Background The risk of microalbuminuria in patients with insulin-dependentdiabetes mellitus (IDDM) is thought to depend on the degreeof hyperglycemia, but the relation between the degree of hyperglycemiaand urinary albumin excretion has not been defined.
Methods We measured urinary albumin excretion in three randomurine samples obtained at least one month apart from 1613 patientswith IDDM. Microalbuminuria or overt albuminuria was consideredto be present if the ratio of albumin (in micrograms) to creatinine(in milligrams) was 17 to 299 or >300, respectively, formen and 25 to 299 or >300, respectively, for women. Measurementsof glycosylated hemoglobin (hemoglobin A1) obtained up to fouryears before the urine testing were used as an index of hyperglycemia.Twelve percent of the patients had overt albuminuria and wereexcluded from subsequent analyses.
Results The prevalence of microalbuminuria was 18 percent inpatients with IDDM. It increased with increasing postpubertalduration of diabetes and, within each six-year interval of diseaseduration, it increased nonlinearly with the hemoglobin A1 value.For hemoglobin A1 values below 10.1 percent, the slope of therelation was almost flat, whereas for values above 10.1 percent,the prevalence of microalbuminuria rose steeply (P<0.001).For example, as the hemoglobin A1 value increased from 8.1 to10.1 percent, the odds of microalbuminuria increased by a factorof 1.3, but as the value increased from 10.1 to 12.1 percent,the odds were increased by a factor of 2.4.
Conclusions The risk of microalbuminuria in patients with IDDMincreases abruptly above a hemoglobin A1 value of 10.1 percent(equivalent to a hemoglobin A1c value of 8.1 percent), suggestingthat efforts to reduce the frequency of diabetic nephropathyshould be focused on reducing hemoglobin A1 values that areabove this threshold.
Diabetic nephropathy is the chief cause of morbidity and prematuremortality in patients with insulin-dependent diabetes mellitus(IDDM).1,2 This complication is first manifested as an increasein urinary albumin excretion (microalbuminuria), which progressesto overt albuminuria and then to renal failure.3 Improved glycemiccontrol seems to delay or prevent the onset of microalbuminuria,but the development of cost-effective preventive strategiesrequires knowledge of the increase in the risk of nephropathyassociated with each increase in the degree of hyperglycemia.4,5,6The recently published results of the Diabetes Control and ComplicationsTrial (DCCT) showed that intensive treatment of diabetes reducesthe risk of diabetic complications, including microalbuminuria,but the relation between the degree of hyperglycemia and therisk of microalbuminuria was not examined.7 In this study, weexamined the relation between the degree and duration of hyperglycemiaand the prevalence of microalbuminuria in a large cohort ofpatients with IDDM.
Methods
Characteristics and Evaluation of the Study Subjects
Between January 1, 1991, and March 31, 1992, every other diabeticpatient between the ages of 15 and 44 years who visited theinternal medicine or pediatrics clinic of the Joslin DiabetesCenter was screened for microalbuminuria. Most of the patientswere referred to the center soon after the diagnosis of diabetesand had received most of their care at the center since thattime. Before this study, no systematic screening for microalbuminuriahad been conducted at the center. Patients who visited the pregnancyclinic or who had given birth within the preceding six weekswere excluded. Additional eligibility requirements includedan onset of diabetes before the age of 41 years, Massachusettsresidency, and registration at the center before 1991. The screeningprotocol was approved by the committee on human subjects atthe center.
By March 31, 1992, a total of 1795 patients had been screenedat least once for microalbuminuria. The urine samples were collectedrandomly at the time of the clinic visit, with no advance instructionsconcerning fluid intake or urination. The laboratory continuedto save urine samples from these patients whenever they returnedto the clinic. This analysis is based on results available byDecember 31, 1993.
Urine samples with abnormal sediments on routine urinalysiswere discarded. All others were assayed for albumin within sevendays after collection (samples not analyzed immediately wererefrigerated) with the use of reagent strips (Multistix; AmesDivision, Miles Laboratories, Elkhart, Ind.), which were readby an optical scanner. If the reading was strongly positive(>2+; albumin concentration, >1000 µg per milliliter),the patient was considered to have overt albuminuria and thesample was not analyzed further. In the remaining samples, theurinary albumin concentration was measured by immunonephelometrywith N Albumin kits (Behring, Somerville, N.J.) normally usedto measure serum albumin and a manufacturer-supplied protocolspecifically designed to detect the low concentrations of albuminin urine.8 The intraassay and interassay coefficients of variationwere less than 2 percent and less than 4 percent, respectively.Urinary creatinine concentrations were measured by colorimetry(modified Jaffé reaction) on an Astra-7 automated system(Beckman Instruments, Brea, Calif.).
For male patients normoalbuminuria was defined as a ratio ofalbumin (measured in micrograms) to creatinine (measured inmilligrams) of less than 17 and for female patients as a ratioof less than 25. These sex-specific values are equivalent toa urinary albumin excretion rate of 30 µg per minute (unpublisheddata). A ratio of albumin to creatinine of 300 or higher, regardlessof sex, was considered to indicate overt albuminuria. Microalbuminuriawas defined as a ratio of albumin to creatinine in the intermediaterange: 17 to 299 for male patients and 25 to 299 for femalepatients. The results of one or two subsequent measurementsin 80 percent of the patients one or more months later (median,five) were very similar to the initial results (Spearman correlation,0.81), confirming that the majority of patients were properlycategorized according to urinary albumin excretion in the firstsample.
If the results of a second measurement placed the patient ina different category from that based on the first measurement,a third urine sample was obtained to confirm either the firstor second measurement. If the results of all three measurementswere different, the patient was considered to have microalbuminuria.If a third urine sample was not obtained (as was true for 5percent of the group), the assignment was based on the geometricmean of the first two measurements. If the results of only onemeasurement were available (15 percent of the entire group),they were used to make the classification. The 20 percent ofpatients classified on the basis of one assay or two discordantassays did not differ from the rest with respect to age, sex,duration of diabetes, glycosylated hemoglobin (hemoglobin A1)value, or urinary albumin excretion. The 24 patients who hadreceived renal transplants were classified as having overt albuminuriaregardless of assay results.
We extracted from the records of the clinical laboratory allinformation on measurements of hemoglobin A1 in these patientsduring two periods: the 12 months of 1988 and the 24 monthsof 1990 and 1991. The degree of hyperglycemia in each patientwas determined by calculating the geometric mean of all thehemoglobin A1 values obtained in 1988 and the geometric meanof three values (separated by at least a month) obtained in1990 and 1991 (values for 1989 could not be readily abstractedfrom the patients' records). Hemoglobin A1 (normal range, 5.4to 7.4 percent)was measured electrophoretically (Corning Medicaland Scientific, Corning, N.Y.).9 To facilitate comparisons withthe results of the DCCT, 52 blood samples were analyzed in thestandard manner by our laboratory and according to the Diamathigh-performance liquid chromatographic method (Bio-Rad Laboratories,Hercules, Calif.10) by the Department of Pathology, Universityof Missouri School of Medicine, Columbia. The latter assay wascalibrated to match the reference system used by the centralhemoglobin A1c laboratory of the DCCT.11 Hemoglobin A1c, a componentof hemoglobin A1, is the most chemically specific glycated hemoglobinthat can be measured.10 The correlation between the two setsof results was 0.98, and linear regression analysis was usedto determine a conversion formula that would yield hemoglobinA1c values that corresponded approximately to the hemoglobinA1 values obtained in our laboratory (hemoglobin A1c = [hemoglobinA1 - 0.14]/1.23).
The date of diagnosis of diabetes was abstracted from the medicalrecords or obtained from the patients. All cases of diabetesdiagnosed in patients less than 21 years old were classifiedas insulin-dependent. If diabetes was diagnosed between theages of 21 and 40 years, the medical record was reviewed bya physician to classify the type of diabetes. Patients who beganinsulin therapy within two years after the diagnosis of diabetesand continued to receive it were considered to have IDDM. Anage of less than 41 years at diagnosis and a need for continuoustreatment with insulin were found to be accurate criteria foran operational definition of IDDM in other studies.12,13
Of 1795 patients screened, 177 (10 percent) had non-insulin-dependentdiabetes and were excluded from the analysis. Another five patientswere excluded because they had nondiabetic renal disease predatingthe diagnosis of diabetes. Ninety-two percent of the remaining1613 patients reported their racial or ethnic origin as non-Hispanicwhite. The remaining 8 percent of the group was composed ofnearly equal proportions of blacks, Hispanics, and Asians; allpatients in these groups were considered together for this analysis.
Age, the duration of diabetes, and the duration of follow-upat the Joslin Diabetes Center were calculated as of the datethe second urine sample was obtained. For patients given a diagnosisof diabetes before the age of 10 years, the duration of thedisease after puberty was calculated beginning after the 10thbirthday, since the years of diabetes before puberty do notcontribute to the development of diabetic nephropathy.14,15
Statistical Analysis
Analysis of variance and analysis of covariance were used tocompare the subgroups of patients with different degrees ofalbuminuria; Tukey's studentized range test was used to assessstatistical significance. The relations between the prevalenceof microalbuminuria and the hemoglobin A1 value and the postpubertalduration of diabetes were evaluated by calculations of oddsratios.16 The statistical significance of these associations,after adjustment for covariates, was evaluated with logistic-regressionmodels in which hemoglobin A1 was treated as a grouped variableor expressed as a logarithm.17,18
To evaluate the relation between the hemoglobin A1 level andthe risk of microalbuminuria, three alternative models wereexamined: a simple exponential,18 a threshold,19 and a changepoint20model.
Results
Among the 1613 patients (duration of IDDM, 1 to 39 years) whowere screened for microalbuminuria during the 36-month studyperiod, 295 (18 percent) had microalbuminuria and 201 (12 percent)had overt albuminuria. The characteristics of the patients aresummarized in Table 1 according to the degree of albuminuria.Both the patients with microalbuminuria and those with overtalbuminuria had had diabetes longer and had higher hemoglobinA1 values in both 1988 and 1990 to 1991 than the patients withnormoalbuminuria. The hemoglobin A1 values in the group withmicroalbuminuria and the group with overt albuminuria were similarduring both periods. The correlation between the hemoglobinA1 values in 1988 and those in 1990 to 1991 was high in thepatients with normoalbuminuria (r = 0.60) and in the patientswith microalbuminuria (r = 0.60), but not in those with overtalbuminuria (r = 0.34), indicating considerable stability inthe degree of hyperglycemia in the first two groups. By contrast,the low correlation in the patients with overt albuminuria suggestsgreater variation in the degree of hyperglycemia, as a resultof intensified efforts to control glycemia prompted by the clinicaldiagnosis or deteriorating renal function. For this reason,the patients with overt albuminuria were excluded from furtheranalyses.
Table 1. Clinical Characteristics of Patients with IDDM, According to the Degree of Albuminuria.
To examine the association between microalbuminuria and thedegree of hyperglycemia, the patients with normoalbuminuriaand microalbuminuria were subdivided into quintiles based onthe distribution of the hemoglobin A1 values in 1990 to 1991(5.9 to 8.8 percent, 8.9 to 9.8 percent, 9.9 to 10.7 percent,10.8 to 11.9 percent, and 12.0 to 21.3 percent). The prevalenceof microalbuminuria was 11.7 percent in the lowest quintileof hemoglobin A1 and increased progressively with each quintileto 15.3 percent, 17.4 percent, 24.5 percent, and 36.1 percent.To control for the effect of the duration of diabetes, the prevalenceof microalbuminuria in each of these quintiles was examinedaccording to the postpubertal duration of diabetes (Table 2).Among patients who had had diabetes six years or less, the prevalenceof microalbuminuria in the lowest quintile of hemoglobin A1was 3.8 percent. This prevalence, which is much higher thanthe prevalence of 0.9 percent reported in normal subjects (unpublisheddata), was used as the reference point for calculating the oddsratios for other combinations of hemoglobin A1 values and durationsof diabetes. The pattern of results was similar when the 1988and 1990 to 1991 hemoglobin A1 values were combined (data notshown).
Table 2. Odds Ratios for the Effect of Variations in Hemoglobin A1 Values on the Development of Microalbuminuria, According to the Postpubertal Duration of IDDM.
The risk of microalbuminuria increased with the duration ofIDDM (Table 2) except among patients who had had diabetes for25 to 32 years. In a multiple logistic-regression model withadjustment for hemoglobin A1 value, sex, and age at the timeof diagnosis, each successive 6-year interval of disease durationthrough the 19-to-24-year interval was associated with an increasein risk of approximately 80 percent (P<0.001). The absenceof any additional increase in risk with a longer duration ofdisease may be due to the exhaustion of the pool of susceptiblepatients after 24 years.21
The risk of microalbuminuria also increased with the level ofhemoglobin A1 (Table 2). The risk rose moderately between thefirst and fourth quintiles and then steeply in the fifth quintile.This pattern was almost identical in each category of diseaseduration except for that spanning 25 to 32 years. The incrementin risk between the first and fifth quintiles of hemoglobinA1 was estimated in a multiple logistic-regression model thatadjusted for the duration of diabetes, sex, and age at diagnosis.As compared with the first quintile, the risk rose 34 percent(P = 0.29) in the second quintile, 33 percent (P = 0.31) inthe third quintile, 101 percent (P = 0.009) in the fourth quintile,and 412 percent (P<0.001) in the fifth quintile. HemoglobinA1 was then modeled as a continuous variable, and the fifthquintile was treated as an outlier. The relative risk increasedby a factor of 1.25 with each increase of 1 percentage pointon the hemoglobin A1 scale (P<0.001), but in the fifth quintile,the relative risk was higher by a factor of 1.67 (P = 0.04)than that predicted by the regression line.
We examined the evidence of a nonlinear relation between therisk of microalbuminuria and the hemoglobin A1 value more closelyby grouping the hemoglobin A1 values in small intervals of equalwidth (0.45 percent) (Figure 1). The intervals in the tailsof the distribution were combined as necessary to maintain thesample size. The hemoglobin A1 groups were modeled with indicatorvariables in a logistic-regression model of the prevalence ofmicroalbuminuria that included covariates to adjust for ageat onset of diabetes, the duration of diabetes, and sex. Asbefore, patients who had had diabetes for at least 25 yearswere excluded. The reference group for the adjusted relativeodds was the group of patients with the lowest hemoglobin A1values (range, 5.9 to 7.9 percent; mean, 7.3 percent). To finda suitable model for the relation between the risk of microalbuminuriaand the degree of hyperglycemia, we tested three alternativelogistic-regression models: a simple exponential,18 a threshold,19and a changepoint20 model. The results were similar. The simpleexponential model confirmed the earlier indication of nonlinearitybecause the resulting curve bent sharply upward, in a mannerconsistent with the occurrence of a threshold. The results ofthe threshold test were significant (P = 0.03), with an estimatedthreshold value for hemoglobin A1 of 9.9 percent. The changepointmodel estimated the threshold at 10.1 percent and allowed anon-zero slope for the line below the threshold a modelthat seemed to represent the data most closely (Figure 1).
Figure 1. Relation between Mean Hemoglobin A1 Values and the Risk of Microalbuminuria in Patients with IDDM.
Hemoglobin A1 values were grouped into small intervals of equal width (0.45 percent, or 0.90 percent in the tails) and modeled with indicator variables in a logistic-regression model of the prevalence of microalbuminuria with covariates to adjust for the age at onset of diabetes, the duration of diabetes, and sex. The reference group for the adjusted relative odds (circles) was the group of patients with hemoglobin A1 values ranging from 5.9 to 7.9 percent. A changepoint model, including the same covariates, was fitted to the logarithm of hemoglobin A1 as a continuous variable to estimate the location of the changepoint and the regression slopes below and above the changepoint (continuous line). The horizontal axis shows hemoglobin A1 values as well as the equivalent values of hemoglobin A1c (see the Methods section) and the blood glucose profile.11 In the DCCT, the blood glucose profile was determined by measuring the capillary-blood glucose concentration seven times in a 24-hour period (before and 90 minutes after each of the three major meals and before bedtime). The results of quarterly determinations of the blood glucose profile over a one-year period were averaged, and the line of regression was determined on the basis of the average of quarterly determinations of hemoglobin A1c during the same year. To convert values for blood glucose to millimoles per liter, multiply by 0.05551.
For hemoglobin A1 values below 10.1 percent, the slope of therelation was almost flat (P = 0.25), whereas for values above10.1 percent the prevalence of microalbuminuria rose steeply(P<0.001). For example, as the hemoglobin A1 value increasedfrom 8.1 to 10.1 percent, the odds of microalbuminuria wereincreased by a factor of 1.3 (95 percent confidence interval,0.8 to 2.0), but as the value increased from 10.1 to 12.1 percent,the odds were increased by a factor of 2.4 (95 percent confidenceinterval, 1.9 to 3.0).
Discussion
We found that the risk of microalbuminuria in patients withIDDM is strongly related to both the duration of diabetes andthe degree of hyperglycemia, measured as the prevailing levelof hemoglobin A1 during the preceding two to four years. Inpatients who had had diabetes for less than 25 years and whosehemoglobin A1 values were below 10.1 percent, the risk of persistentmicroalbuminuria varied little, although it was higher thanin normal subjects. In contrast, in patients with hemoglobinA1 values above 10.1 percent, the risk of microalbuminuria rosesteeply. This nonlinear pattern for the relation between hemoglobinA1 values and the risk of microalbuminuria was independent ofthe effect of the duration of diabetes; the latter was an independentrisk factor for which no threshold effect could be found.
Our results are consistent with those of the DCCT.7 In boththe primary- and secondary-prevention components of that trial,the patients who received intensive treatment had a statisticallysignificant reduction in the cumulative incidence of microalbuminuriaas compared with the patients who received conventional treatment.The lower risk of microalbuminuria in the former group was attributedto improved glycemic control, but the relation was not furtherdefined. A relation between elevated hemoglobin A1 values andan increased risk of microalbuminuria has been reported previously,but none of the studies had a sample size sufficient to examinethe relation more closely.22,23,24,25 Small sample size hasnot been the only obstacle to the detection of a threshold inthe relation. An emphasis on models that smooth the data hasbeen a factor,7,26 and there are limitations inherent in thestatistical procedures used for this purpose.19,27 The findingin the DCCT of a high risk of hypoglycemia associated with intensivetreatment prompted our scrutiny of the evidence of a thresholdin the relation between the hemoglobin A1 value and the riskof microalbuminuria. There seems to be a similar threshold inthe relation between the degree of hyperglycemia and the developmentof diabetic retinopathy.26
The distinctly different risks of microalbuminuria in patientswith low hemoglobin A1 values and those with high values suggestthat diabetes damages the kidney through several mechanisms.The mechanisms operating below the threshold hemoglobin A1 valueof less than 10.1 percent (which corresponds to prevailing bloodglucose concentrations of less than 200 mg per deciliter [11.1mmol per liter]) seem to be independent of the level of hyperglycemiaand may be influenced by other components of the diabetic milieu for example, abnormalities in plasma insulin concentrations.28,29At high hemoglobin A1 values, which are indicative of high bloodglucose concentrations, microalbuminuria is most likely causedby the deleterious effects of hyperglycemia on cell functionsand extracellular structures such as the basement membrane andmesangial matrix.30,31
Our data have several shortcomings. First, the measurementsof hemoglobin A1 determined before the screening for microalbuminuriacan only be considered as an approximation of the degree ofhyperglycemia during the interval in which microalbuminuriadeveloped. However, the resulting misclassification of the degreeof hyperglycemia most likely increased the random variationof hemoglobin A1 values, making the detection of a thresholdvalue for hemoglobin A1 more difficult.32 Second, since measurementsof hemoglobin A1 (and hemoglobin A1c) vary among laboratories,the threshold value reported here cannot be generalized, exceptto laboratories that have calibrated their values to a referencemethod such as that used in the DCCT.10,11 Third, our findingswere obtained in patients whose IDDM had been diagnosed beforethe age of 41 years. Whether there is a similar relation betweenthe degree of hyperglycemia and the development of microalbuminuriain older patients with IDDM and in patients with non-insulin-dependentdiabetes mellitus is not known.
In conclusion, our findings have implications for the care ofpatients with IDDM. Patients and care providers should givethe highest priority to improving glycemic control sufficientlyto maintain hemoglobin A1 values below 10.1 percent (equivalentto hemoglobin A1c values below 8.1 percent). If this can beachieved, the number of patients in whom microalbuminuria developsshould decline substantially, which should, in turn, lower thenumber in whom overt macroalbuminuria and end-stage renal diseasedevelop.
Supported by a grant (RO1-DK41526) from the National Institutesof Health.
We are indebted to Drs. A.R. Christlieb and K. Quickel for assistancein starting the study; to the clinic staff for help in implementingthe screening; to Dr. J. Robins for comments regarding the thresholdmodel; to Drs. R.R. Little and D.E. Goldstein for assistancein developing a conversion formula for expressing the valuefor hemoglobin A1 as an approximate hemoglobin A1c value; andto Mr. G. Gearin, Mr. F. Denri, Mr. S. Federman, Mr. M. Wantman,and Ms. W. Fisher of the Section on Epidemiology and Genetics,Joslin Diabetes Center, for making the study possible.
Source Information
From the Epidemiology and Genetics Section, Research Division, Joslin Diabetes Center (A.S.K., L.M.B.L., M.K., M.Q., J.H.W.); the Departments of Medicine (A.S.K.) and Pediatrics (L.M.B.L., M.Q.), Harvard Medical School; and the Department of Epidemiology, Harvard School of Public Health (A.S.K., J.H.W.) all in Boston.
Address reprint requests to Dr. Krolewski at the Epidemiology and Genetics Section, Joslin Diabetes Center, 1 Joslin Pl., Boston, MA 02215-5397.
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