Collecting urine for microalbumin assay

Frequency and determinants of microalbuminuria in mild hypertension: a primary-care-based study

OBJECTIVE

To evaluate the frequency of microalbuminuria and its relationship with several risk factors and left ventricular mass in a population of mildly hypertensive subjects attended in a primary-care setting.

DESIGN

Cross-sectional study.

SETTING

Eight primary-care centres.

PATIENTS

Two hundred and twenty-three non-diabetic patients recently diagnosed with mild hypertension were included in the study. None of them had clinical evidence of target-organ damage or had received prior antihypertensive treatment.

INTERVENTIONS

Subjects included in the study underwent clinical interview, measurement of blood pressure (BP) on three visits, blood analysis, measurement of albumin by immunonephelometry in three overnight urine collections, 24 h BP monitoring and M-mode and Doppler echocardiography.

MAIN OUTCOME MEASURES

Tobacco habit, clinic BP, body mass index, serum lipids and uric acid, glycaemia, urinary albumin excretion (UAE), ambulatory BP and left ventricular mass index.

RESULTS

The frequency of microalbuminuria was 7.2%. Microalbuminuric patients were more likely to be men and to be characterized by higher ambulatory BP, body mass index and uric acid levels. Regression analysis demonstrated that male sex and 24 h systolic BP were determinants of UAE. Patients with white-coat hypertension showed lower UAE than did subjects with sustained hypertension. Although a certain relationship between UAE and left ventricular mass index was found, these variables were not significantly correlated.

CONCLUSIONS

A low proportion of mildly hypertensive patients attended in a primary care setting are microalbuminuric. In this population, UAE is an expression of BP values over 24 h and correlates with several risk factors.

Near-normal urinary albumin concentrations predict progression to diabetic nephropathy in Type 1 diabetes mellitus

AIMS

To determine urinary albumin concentrations that predict progression to diabetic nephropathy and sight-threatening diabetic retinopathy and identify baseline parameters associated with progression.

METHODS

One thousand two hundred and one Type 1 diabetic patients aged 35 years or younger at diagnosis attending six hospital diabetes clinics in Scotland and included on the Royal College of Physicians of Edinburgh Diabetes Register were followed for a median (interquartile range) of 4.0 (2.5-5.5) years. Diabetic nephropathy was defined as the geometric mean of two consecutive urinary albumin values > 200 mg/l or a single value > 1,000 mg/l. Retinopathy was defined as clinician-determined maculopathy or proliferative retinopathy.

RESULTS

Forty-six patients developed nephropathy (4%) and 98 retinopathy (8%). Cox proportional hazards analyses demonstrated that a baseline urinary albumin concentration above 7.4 mg/l, longer duration of diabetes and higher HbA1c levels predicted the development of nephropathy. Higher baseline urinary albumin concentrations were the most powerful predictor for the development of nephropathy. Longer duration of diabetes, baseline blood pressure > 140/90 mmHg and higher HbA1c levels all predicted the development of sight-threatening retinopathy whereas baseline urinary albumin concentration did not.

CONCLUSIONS

Elevation of urinary albumin concentration just above the normal range is associated with an increased risk of developing diabetic nephropathy. Identifying patients with any abnormalities of urinary albumin excretion will provide a clear rationale for early therapeutic interventions.

Comparison of vasodilator effects of substance P in human forearm vessels of normoalbuminuric Type 1 diabetic and non-diabetic subjects

AIMS

To compare the vasodilatory responses to substance P in human forearm vessels in Type 1 normoalbuminuric diabetic and non-diabetic subjects.

METHODS

Forearm blood flow (FBF) was measured using a plethysmography technique in 12 normoalbuminuric Type 1 diabetic subjects (six males, six females) (HbA(1c) 8.2 +/- 0.3% (mean +/- SEM)) and 12 non-diabetic healthy control subjects in response to the infusion of the vasodilators substance P (SP), acetylcholine (ACh) and nitroprusside.

RESULTS

There was no significant difference in baseline FBF between the two groups (2.80 +/- 0.29 ml/min per 100 ml forearm tissue (diabetic group) vs. 2.85 +/- 0.37 ml/min per 100 ml (non-diabetic group), P = 0.45). Infusion of SP was associated with an incremental increase in FBF in the diabetic (0.6, 2 and 6 ng/min - 6.08 +/- 1.07, 7.82 +/- 1.08 and 9.48 +/- 1.14 ml/min per 100 ml, respectively) and the non-diabetic group (0.6, 2 and 6 ng/min - 5.41 +/- 0.80, 6.93 +/- 0.96 and 9.25 +/- 1.11 ml/min per 100 ml, respectively). Similarly, an incremental rise in FBF was observed during infusion of ACh (diabetic group: 7.5, 15 and 30 microg/min - 7.14 +/- 1.22, 8.91 +/- 1.40 and 11.67 +/- 1.93 ml/min per 100 ml, respectively; non-diabetic group: 7.5, 15 and 30 microg/min - 5.87 +/- 0.81, 7.49 +/- 0.96 and 10.74 +/- 1.29 ml/min per 100 ml, respectively). When FBF was expressed as percentage change from baseline, there was no significant difference in vasodilatory responses between the two groups for SP (0.6 ng/min, P = 0.21; 2 ng/min, P = 0.19; 6 ng/min, P = 0.19) or ACh (7.5 microg/min, P = 0.20; 15 microg/min, P = 0.20; 30 microg/min, P = 0.35).

CONCLUSIONS

This study suggests that endothelium-dependent vasodilatory responses to SP (and ACh) are not impaired in Type 1 diabetic subjects with normal urinary albumin excretion.

Microalbuminuria in essential hypertension: significance, pathophysiology, and therapeutic implications

Some patients with essential hypertension manifest greater than normal urinary albumin excretion (UAE). The significance of this association, which is the object of this review, is not well established. Hypertensive patients with microalbuminuria manifest greater levels of blood pressure, particularly at night, and higher serum levels of cholesterol, triglycerides, and uric acid than patients with normal UAE. Levels of high-density lipoprotein cholesterol, on the other hand, were lower in patients with microalbuminuria than in those with normal UAE. Patients with microalbuminuria manifested greater incidence of insulin resistance and thicker carotid arteries than patients with normal UAE. After a follow-up of 7 years, we observed that 12 cardiovascular events occurred among 54 (21.3%) patients with microalbuminuria and only two such events among 87 patients with normal UAE (P < 0.0002). Stepwise logistic regression analysis showed that UAE, cholesterol level, and diastolic blood pressure were independent predictors of the cardiovascular outcome. Rate of creatinine clearance from patients with microalbuminuria decreased more than that from those with normal UAE. In conclusion, these studies suggest that hypertensive individuals with microalbuminuria manifest a variety of biochemical and hormonal derangements with pathogenic potential, which results in hypertensive patients having a greater incidence of cardiovascular events and a greater decline in renal function than patients with normal UAE.

Endothelial dysfunction in Type 2 diabetic subjects with and without microalbuminuria

AIMS

The primary aim of this study was to determine whether microalbuminuria is associated with endothelial dysfunction in Type 1 diabetes mellitus. The secondary aim was to determine whether any reported biochemical markers of cardiovascular risk are associated with endothelial dysfunction in this group.

METHODS

Measurements were made of the vasodilatory responses of the brachial artery to post-ischaemic hyperaemia and to sublingual glyceryl trinitrate (GTN) (causing endothelium-dependent and endothelium-independent dilation, respectively) using a high-resolution ultrasound technique in 18 Type 1 diabetic patients with microalbuminuria, 18 age and sex-matched normoalbuminuric Type 1 diabetic patients and 18 non-diabetic control subjects.

RESULTS

There was a significant reduction in flow-mediated dilation (FMD) in microalbuminuric and normoalbuminuric diabetic patients compared with control subjects (2.4% (95% confidence interval (CI) 1.0-3.8%) and 2.3% (95% CI 0.7-3.9%) respectively vs. 6.3% (95% CI 5.1-7.5%), P<0.0001) but no difference in GTN-mediated dilation (14.7% (95% CI 10.7-18.7%) and 15.2% (95% CI 11.2-19.2%) vs. 18.7% (95% CI 16.1-21.3%), P = 0.09). There was no significant difference in FMD, however, between the microalbuminuric group and normoalbuminuric group (P=0.45). FMD was not significantly associated with urinary albumin-creatinine ratio, glycosylated haemoglobin, plasma glucose, lipid or lipoprotein concentrations in diabetic patients. There was a positive correlation between active transforming growth factor (TGF)-beta concentration, a novel biochemical marker of macrovascular disease, and FMD in diabetic patients (r=0.36, P<0.05). GTN-mediated dilation was positively associated with HDL-cholesterol concentration (r = 0.49, P = 0.002) but not with other biochemical variables (including active TGF-beta concentration). Active TGF-beta concentration was not associated with degree of microalbuminuria or other biochemical parameters.

CONCLUSIONS

These data suggest that endothelial dysfunction occurs in Type 1 diabetic patients regardless of urine albumin status. Endothelial dysfunction appears therefore to predate the development of microalbuminuria as a marker for the development of coronary artery disease. It is also concluded that low plasma levels of active TGF-beta are associated with an impaired endothelial response and this may provide a useful tool for identifying Type 1 diabetic patients at a greater risk of coronary artery disease.

Consistency of microvascular and autonomic abnormalities in diabetes

The progression of early measures of microvascular disease and autonomic neuropathy were studied in a group of 81 children with insulin dependent diabetes mellitus over a mean interval of 4.2 years. Repeated measurements were made of blood pressure, albumin excretion, joint mobility, and pupillary dilatation in darkness. Over the years between the first and the second study, systolic and diastolic blood pressure showed positive tracking correlations (r = 0.38 and r = 0.32) with a small but significant deviation from normality; albumin/creatinine ratio was significantly increased (0.79 v 0.55); a greater number of children were identified in the second study as having limitation of mobility of the fifth metacarpophalangeal joint; and pupillary dilatation in darkness significantly decreased (61.5% v 62.9%); 62% of the children with one or more abnormal measurements in the first study were found to have measurements outside the normal ranges in the second study, indicating a consistency in observations over time. It remains to be seen with what accuracy these measurements predict adult onset clinical disease.

Prevalence of microalbuminuria, lipoprotein (a) and coronary artery disease in the lipid clinic

AIMS

To assess the prevalence of microalbuminuria (MA) and elevated serum lipoprotein (a) (Lp (a)) concentration, and their association with coronary artery disease (CAD) and other conventional cardiovascular risk factors in non-diabetic patients attending a lipid clinic.

METHODS

Clinical details were obtained from 96 consecutive non-diabetic patients from whom a fasting blood sample was taken to measure serum lipid, lipoprotein, apolipoprotein and plasma glucose, urea, and electrolyte concentrations. The urine albumin/creatinine ratio (Ua/Uc) was estimated from a random clinic sample.

RESULTS

Of the patients, 26% had MA (defined as a Ua/Uc > 2.2 mg/mumol), 38% had an elevated Lp (a) concentration (defined as > 0.4 g/l), 36% were hypertensive (blood pressure > 160/95) or were taking antihypertensive medication, and 25% had established CAD defined on clinical criteria. In men the Ua/Uc ratio was highly associated with age, plasma low density lipoprotein cholesterol, and triglyceride concentrations. In women there was no association between the Ua/Uc ratio and variables examined. Lp (a) concentration was not associated with variables examined in either sex. In multiple logistic regression analysis adjusted for age and sex, serum Lp (a) concentration, diastolic blood pressure and treatment of hyperlipidaemia were highly associated with CAD. MA was not, however, associated with CAD.

CONCLUSIONS

MA is common in a lipid clinic and is more likely to be found among older male patients with hyperlipidaemia. However, in contrast with Lp (a) concentrations, MA is not a risk factor for CAD in this high risk population. Lp (a) concentration may be a useful tool in the lipid clinic, but there does not seem to be a justification for measuring the Ua/Uc ratio, at least in non-diabetic subjects.

Microalbuminuria in 411 untreated individuals with established hypertension, white coat hypertension, and normotension

We compared urinary albumin excretion in normotensive subjects and patients with white coat and established hypertension. The study involved prospective comparison of office blood pressure, daytime ambulatory blood pressure, and urinary albumin excretion in consecutive patients (n = 284) who were selected from general practice with newly diagnosed mild to moderate hypertension before the institution of pharmacologic antihypertensive therapy. All patients had a diastolic office blood pressure above 90 mm Hg; 173 had a consistently elevated diastolic blood pressure (established hypertension), and 111 had an average daytime ambulatory blood pressure below 90 mm Hg (white coat hypertension). A sample of 127 subjects drawn from the Danish national register served as a normotensive control group. The main outcome measure was the ratio of early morning urinary albumin to creatinine. This ratio differed significantly among the three groups, being (on a molar basis) 21 +/- 69 x 10(-6) in the normotensive subjects, 22 +/- 39 x 10(-6) in the white coat hypertensive patients, and 51 +/- 177 x 10(-6) in patients with established hypertension. The difference remained significant after correction for covariables. The ratio of early morning urinary albumin to creatinine was weakly but significantly correlated to blood pressure, was more pronounced for ambulatory than for office measurements, was more pronounced for systolic than for diastolic pressure, and was more pronounced for hypertensive than for normotensive individuals. The ratio was as reproducible a measure as 24-hour albumin excretion. We conclude that white coat hypertensive patients have less renal involvement than patients with established hypertension but more than a normotensive control group.

Epidemiology of microalbuminuria in the general population

Microalbuminuria in the general population is associated with recognized risk factors for cardiovascular disease such as hypertension, hyperglycemia, hyperinsulinemia, and hyperlipidemia; and it is an independent predictor of subsequent cardiovascular mortality in hypertensive, diabetic, and elderly populations. Although different methods have been used for measuring and expressing urinary albumin excretion and a variety of cutoff levels have been used for defining microalbuminuria, prevalence of microalbuminuria appears to be higher in non-Europeans (8%-28%) than in Europeans (2%-10%). However, because of the large within-individual variability of urinary albumin excretion and the relatively low prevalence of microalbuminuria, large studies are required to detect statistically significant associations between albuminuria and cardiovascular risk factors. Evidence presented here supports the proposition that microalbuminuria represents a marker of cardiovascular disease risk in nondiabetic individuals as well as diabetic individuals. Moreover, because of a high sensitivity of the test and because albuminuria is a concomitant of many forms of renal disease, microalbuminuria also has a role in detecting patients with renal involvement associated with essential hypertension, lupus erythematosus, women with pre-eclampsia, and subjects with unsuspected primary and secondary nephropathies.

Urinary albumin excretion rate and puberty in non-diabetic children and adolescents

Slightly elevated urinary albumin excretion rate (microalbuminuria) is a marker of early diabetic nephropathy, but it is unclear if the established definition of microalbuminuria (20-200 micrograms/min) is correct for children and adolescents. We investigated the albumin excretion rate, albumin/creatinine ratio and urinary albumin concentration in 150 healthy schoolchildren and adolescents to (a) obtain a reference value for albumin excretion rate, (b) relate albumin excretion to pubertal stages and (c) evaluate albumin/creatinine ratio and morning albumin concentration as screening methods for elevated albumin excretion rate. Albumin concentration was measured by immunoturbidimetry in timed overnight urine samples. The albumin excretion showed a skewed distribution (geometric mean 3.2 micrograms/min, 95 percentile 15.1 micrograms/min). In girls, a peak in the albumin excretion rate was found at the pubertal stage 4 (Tanner) and in boys at stage 5. Albumin/creatinine ratio of 2.5 mg/mmol as a screening level for elevated albumin excretion (15 micrograms/min) showed a high positive (0.88) and negative (0.99) predictive value.

The relationship between the development and progression of microalbuminuria and arterial blood pressure in type 1 (insulin-dependent) diabetes mellitus

OBJECTIVE

to investigate the association between urinary albumin excretion and arterial blood pressure in type 1 (insulin-dependent) diabetes.

RESEARCH DESIGN AND METHODS

urinary albumin excretion and blood pressures were followed prospectively for a mean period of 26 months (range 18-29 months) in 46 young type 1 (insulin-dependent) diabetic subjects without overt nephropathy. Supine blood pressures (BP) were measured by a single observer using a random zero sphygmomanometer. Albumin excretion was assessed at baseline by a timed clinic excretion rate (AER; microalbuminuria = AER greater than 33 micrograms/min), and at follow-up in at least two urine specimens by the albumin/creatinine (A/Cr) ratio (micro-albuminuria = A/Cr greater than 3.7 mg/mmol).

RESULTS

39 subjects initially had normal AERs. Seven had developed microalbuminuria at follow-up: their mean BP rose from 114 +/- 13/62 +/- 13 to 119 +/- 7/77 +/- 5 mmHg (for diastolic BP, P less than 0.05), while there was no change in the mean BP in the remaining 32 patients. A rise in diastolic BP of greater than 10 mmHg occurred in five of the seven subjects who developed microalbuminuria, and in only seven of 32 who did not (P = 0.02). In the seven patients in whom microalbuminuria persisted (n = 3) or progressed to overt proteinuria (n = 4), BP increased from 123 +/- 12/70 +/- 14 to 139 +/- 12/88 +/- 10 mmHg (P less than 0.02 for both).

CONCLUSIONS

this study has shown that BP is normal before the onset of microalbuminuria, and that a rise in diastolic BP accompanies the development or progression of microalbuminuria. The rate of rise in BP may be more important than the absolute level in defining 'hypertension' in young diabetic patients with microalbuminuria.

Microalbuminuria and associated cardiovascular risk factors in the community

The prevalence of microalbuminuria and relationship to cardiovascular risk factors was examined in a cross-sectional community survey of cardiovascular risk factors. Microalbuminuria (when classified as albumin concentration greater than 20 micrograms/ml) was present in 6.3% of subjects but in conjunction with an albumin/creatinine ratio greater than 3.5 in only 2.2%. Diastolic blood pressure, prevalence of abnormal electrocardiographs, and to a lesser extent systolic blood pressure and fibrinogen concentration, were greater in those with albuminuria concentrations greater than 20 micrograms/ml. The strongest positive univariate correlates of albumin/creatinine ratios in those with detectable albuminuria were age, fibrinogen, blood pressure, total- and low density lipoprotein-(LDL) cholesterol, apo B and alcohol intake, whereas fasting insulin and insulin resistance were inversely correlated. Multiple regression analysis revealed that age, gender, systolic blood pressure and insulin resistance independently accounted for 37% of the variability in albumin/creatinine ratios. When those 10 subjects with microalbuminuria and albumin/creatinine ratios greater than 3.5 were matched with 20 with normoalbuminuria for age, gender and body mass index, the microalbuminuric subjects had significantly lower LDL cholesterol/apo B ratios and a tendency to lower high density lipoprotein (HDL) cholesterol and HDL cholesterol/apo A1 ratios. Microalbuminuria is uncommon in the general population, and is related to ageing, blood pressure and other vascular risk factors. It may reflect the presence of established cardiovascular disease.

Measuring the albumin excretion rate: agreement between methods and biological variability

A timed urine collection is necessary to determine the excretion rate of albumin (AER) but such specimens are tedious to collect and frequently inaccurate. Albumin excretion can also be quantified by the use of the albumin:creatinine ratio in randomly obtained specimens. In the present study the agreement between AER as measured on a 24-h urine collection and as estimated from the albumin:creatinine ratio is determined. Previously published studies have examined the correlation rather than the agreement between these methods and not taken into account the biological variability of AER. Thirty patients with diabetes who had normal renal function, but varying degrees of albuminuria, produced two 24-h specimens and two random daytime specimens of urine. AER was measured on the former and estimated from the latter by multiplying the albumin:creatinine ratio by an estimate of that individual's creatinine excretion rate. Agreement between the methods and the biological variability was determined by using appropriate statistical methodology, the main outcome measure being the limits of agreement between repeat values for both measurements and both estimates of AER, and between the averages of the measurements and the estimates. The limits of agreement between repeated 24-h measurements were wide, the second specimen being 33 to 490% of the first. The estimates of AER gave values numerically similar to the measurements. The limits of agreement between the two estimates did not differ significantly from those of the measurements, nor did the limits of agreement when the average of the measurements and the average of the estimates were compared (all NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Which method for quantifying "microalbuminuria" in diabetics? Comparison of several immunological methods (immunoturbidimetric assay, immunonephelometric assay, radioimmunoassay and two semiquantitative tests) for measurement of albumin in urine

We have compared the chemical and clinical characteristics of an immunonephelometric assay (INA), two immunoturbidimetric assays (ITA) and two semi-quantitative methods with those of a solid-phase radioimmunoassay (RIA) for measurement of urinary albumin (UA) concentration in 136 diabetic patients. INA and RIA had similar accuracy, and provided comparable results. However, RIA has slightly greater sensitivity than INA, which is easier and faster. Good agreement was also found between RIA and the two ITA methods, although one of these overestimated RIA values in the low-medium range (5-30 mg/l) of urinary albumin. ITA seems suitable for initial screening of albuminuria in diabetic patients but more sensitive procedures (such as RIA and INA) seem preferable for measurement of UA concentrations in the normal range. The two semi-quantitative methods showed high sensitivity but poor specificity, because of the large number of false positive results. About 50% of diabetic patients "positive" by these methods did not have microalbuminuria. The utility of these methods is questionable, because many samples from diabetic patients need to be reassayed by a more specific and sensitive assay such as the RIA, INA or ITA methods.

A simple approach to screening for microalbuminuria in a type 1 (insulin-dependent) diabetic population

The reproducibility of albumin concentration in first-morning samples of urine was assessed in 334 insulin-dependent diabetic patients aged 18-60 years. The albumin excretion rate was determined immunoturbidimetrically in three sterile, Albustix-negative, first-morning urine samples submitted over a week. An abnormally high mean value, greater than or equal to 2.5 mg albumin per mmol creatinine (Ua/Uc), was found in 33 patients (9.9%). These patients were older (mean 42 vs 34 years, P less than 0.01), had longer disease duration (18 vs 14 years, P less than 0.01) and higher HbA1c values (6.8 vs 6.3%, P less than 0.05) than those without microalbuminuria. Although triplicate samples were collected within 7 days, Ua/Uc showed considerable intraindividual variation, with a mean coefficient of variation of 49%. Despite this it was found that Ua/Uc values greater than 1 mg/mmol on the first specimen had a sensitivity of 97% and a specificity of 82% for detecting those with a three-sample mean value greater than 2.5 mg/mmol. Thus virtually all those with microalbuminuria (32/33) had a single first-morning result greater than 1 mg/mmol, and in those with a lower ratio microalbuminuria was excluded with more than 99% certainty.

Construction and use of a template block for radial immunodiffusion

Details for design and construction of template blocks for the increased sensitivity of radial immunodiffusion (RID) assays are described. The sensitivity of the template RID was 10-fold higher for quantifying albumin and 5-fold higher for IgG than the conventional RID with wells cut into the agarose. The system described in this communication resulted from the unit which provided the most consistent results from a variety of trials with different block thicknesses, chamber sizes and depths, and chamber arrangements.

A microalbuminuria assay using bromphenol blue

A simple microalbuminuria assay using bromphenol blue/glycine reagent is described. Urine samples were prepared using gel filtration on Sephadex G-50 minicolumns and absorbance was measured at 610 nm 20 s after mixing 10 parts of eluate and 1 part of reagent. The detection limit of this method was 3 mg/l; within-run and between-run precision was between 0.5 and 4.1% for borderline and raised albumin concentrations. The recovery of albumin added to samples was 98.7 +/- 2.5%. Results obtained by this method correlated closely with values obtained by radial immunodiffusion (r = 0.987). The test is cheap (reagent costs about 5 cents) and suitable for the non-specialist laboratory.