:681, 1974. Cook JD, Finch CA, and Smith NJ: Evaluation of the iron status of a population, Blood 48:449, 1976. Dallman PR: New approaches to screening for iron deficiency, J PEDIATR 90:678, 1977.