Urinary albumin excretion rate and glomerular filtration rate in the prediction of diabetic nephropathy; a long-term follow-up study of childhood onset type-1 diabetic patients

Gisela Dahlquist1, Eva-Lena Stattin1 and Susanne Rudberg2,

1 Department of Pediatrics, Umeå University Hospital, Umeå and 2 Department of Woman and Child Health, Karolinska Institute, Stockholm, Sweden



   Abstract
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Background. Predictors of diabetic nephropathy are only partly known. The role of glomerular hyperfiltration is much discussed. We have studied the cumulative incidence of micro and macroalbuminuria and the predictive value of glomerular filtration rate (GFR) and screening value of albumin excretion rate (AER) in type-1 diabetes.

Methods. A cohort of diabetic children was followed up at a mean duration of 29±3 years. All 75 children treated in one hospital with diabetes duration >=8 years were prospectively followed for 8 years examining GFR, AER, blood pressure and HbA1c. After another 8–10 years, 60 of them were traced for endpoint follow-up.

Results. Seven patients (12%) developed macroalbuminuria, i.e. persistent overnight AER>200 mg/min, 12 (20%) developed persistent microalbuminuria (AER 15–200 mg/min) and 17 (28%) transient microalbuminuria (>15 mg/min on two consecutive occasions, normalized at endpoint). One baseline screening value of 24-h AER>15 mg/min predicted 93% of patients with persistent micro or macroalbuminuria. The negative predictive value was 78%. Six of seven macroalbuminuric and 10 of 12 microalbuminuric patients had a baseline GFR above the normal limit of the method (>=125 ml/min/1.73 m2). When adjusted for diabetes duration, increased GFR predicted macro or microalbuminuria (odds ratios=5.44, P=0.04). The positive predictive value for having an increased baseline GFR was 53%.The negative predictive value was 77%. Stratification for HbA1c did not change the effect of an increased GFR.

Conclusions. At a mean diabetes duration of 29 years the cumulative incidence of macroalbuminuria was 12%; however, another 20% had persistent microalbuminuria. A screening value of 24-h AER >15 mg/min was a strong predictor, whereas increased GFR was a weaker but significant predictor for micro and macroalbuminuria.

Keywords: albuminuria; cumulative incidence; glomerular filtration rate; type-1 diabetes



   Introduction
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Diabetic nephropathy is supposed to develop in about 30% of patients with type-1 diabetes [1] but the incidence may be declining in Sweden [2]. There is a marked decreased risk of renal disease in patients who survive with diabetes for 25 years or more [1]. During the past two decades young onset type-1 diabetic cases have been treated with intensive insulin therapy according to Swedish guidelines, which might have affected the prevalence of overt diabetes nephropathy.

Early diabetes is characterized, both clinically and experimentally, by an increase in glomerular filtration rate (GFR) and renal plasma flow (RPF) [3,4]. Experimental studies in diabetic animals have demonstrated that increased intraglomerular flow and pressure, associated with glomerular hyperfiltration, may damage the renal structure [5]. Studies in human type-1 diabetes have also indicated that glomerular hyperfiltration is a risk factor for nephropathy [68], although this remains controversial [9,10]. The mechanisms responsible for such a possible deleterious effect of early renal functional changes have not been fully evaluated. Glomerular hyperfiltration is promoted by hyperglycaemia, which is also a major risk factor for diabetic nephropathy per se [11]. In a previous study, we found that an increased GFR, independent of the degree of metabolic control, present in early type-1 diabetes predicted the development of incipient diabetic nephropathy 8 years later [12].

The aim of the present study was to determine in the same cohort of patients, the prevalence of diabetic nephropathy and the predictive value of a screening value of albumin excretion rate (AER) and GFR on diabetic nephropathy after another 8–10 years of follow-up, i.e. after a mean diabetes duration of 29 years.



   Subjects and methods
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Subjects
The study was approved by the local Ethics Committee. Between 1979 and 1981 all patients with type-1 diabetes attending St Göran Children's Hospital in Stockholm with a diabetes duration >8 year (n=75), were invited to take part in a prospective follow-up study. All 75 patients (37 males and 38 females) agreed to participate. GFR, AER, blood pressure and HbA1c were measured at baseline and every second year. Sixty-four patients of the original cohort were followed prospectively for 8 years up to a mean duration of diabetes of 20 years [12].

Sixteen to 18 years after the baseline examination, the original group of patients was traced and asked to participate in a final follow-up study. Sixty patients 34.5±2.8 (mean±SD) years old with a mean duration of 29.0 (range 25–35) years were traced and agreed to participate. Two patients from the original cohort had died due to reasons unrelated to diabetes (leukaemia and the Moya Moya disease), nine patients were not traced and four patients declined to participate. One patient had died in renal failure due to overt nephropathy and was, thus, included in the statistical analyses. At baseline, clinical characteristics did not differ between the 15 non-participants and the 60 participants (Table 1Go).


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Table 1. Description of all cases and different subgroups (defined in text)

 
Endpoint information 16–18 years after the start of the study was obtained from hospital medical records, regarding HbA1c, AER, blood pressure levels and the use of antihypertensive medication. In accordance with European Consensus Guidelines in Diabetes, the definition of persistent microalbuminuria was based on at least two positive consecutively collected timed overnight urine samples [13], or on earlier microalbuminuria normalized at endpoint due to antihypertensive treatment. Microalbuminuria was defined as an AER between 15–200 mg/min. We choose 15 mg/min as the lower limit as microalbuminuria was analysed in overnight urine samples that yield approximately 25% lower albumin excretion than 24-h urine [14]. Persistent macroalbuminuria was defined as AER>200 mg/min in at least two consecutive collections, or earlier macroalbuminuria reverted to micro or normoalbuminuria at endpoint due to antihypertensive treatment. Transient albuminuria was defined as AER>15 mg/min on two consecutive occasions during follow-up but not at endpoint, in patients without antihypertensive treatment.

Baseline methods
Initial GFR was estimated using a single inulin injection technique. Polfructosan (Inutest Laevosan Gesellschaft 25%; Linz, Austria) was injected at a dose corresponding to 12 ml/m2 body surface area and several blood samples were taken between 5 and 180 min after the injection. Polyfructosan in blood was determined by the anthron method. Clearance was calculated from a two-compartment analysis [15]. GFR was measured after ordinary insulin dose and breakfast between 08:00 and 12:00. All samples were fermented to reduce the possible interference by blood glucose with inulin values.

AER was analysed in 24-h urine collections by an RIA method (Phadebas®, Pharmacia, Uppsala, Sweden). In a previous study in children and adolescents, we found that the upper normal limit of AER was 18.5 mg/min in 24-h urine collection [16]. To increase the power of the present study we arbitrarily chose 15 mg/min as the screening value. At 8-years of follow up AER was analysed from both 24-h urine and at least two overnight urine samples.

Blood pressure was measured prior to the GFR examination after 15 min of rest in a supine position, using a standard mercury manometer with an appropriately sized cuff. Diastolic blood pressure was read at the disappearance of the Korotkoff sounds. All blood pressure measurements were performed by the same observer.

HbA1c was (initially) analysed by an isoelectric focusing technique (LKB Ampholine PAGplate, LKB, Stockholm, Sweden).

Endpoint methods
For measurement of AER all patients were asked to deliver, by mail, two timed overnight urine samples to the Department of Clinical Chemistry at Norrlands University Hospital in Umeå. AER was analysed by an immunoturbidimetric method [17] using an automated spectrophotometer (Hitachi 911). If only one urinary test was positive, the patient was asked for a third sample. Forty-three of the 60 patients delivered urinary samples. The rest of the patients data on AER was obtained from hospital files and estimated with either RIA or an immunoturbidimetric method. These methods are both very sensitive and have very small methodological variations. We have previously compared these two methods and found them to have an excellent correlation (r=0.91) [16], thus, making a systematic bias unlikely.

HbA1c was collected from medical records. Laboratory methods of HbA1c differ in different medical centres. However, the three laboratories used by the respective diabetic clinics collaborated in a national quality assessment programme. During the year of this study the mean coefficient of variation between the methods was only 3% (EQUALIS reference laboratory).

Data on blood pressure levels, antihypertensive medication and prevalence of retinopathy were collected from medical records. Thus, the definition on simplex or proliferative retinopathy was defined locally. All patients defined as having proliferative or pre-proliferative retinopathy had been treated with photocoagulation. Table 1Go shows that all characteristics, except for the AER were similar between groups.

Statistical methods
The Student's t-test for unmatched design or {chi}2 analysis was used for comparison between groups. Paired t-test was used for comparisons between GFR-values at baseline and at 8 years of follow-up. Crude odds ratios and stratifications and 95% confidence limits were calculated using the Mantel-Haenszel method. For comparisons of survival time (i.e. remaining normoalbuminuric at the end of the study) the log rank procedure was used. In all analyses patients with transient microalbuminuria were excluded. The positive and negative predictive values of AER and GFR were calculated. The positive predictive value here means the fraction of patients who initially had an increased AER or GFR screening value, and who had developed persistent micro- or macroalbuminuria at endpoint. The negative predictive value means the fraction of patients who initially had a normal AER or GFR value and did not develop micro or macroalbuminuria. Mean (95% confidence interval) is given if not otherwise stated.



   Results
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 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
Seven patients (12%) developed persistent macroalbuminuria. Six of them were treated with ACE inhibitors. There was no information on antihypertensive treatment in the records of the patient who died with macroalbuminuria. None of the macroalbuminuric patients, receiving antihypertensive treatment, reverted to micro or normoalbuminuria. Twelve patients (20%) developed persistent microalbuminuria. Nine of them were treated with antihypertensives (eight with ACE inhibitors and one with beta-blocker). These patients had three to five urinary samples with elevated AER, prior to antihypertensive treatment. Four of these became normoalbuminuric. Seventeen patients (28%) had had transient microalbuminuria during follow-up. None of them had antihypertensive treatment per definition.

Baseline values of sex, age at onset, blood pressure, blood glucose 90 min after breakfast and HbA1c were similar in patients developing macro or microalbuminuria as in those that remained normoalbuminuric. Baseline AER was significantly higher among patients developing micro and macroalbuminuria compared with the normoalbuminuric group (Table 1Go). Baseline mean GFR tended to be higher in patients with persistent macro or microalbuminuria at endpoint than in normoalbuminuric patients. GFR-values at baseline, 4 and 8 years of follow-up are presented for each group in Table 2Go. A significant decline in GFR was seen after 8 years in patients who had developed macroalbuminuria at endpoint.


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Table 2. GFR at baseline, after 4 and 8 years of follow-up in normo, micro, macro and transient microalbuminuric patients; GFR (ml/minx1.73 m2)

 
Prediction of incipient and overt diabetic nephropathy
When estimating a screening value of AER>15 mg/min as a predictor for persistent macro or microalbuminuria at a mean diabetes duration of 29 years the positive predictive value was 93% (13/14) and the negative predictive value was 78% (21/27). Two baseline AER values were missing (both from the group who remained normoalbuminuric). The odds to develop persistent micro or macroalbuminuria when having a screening value of 24-h AER>15 mg/min was 45.5 (95% confidence limits 7.82; 264.83), P<0.001. Patients with transient microalbuminuria were excluded from all calculations.

Six of the seven patients who developed persistent macroalbuminuria and 10 of the 12 with persistent microalbuminuria, had initial GFR exceeding the upper normal limit (>125 ml/minx1.73 m2 b.s). As 30 of the patients had initially increased GFR (according to this definition) the positive predictive value of screening value of GFR, exceeding the upper normal limit was 23% for macroalbuminuria and 53% (16/30) for micro or macroalbuminuria. The negative predictive value was 77% (10/13).

When looking at a GFR above 125 ml/min/1.73m2 as a risk factor for micro or macroalbuminuria the crude odds ratio (95% confidence limits) was 3.81 (0.90; 16.1), P=0.063. When stratifying for initial HbA1c dichotomized according to the mean value of normoalbuminuric patients (12.0%) the Mantel-Haenzsel rate ratio was similarly, 3.67 (0.90; 15), P=0.07. It was also necessary to stratify for duration of diabetes, which ranged between 25 and 35 years, and it was found that the odds ratio for having an elevated baseline GFR then increased to 5.44 (1.02; 28.6, P=0.04). There was no significant heterogeneity between strata for any of the stratification procedures. Figure 1Go shows a Kaplan–Meier plot of the percentage remaining normoalbuminuric and with baseline GFR above the normal range compared to those with a normal GFR (log rank test P=0.05).



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Fig. 1. Kaplan–Meier plot of percentage remaining normoalbuminuric, with baseline GFR above the normal range (—) and those with baseline GFR normal. Log rank test, P=0.05.

 



   Discussion
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 
This study attempts to assess the occurrence of macro and microalbuminuria in a cohort of hospital based young diabetic patients, after a follow-up time long enough to be close to endpoint assessment [1]. At a mean diabetes duration of 29 years, only seven (12%) developed persistent macroalbuminuria which is close to a previous report from Sweden [2]. Twelve patients (20%) were defined as having persistent microalbuminuria, most of them were treated with ACE inhibitors that might have masked or prohibited progress into macroalbuminuria. Interestingly, 17 (28%) had transient microalbuminuria, i.e. two consecutive samples of microalbuminuria which were normalized without antihypertensive treatment at endpoint follow-up. These findings concert with a previous study of ours, which reported that approximately 30% of type-1 diabetic adolescents exhibit transient microalbuminuria over shorter periods of time [18]. The renal fate of these patients is unknown and it is possible that under unfavourable conditions such as deteriorated metabolic control or increasing blood pressure, these patients may still progress to diabetic nephropathy. Thus, due to the unknown fate of these patients, they were excluded from our risk analysis. It is also interesting to note that 17 of 60 patients were diagnosed with proliferative or pre-proliferative retinopathy and received treatment.

In agreement with our previous report [12] half of the patients with initial glomerular hyperfiltration developed micro or macroalbuminuria. In terms of prediction in this rather limited group of patients, GFR was clearly a less strong predictor than only one screening value of AER>15 mg/min in 24-h urine 18 years before. It should be kept in mind though, that using an elevated screening value of AER increases the probability that it will be a strong predictor, as these patients already at baseline are closer to the endpoint chosen. The crude odds ratio for an increased GFR was only borderline statistically significant. This may partly be explained by the small sample size of this study, yielding a low power. However, when adjusting for diabetes duration, a clearly significant increase in risk was noted. When stratifying for the other obvious confounder, a poor metabolic control at baseline, the odds ratio was not significantly changed. The positive predictive value of an increased GFR was 53%. Thus, hyperfiltration is not a sufficient determinant for nephropathy but might—when accompanied by other metabolic or hemodynamic characteristics of diabetes—be a contributor of this complication. Our results agree with the retrospective studies by Mogensen et al. [6,7] as well as a 10-year follow-up study by Chiarelli et al. [8] showing a strong predictive value for a GFR >140 ml/min/173 m2 on the occurrence of microalbuminuria. Our clinical findings are also in accordance with the findings of Berg et al. [19], showing that in normoalbuminuric patients early diabetic structural changes in kidney biopsies were related to long standing hyperfiltration in young diabetic patients. The findings are also in accordance with our own results in microalbuminuric type-1 diabetic patients showing independent influence of long-term HbA1c, diabetes duration and preceding glomerular hyperfiltration on basement membrane thickness and overall glomerulopathy changes [20]. In contrast, the retrospective study by Lervang et al. [9] found no difference in the initial median GFR between patients with increased compared with normal AER at follow-up 18 years later. One explanation to the diverging results may be that in some of the patients in the latter study, initial data were collected at diagnosis of diabetes i.e. prior to insulin treatment. It is known that, at onset of type-1 diabetes, GFR may be substantially elevated before start of insulin treatment [3]. In a 10-year follow-up study by Yip et al. [10] glomerular hyperfiltration was not found to be a risk factor for diabetes nephropathy. However, our study populations are not comparable regarding age and diabetes duration and the follow-up period in our study is clearly much longer.

The reducing effect of antihypertensive treatment on elevated AER is well known [21]. Therefore, we believe it is justified to classify patients as having micro or macroalbuminuria although AER levels have been normalized due to antihypertensive treatment. In agreement with our previous report [12], the initial high mean HbA1c values had decreased over the years with a rather wide variability of HbA1c. This study had a limited power to detect a clear-cut association between initial HbA1c values and the development of micro or macroalbuminuria. Nor could we find a significant correlation between baseline GFR and baseline HbA1c. However we still adjusted in the analysis by stratification procedures for HbA1c when studying GFR as a predictor.

In conclusion, after a mean diabetes duration of 29 years in this hospital based cohort of Swedish childhood onset diabetic patients, only 12% developed persistent macroalbuminuria and 20% developed microalbuminuria. A screening value of 24-h AER >15 mg/min 18 years before was a strong predictor. Although the present study does not enable us to identify the pathogenic role of glomerular hyperfiltration, a high baseline GFR was also a significant risk factor for the development of macro or microalbuminuria when adjusting for duration.



   Acknowledgments
 
This study was supported by grants from the Swedish Medical Research Council (project number 07531), the Swedish Diabetes Association and the NOVO Nordic Foundation.



   Notes
 
Correspondence and offprint requests to: Susanne Rudberg, MD, PhD, Astrid Lindgren Children's Hospital/Karolinska Hospital, S-171 76 Stockholm, Sweden. Back



   References
 Top
 Abstract
 Introduction
 Subjects and methods
 Results
 Discussion
 References
 

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Received for publication: 17. 4.00
Accepted in revised form: 6. 2.01