1 Department of Preventive Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
2 Department of Epidemiology and Public Health, Faculty of Medicine, St. Marys Campus, Imperial College London, London, United Kingdom.
3 Akademisch Ziekenhuis St. Rafael, Leuven, Belgium.
4 Department of Health Science, Shiga University of Medical Science, Otsu, Japan.
Received for publication December 9, 2003; accepted for publication June 22, 2004.
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ABSTRACT |
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albumins; albuminuria; blood pressure; creatinine; epidemiologic methods
Abbreviations: Abbreviations: ACR, albumin:creatinine ratio; CV, coefficient of variation; INTERMAP, International Study of Macronutrients and Blood Pressure.
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INTRODUCTION |
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The amount of albumin excreted in the urine over a 24-hour period is considered the "gold standard" for assessing albumin level and defining microalbuminuria (13). However, 24-hour urine collections are cumbersome and subject to error due to inaccurate timing and/or incompleteness. American Diabetes Association guidelines for detection of microalbuminuria permit use of 24-hour collections, timed specimens taken over a period of less than 24 hours, and untimed random specimens (13). For random specimens, results and cutoff points for microalbuminuria must be based on either urinary albumin concentration or the albumin:creatinine ratio (ACR). The American Diabetes Association considers the daily within-person variability of albumin concentration to be too high to permit use of albumin concentration alone in defining microalbuminuria. Thus, researchers have typically used the ACR for defining microalbuminuria (3, 69, 1416) in spot or random collections, although some have also used albumin concentration (4, 17, 18). Use of the ACR is also problematic, however, since it requires measurement of creatinine excretion, which varies by gender, age, and ethnicity (5, 1922). This has led some investigators to propose gender-specific cutoff points for microalbuminuria when using the ACR (5, 1925). In addition, albumin excretion has been found to be highly variable within individuals, with the within-person coefficient of variation (CV) generally averaging 4060 percent (2631). Thus, there are strengths and weaknesses in using 24-hour albumin excretion, ACR, or albumin concentration that are relevant for investigators conducting epidemiologic studies on albumin level and microalbuminuria.
The purpose of this study was to further assess the strengths and weaknesses of the ACR and albumin concentration as alternatives to albumin excretion using albumin measurements from two 24-hour urine collections in men and women in the International Study of Macronutrients and Blood Pressure (INTERMAP). In particular, we examined the following questions: 1) Is ACR or albumin concentration more strongly related to albumin excretion in a 24-hour collection? 2) How does daily within-person variation, as assessed by the within-person CV, compare for 24-hour albumin excretion, ACR, and albumin concentration and for other urinary variables such as sodium, potassium, and creatinine? 3) Are known predictors of albumin excretion, such as blood pressure and body mass index (1416, 32, 33), more strongly associated with 24-hour ACR or albumin concentration?
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MATERIALS AND METHODS |
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Between 1996 and 1999, each participant visited the local INTERMAP research center on four occasions. Two visits were undertaken on consecutive days, with a further two visits made on consecutive days 36 weeks later.
Data collection
All data were collected by trained and certified staff. Dietary data were collected at each visit with the 24-hour recall method. Blood pressure was measured twice at each visit with a random-zero sphygmomanometer while the participant was seated. Height and weight without shoes were measured at the first and third visits, and body mass index was calculated as weight divided by height squared (kg/m2). Data on demographic and other factors were collected by interviewer-administered questionnaire.
Two timed 24-hour urine specimens were collected. Collections were started at the research center on the first and third visits and completed at the center the following day. Urine aliquots were stored frozen at 20°C before and after being shipped frozen to the Central Laboratory, where analyses were performed with strict internal and external quality control. Levels of sodium, potassium, creatinine, urea, magnesium, and calcium were analyzed within 3 years of receipt of aliquots at the Central Laboratory, while albumin was analyzed in aliquots that had been frozen for 3 or more years, with completion in 2002. Urinary sodium and potassium concentrations were measured by emission flame photometry. Standard methods were used for analyses of other urinary variables (3538). Individual excretion values were calculated as the product of concentrations in the urine and urinary volume corrected to 24 hours.
Among the 4,678 men and women with albumin measurements, there were 2,745 persons for whom one or both albumin concentration values were below the detection limit of the assay (1 mg/liter).
Statistical methods
The American Diabetes Association defines microalbuminuria as excretion of 30299 mg of albumin in a 24-hour collection, with values 300 being defined as macroalbuminuria (13). To derive comparable gender/country-specific INTERMAP cutoffs for ACR and albumin concentration, we divided 30 by each subgroups mean creatinine excretion and mean urinary volume to obtain ACR and albumin concentration cutoff points for microalbuminuria; we multiplied these by 10 to obtain cutoff points for macroalbuminuria.
To assess associations of ACR and albumin concentration with albumin excretion, we calculated Spearmans rank-order correlations of ACR and albumin concentration with albumin excretion for the 1,933 men and women with albumin concentrations 1 mg/liter in both collections and kappa statistics to assess their overall agreement with albumin excretion in classifying all 4,678 INTERMAP participants as having micro- or macroalbuminuria (39). Because the use of INTERMAP-specific cutoff points for micro- and macroalbuminuria defined by ACR and albumin concentration are likely to maximize agreement with 24-hour albumin excretion, we also calculated overall agreement with albumin excretion for 1) the American Diabetes Association cutoffs of 30299 mg/g for ACR, 2) the gender-specific ACR cutoffs of 17249 mg/g for men and 25354 mg/g for women that have been proposed by Warram et al. (20) and used by some investigators (16, 21, 22), and 3) the albumin concentration cutoffs of 16159 mg/liter suggested by Bakker (19).
Intraindividual variability in albumin excretion is typically assessed by means of the within-person CV (2631, 40), which is calculated from multiple measurements made in the same individual as 100 x standard deviation/mean (40). Hence, to estimate daily variability for each urinary measure, we calculated each individuals CV for the first collection and the repeat collection and then used the mean and median values among participants to summarize daily within-person variability in INTERMAP. We also computed the Spearman rank-order correlation between the first and repeat measurements for each urinary measure as a further assessment of within-person variation. These analyses were carried out for all 4,678 participants, for the 1,933 participants with albumin concentrations 1 mg/liter in both collections, for the 1,709 participants with an average albumin excretion less than 30 mg and an albumin concentration
1 mg/liter in both collections, and for the 204 participants with an average albumin excretion of 30299 mg.
To examine associations of albumin excretion, ACR, and albumin concentration with blood pressure, body mass index, and urinary volume as continuous variables, we calculated Spearmans rank-order correlations separately by country and gender and then computed weighted averages by gender and for men and women combined, with weights equal to sample size. Participants for whom either albumin concentration value was below the detection limit were excluded from these analyses.
We also examined associations of these same variables with microalbuminuria, defined on the basis of each of albumin excretion, ACR, and albumin concentration as described above, using logistic regression with adjustment for age, sex, and INTERMAP sample.
In all analyses except those related to assessment of intraindividual variability of urinary variables, we used the average of all measurements for each variable.
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RESULTS |
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Table 2 shows the associations of ACR and albumin concentration with 24-hour albumin excretion based on classification of participants as having micro- or macroalbuminuria. The INTERMAP ACR cutoffs show better agreement with albumin excretion than the INTERMAP albumin concentration cutoffs ( = 0.935 and
= 0.872, respectively). The kappa value of 0.908 for the gender-specific ACR cutoffs is only slightly smaller than the value of 0.935 for the INTERMAP ACR criteria. Similarly, the kappa value of 0.855 for the albumin concentration cutoffs suggested by Bakker (19) is only slightly smaller than the value of 0.872 for the INTERMAP albumin concentration criteria. The ACR cutoffs used by the American Diabetes Association yield a kappa value of 0.857 and a sensitivity of 78.9 percent, which is substantially lower than the sensitivities for the other two sets of ACR criteria (i.e., 92.6 percent and 95.1 percent). This is due primarily to the 30 mg/g cutoffs being substantially higher than the cutoffs for INTERMAP men in table 1 and the 17 mg/g for men proposed by Warram et al. (20).
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Mean and median CVs differ little by gender, except for persons with an average albumin excretion of 30299 mg. Mean and median CVs were also generally similar across the four countries (data not shown). Among persons with both albumin concentration values 1 mg/liter, the mean CV for albumin excretion ranged from 31 percent to 38 percent in men and from 29 percent to 41 percent in women.
Rank-order correlations for first and repeat measurements of the albumin measures are similar to those for several other urinary variables for all men and all women. For men with both albumin concentration values 1 mg/liter, the correlations for albumin excretion, ACR, and albumin concentration of 0.6950.700 are larger than the correlations for all men for urinary urea, volume, and sodium and similar to the correlations for potassium and sodium:potassium ratio. However, for women with both albumin concentration values
1 mg/liter, the correlations of 0.5160.533 for the three albumin measures are smaller than those for the other variables for all women. Correlations for the albumin measures are smallest among persons with an average albumin excretion of 30299 mg, ranging from 0.339 to 0.427 for men and being less than zero for women.
Associations of other variables with albumin excretion
Table 4 presents weighted averages of the rank-order correlations of albumin excretion, ACR, and albumin concentration with blood pressure, body mass index, and urinary volume for INTERMAP participants with albumin concentrations 1 mg/liter in both collections, as well as odds ratios for associations of these variables with microalbuminuria defined by albumin excretion, ACR, and albumin concentration. We included urinary volume here to assess whether multiplication of the concentration by the 24-hour volume to obtain albumin excretion resulted in stronger associations for volume and albumin excretion than for volume and ACR.
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Twenty-four-hour albumin excretion does not have a substantially larger correlation with 24-hour urinary volume than ACR in men or women. This suggests that multiplying albumin concentration by urinary volume does not markedly increase the correlation with albumin excretion relative to that observed for ACR, since ACR might be expected to be independent of volume unless volume is itself related to albumin excretion. The correlations of albumin concentration with volume are less than zero, which is not unexpected, since on average we would expect the concentration to be higher with lower total volume.
For associations with microalbuminuria, odds ratios are given for a difference of one standard deviation. Participants with macroalbuminuria defined by albumin excretion, ACR, or albumin concentration were excluded from these analyses (37, 41, and 39 participants, respectively). Odds ratios for systolic and diastolic blood pressure and body mass index are the same or nearly the same for all three definitions of microalbuminuria, while the odds ratio for urinary volume is slightly higher for albumin excretion than for ACR, and the odds ratio for albumin concentration and urinary volume does not differ significantly from 1.0. Logistic regression analyses with ACR microalbuminuria defined on the basis of Warram et al.s (20) criteria and albumin concentration microalbuminuria defined on the basis of Bakkers (19) criteria also gave very similar odds ratios: 1.90 and 1.92, respectively, for systolic pressure, 1.69 and 1.71 for diastolic pressure, 1.46 and 1.52 for body mass index, and 1.26 and 1.05 for urinary volume.
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DISCUSSION |
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The major weakness of ACR is that creatinine excretion is higher in men than in women, varies by ethnicity, and declines with age (5, 1925), leading to problems in defining appropriate ACR cutoffs for microalbuminuria. While the American Diabetes Association uses a single set of cutoffs to define microalbuminuria for ACR (13), some investigators have proposed gender-specific cutoffs to help alleviate this problem (5, 1925). The cost of using ACR is also higher than that for albumin concentration because of the added cost of taking creatinine measurements. The primary weakness of albumin concentration is that it is affected by urinary flow rate (19, 26) and is therefore expected to have greater within-person variability than ACR, which is assumed to be affected less by daily variation in creatinine excretion than albumin concentration is by urinary flow rate (19). The American Diabetes Association considers the variability in albumin concentration to be too high to permit its use in defining microalbuminuria (13). When the validities of albumin concentration and ACR as alternatives to albumin excretion have been compared (19, 23, 42, 4649), those studies that have compared results from a random specimen with 24-hour albumin excretion have generally found ACR to have no clear advantage over albumin concentration (23, 42, 46, 47), while studies that have compared results based on timed overnight collections (19, 48, 49) have favored ACR. However, the results of the latter studies may have been affected by diurnal variation in albumin excretion (5, 26, 5053), and some investigators consider such collections less sensitive than 24-hour collections (52).
For this report, we used measurements of albumin concentration, ACR, and albumin excretion from two 24-hour urine collections carried out among INTERMAP men and women aged 4059 years to further assess the relative strengths and weaknesses of ACR and albumin concentration as alternatives to 24-hour albumin excretion. We compared ACR and albumin concentration with regard to associations with albumin excretion, daily within-person variability, and associations with blood pressure and body mass index.
With respect to associations with albumin excretion, rank-order correlations of ACR with albumin excretion were larger for both men and women (0.949 and 0.942, respectively) than were correlations of albumin concentration with albumin excretion (0.881 and 0.816, respectively). When micro- and macroalbuminuria were defined by INTERMAP-specific gender/country cutoffs for both ACR and albumin concentration, ACR showed better agreement with albumin excretion than did albumin concentration ( = 0.935 vs.
= 0.872). The ACR also showed better agreement with albumin excretion than albumin concentration (i.e., 0.908) when micro- and macroalbuminuria were defined according to the gender-specific cutoffs of 17249 mg/g for men and 25354 mg/g for women that were used in several other studies (16, 2022). When the single albumin concentration cutoff of 16159 mg/liter suggested by Bakker (19) was used, the rate of agreement with albumin excretion was 0.855, which is similar to the rate of agreement of 0.857 obtained with the American Diabetes Association ACR cutoff of 30299 mg/g.
In characterizing the daily variability of albumin excretion in individuals, investigators generally use either the mean within-person CV or the median within-person CV, which is typically in the range of 4060 percent (2631). Results for INTERMAP participants were consistent with these findings: Mean CVs were almost 60 percent when values below the detection limit were included and 3440 percent when such values were excluded. While the differences in mean and median CVs among albumin excretion, ACR, and albumin concentration were not large, the smallest values were generally those for albumin concentration. The three urinary albumin measures also had substantially larger mean and median within-person CVs than other urinary variables in INTERMAP, including sodium, potassium, urea, creatinine, and volume. When values below the assay detection limit were excluded, rank-order correlations of first and repeat values for albumin excretion, ACR, and albumin concentration were generally similar and comparable to those of other urinary variables in men but smaller in women.
INTERMAP urine samples were frozen for approximately 3 years at 20°C before being analyzed. Several investigators have reported that long-term freezing at this temperature can affect estimates of concentration, with the impact generally being greater at higher concentrations (5458). In a study of persons with type II diabetes mellitus, median ACR decreased by 40 percent in urine samples frozen at 20°C for 2 years (56). While within-person CVs for albumin excretion in INTERMAP are consistent with those of other studies, it is likely that they are larger than they might have been if albumin had been measured in fresh urine samples, given the possibility that the loss of measured albumin was differential between the two collections for each person in INTERMAP.
While rank-order correlations of ACR and albumin concentration with systolic and diastolic blood pressure were similar to those for 24-hour albumin excretion, correlations of ACR with body mass index were smaller than those for albumin excretion and albumin concentration. This represents a weakness of ACR for epidemiologic studies that include measures of body size and obesity as key variables, since it raises questions as to whether or not ACR can be used to assess associations of albumin excretion with body mass index and other measures of body size and obesity, and whether investigators in studies that use ACR who wish to adjust for these variables can correctly do so.
Associations of blood pressure and body mass index with micro- and macroalbuminuria were essentially identical whether defined by albumin excretion, the INTERMAP gender/country-specific cutoffs for ACR or albumin concentration, the gender-specific ACR cutoffs proposed by Warram et al. (20), or the single albumin concentration cutoff of 16159 mg/liter suggested by Bakker (19). This almost certainly reflects the high rate of agreement among the various categorizations, as well as the fact that persons incorrectly classified as microalbuminuric on the basis of ACR or albumin concentration have blood pressure levels similar to those of persons incorrectly classified as normoalbuminuric or correctly classified as microalbuminuric.
Limitations of this study include the problem of measuring albumin excretion in urine samples that have been frozen for several years and the issue of whether the similar within-person CVs of ACR and albumin concentration from 24-hour urine samples and the similarity of associations with blood pressure can be expected for other types of urine collectionfor example, spot or random collections. The latter concern is lessened by findings that albumin concentration measured in a first morning sample is related to stroke risk (4) and cardiovascular disease risk factors and morbidity (18) and that it correlates as well as or better than ACR with 24-hour albumin excretion in studies measuring albumin concentration and ACR in a spot urine sample following completion of a 24-hour urine measurement (23, 42, 46, 47).
In summary, both albumin concentration and ACR appear to be reasonable alternatives to 24-hour albumin excretion on the basis of their relative strengths and weaknesses and the results presented here (see table 5). However, there are circumstances in which each measure would be favored. In particular, albumin concentration is the better alternative to 24-hour albumin excretion for epidemiologic studies in which measures of body size and obesity are key variables or the cost of creatinine measurement is an important consideration. When these concerns do not apply, ACR appears to be the better alternative to albumin excretion, since it has stronger associations with albumin excretion than albumin concentration. If the studys focus is on microalbuminuria, gender-specific cutoffs such as those proposed by Warram et al. (20) may perform almost as well as those defined specifically for the population under study. However, as Houlihan et al. (24) noted, even gender-specific cutoffs may not be suitable for studies with a broad age range, such as 4080 years.
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ACKNOWLEDGMENTS |
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The INTERMAP Study was accomplished through the fine work of staff at the local, national, and international centers. A listing of many of these colleagues is given in the paper by Stamler et al. (34).
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NOTES |
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REFERENCES |
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