J Nephrol. 2010 Mar-Apr;23(2):125-32.

Abstract
Kidney function should be evaluated by procedures including the calculation of glomerular filtration rate (GFR) estimates and the assessment of albuminuria or proteinuria as creatinine-normalized urinary ratios for albumin or total protein. GFR estimates are an approximation of true GFR, which circumvent the limitations of serum creatinine and creatinine clearance without increasing costs and time of diagnostic work-up. Estimates by Cockcroft-Gault equation tend to be higher than true GFR and estimates by other equations, because this equation predicts creatinine clearance, hence true GFR plus creatinine excretion via tubular secretion. The inclusion of a weight coefficient in the equation causes a GFR overestimation in the presence of large adiposity or edema. Estimates by equations of the Modification of Diet in Renal Disease (MDRD) study can be unreliable for high-normal GFR because that study did not enroll individuals without kidney disease. The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) group has reported a new equation to overcome this limitation. GFR estimates can be biased by interassay creatinine differences or unusual levels of creatinine generation (muscle mass) or of renal tubular creatinine secretion. The urinary ratio of albumin (or total protein) to creatinine is measurable in untimed spot urine and reflects the urinary excretion rate of albumin (or total protein). Low muscle mass could imply borderline elevation in the ratio merely because of low urinary creatinine. Vice versa, high muscle mass could imply normal ratios even in the presence of high urinary albumin, because of high urinary creatinine due to high creatinine generation.

Clinical Chemistry. 2009;55:24-38

Background: Urinary excretion of albumin indicates kidney damage and is recognized as a risk factor for progression of kidney disease and cardiovascular disease. The role of urinary albumin measurements has focused attention on the clinical need for accurate and clearly reported results. The National Kidney Disease Education Program and the IFCC convened a conference to assess the current state of preanalytical, analytical, and postanalytical issues affecting urine albumin measurements and to identify areas needing improvement.
Content: The chemistry of albumin in urine is incompletely understood. Current guidelines recommend the use of the albumin/creatinine ratio (ACR) as a surrogate for the error-prone collection of timed urine samples. Although ACR results are affected by patient preparation and time of day of sample collection, neither is standardized. Considerable intermethod differences have been reported for both albumin and creatinine measurement, but trueness is unknown because there are no reference measurement procedures for albumin and no reference materials for either analyte in urine. The recommended reference intervals for the ACR do not take into account the large intergroup differences in creatinine excretion (e.g., related to differences in age, sex, and ethnicity) nor the continuous increase in risk related to albumin excretion.

Discussion: Clinical needs have been identified for standardization of (a) urine collection methods, (b) urine albumin and creatinine measurements based on a complete reference system, (c) reporting of test results, and (d) reference intervals for the ACR.

current practices that reflect the consensus opinions of the conference participants
1. Use of the term “urine albumin” is recommended; “microalbumin” is discouraged.

2. First morning void urine samples provide lower variability than random samples.

3. Second morning void urine samples may also be acceptable but there is no evidence to support this practice as superior to first morning void.

4. Urine albumin should be measured in urine that has not been frozen. Albumin in urine is adequately stable when stored at 2–8 °C for 7 days before measurement. Any cloudiness due to precipitate or cellular components should be removed by centrifugation before refrigerated storage.

5. Refrigerated urine should be warmed to room temperature before measurement to dissolve precipitates that may have formed and adsorbed albumin. The urine should be visually examined for precipitate and centrifuged if necessary to remove residual precipitate.

6. If urine is to be frozen before measurement, it should be frozen at –70 °C. Any cloudiness due to precipitate or cellular components should be removed by centrifugation before frozen storage. Thawed samples should be warmed to room temperature and mixed thoroughly before measurement. The effect of freezing and thawing on albumin molecular forms is not thoroughly understood.

7. An albumin/creatinine ratio should be reported with all urine albumin measurements.

8. Confusion arises from reporting results in units of “mg albumin/mmol creatinine,” “g albumin/mol creatinine,” “mg albumin/g creatinine,” or “µg albumin/mg creatinine.” This situation reflects national or regional preferences and is not likely to be resolved. Ideally, International System of Units should be adopted. In the interim, uniform guidelines should be followed within a country or region.

9. Albumin concentrations reported in milligrams per liter are difficult to interpret and concentrations in these units should not be the only value reported.

issues requiring further investigation for standardization of measurement and reporting of urine albumin
1. Clarification of preanalytical requirements.

Influence of container type.

Influence of timing of collection (first morning, second morning, random, 24 h) related to biological variability.

Influence of blood (menstrual or urinary bleeding), seminal fluid, and other physiologic contaminants of urine.

2. Clarification of the molecular forms of albumin in freshly voided urine, and the definition of the measurand.

3. Clarification of the degree of urine albumin degradation under various conditions of storage.

4. Clarification regarding the variation in urinary matrix composition over which urine albumin measurement procedures must operate.

5. Clarification of the clinical requirements for total error in measurement of urine albumin.

6. Development of a reference measurement procedure.

7. Development of a urine albumin secondary reference material including its commutability validation and credentialing by JCTLM.

8. Development of a urine creatinine secondary reference material including its commutability validation and credentialing by JCTLM.

9. Identification of appropriate EQAS materials that will allow performance of routine methods to be compared.

10. Standardized measurement results are necessary to enable clinical studies to determine the optimal decision thresholds for AER and ACR.

11. Different decision limits may be needed for random vs first morning or other standardized collection time, owing to the increased variability of randomly collected samples.

12. The ACR varies with age, sex, and ethnicity. Decision thresholds suitable for these subgroups need further investigation. A single decision threshold may not be adequately sensitive for each subgroup.

13. Risk of chronic kidney disease and cardiovascular disease are continuous functions of urine albumin concentration. The appropriate thresholds for risk for given populations (e.g., general population or high risk groups such as diabetes, hypertension or cardiovascular disease) need to be determined.

14. Investigation of the usefulness of age- and sex-specific equations to convert ACR to an estimated AER for which a single reference limit may be appropriate.

Health Technol Assess. 2005 Aug;9(30):iii-vi, xiii-163.

OBJECTIVES: To determine whether microalbuminuria is an independent prognostic factor for the development of diabetic complications and whether improved glycaemic or blood pressure control has a greater influence on the development of diabetic complications in those with microalbuminuria than in those with normoalbuminuria.

DATA SOURCES: Electronic databases up until January 2002.

REVIEW METHODS: A protocol for peer review by an external expert panel was prepared that included selection criteria for data extraction and required two independent reviewers to undertake article selection and review. Completeness was assessed using hand-searching of major journals. Random effects meta-analysis was used to obtain combined estimates of relative risk (RR). Funnel plots, trim and fill methods and meta-regression were used to assess publication bias and sources of heterogeneity.

RESULTS: In patients with type 1 or type 2 DM and microalbuminuria there is a RR of all-cause mortality of 1.8 [95% confidence interval (CI) 1.5 to 2.1] and 1.9 (95% CI 1.7 to 2.1) respectively. Similar RRs were found for other mortality end-points, with age of cohort being inversely related to the RR in type 2 DM. In patients with type 1 DM, there is evidence that microalbuminuria or raised albumin excretion rate has only weak, if any, independent prognostic significance for the incidence of retinopathy and no evidence that it predicts progression of retinopathy, although strong evidence exists for the independent prognostic significance of microalbuminuria or raised albumin excretion rate for the development of proliferative retinopathy (crude RR of 4.1, 95% CI 1.8 to 9.4). For type 2 DM, there is no evidence of any independent prognostic significance for the incidence of retinopathy and little, if any, prognostic relationship between microalbuminuria and the progression of retinopathy or development of proliferative retinopathy. In patients with type 1 DM and microalbuminuria there is an RR of developing end-stage renal disease (ESRD) of 4.8 (95% CI 3.0 to 7.5) and a higher RR (7.5, 95% CI 5.4 to 10.5) of developing clinical proteinuria, with a significantly greater fall in glomerular filtration rate (GFR) in patients with microalbuminuria. In patients with type 2 DM, similar RRs were observed: 3.6 (95% CI 1.6 to 8.4) for developing ESRD and 7.5 (95% CI 5.2 to 10.9) for developing clinical proteinuria, with a significantly greater decline in GFR in the microalbuminuria group of 1.7 (95% CI 0.1 to 3.2) ml per minute per year compared with those who were normoalbuminuric. In adults with type 1 or type 2 DM and microalbuminuria at baseline, the numbers progressing to clinical proteinuria (19% and 24%, respectively) and those regressing to normoalbuminuria (26% and 18%, respectively) did not differ significantly. In children with type 1 DM, regression (44%) was significantly more frequent than progression (15%). In patients with type 1 or type 2 DM and microalbuminuria, there is scarce evidence as to whether improved glycaemic control has any effect on the incidence of cardiovascular disease (CVD), the incidence or progression of retinopathy, or the development of renal complications. However, among patients not stratified by albuminuria, improved glycaemic control benefits retinal and renal complications and may benefit CVD. In the effects of angiotensin-converting enzyme (ACE) inhibitors on GFR in normotensive microalbuminuric patients with type 1 DM, there was no evidence of a consistent treatment effect. There is strong evidence from 11 trials in normotensive type 1 patients with microalbuminuria of a beneficial effect of ACE inhibitor treatment on the risk of developing clinical proteinuria and on the risk of regression to normoalbuminuria. Patients with type 2 DM and microalbuminuria, whether hypertensive or not, may obtain additional cardiovascular benefit from an ACE inhibitor and there may be a beneficial effect on the development of retinopathy in normotensive patients irrespective of albuminuria. There is limited evidence that treatment of hypertensive microalbuminuric type 2 diabetic patients with blockers of the renin–angiotensin system is associated with preserved GFR, but also evidence of no differences in GFR in comparisons with other antihypertensive agents. The data on GFR in normotensive cohorts are inconclusive. In normotensive type 2 patients with microalbuminuria there is evidence from three trials (all enalapril) of a reduction in risk of developing clinical proteinuria; in hypertensive patients there is evidence from one placebo-controlled trial (irbesartan) of a reduction in this risk. Intensive compared with moderate blood pressure control did not affect the rate of progression of microalbuminuria to clinical proteinuria in the one available study. There is inconclusive evidence from four trials of any difference in the proportions of hypertensive patients progressing from microalbuminuria to clinical proteinuria when ACE inhibitors are compared with other antihypertensive agents, and in one trial regression was two-fold higher with lisinopril than with nifedipine.

CONCLUSIONS: The most pronounced benefits of glycaemic control identified in this review are on retinal and renal complications in both normoalbuminuric and microalbuminuric patients considered together, with little or no evidence of any greater benefit in those with microalbuminuria. Hence, microalbuminuric status may be a false boundary when considering the benefits of glycaemic control. Classification of a person as normoalbuminuric must not serve to suggest that they will derive less benefit from optimal glycaemic control than a person who is microalbuminuric. All hypertensive patients benefit from blood pressure lowering and there is little evidence of additional benefit in those with microalbuminuria. Antihypertensive therapy with an ACE inhibitor in normotensive patients with microalbuminuria is beneficial. Monitoring microalbuminuria does not have a proven role in modulating antihypertensive therapy while the patient remains hypertensive. Recommendations for microalbuminuria research include: determining rate and predictors of development and factors involved in regression; carrying out economic evaluations of different screening strategies; investigating the effects of screening on patients; standardising screening tests to enable use of common reference ranges; evaluating the effects of lipid-lowering therapy; and using to modulate antihypertensive therapy.

Diabetes Metab. 2007 Sep;33(4):303-9.

Urinary albumin excretion (UAE) may be assayed on a morning urinary sample or a 24 h-urine sample. Values defining microalbuminuria are: 1) 24-h urine sample: 30-300 mg/24 h; 2) morning urine sample: 20-200 mg/ml or 30-300 mg/g creatinine or 2.5-25 mg/mmol creatinine (men) or 3.5-35 mg/mmol (women); 3) timed urine sample: 20-200 mug/min. The optimal use of semi-quantitative urine test-strip is not clearly defined. It is generally believed that microalbuminuria reflects a generalized impairment of the endothelium; however, no definite proof has been obtained in humans. IN DIABETIC SUBJECTS: Microalbuminuria is a marker of increased risk of cardiovascular (CV) and renal morbidity and mortality in type 1 and type 2 diabetic subjects. The increase in UAE during follow-up is associated with greater CV and renal risks in type 1 and type 2 diabetic subjects; its decrease during follow-up is associated with lower risks. IN NON-DIABETIC SUBJECTS: Microalbuminuria is a marker of increased risk for diabetes mellitus, deterioration of renal function, CV morbidity and all-cause mortality. It is a marker of increased risk for the development of hypertension in normotensive subjects, and is associated with unfavorable outcome in patients with cancer and lymphoma. Persistence of elevated UAE during follow-up is associated with poor outcome in some hypertensive subjects. Measurement of UAE may be recommended in hypertensive medium-risk subjects with 1 or 2 CV risk factors in whom CV risk remains difficult to assess, and in those with refractory hypertension: microalbuminuria indicates a high CV risk and must lead to strict control of arterial pressure. Studies focused on microalbuminuria in non-diabetic non-hypertensive subjects are limited; most of them suggest that microalbuminuria predicts CV complications and deleterious outcome. Subjects with a history of CV or cerebrovascular disease have an even greater CV risk if microalbuminuria is present than if it is not; however, in all cases, therapeutic intervention must be aggressive regardless of whether microalbuminuria is present or not. It is not recommended to measure UAE in non-diabetic non-hypertensive subjects in the absence of history of renal disease. Monitoring of renal function (UAE, serum creatinine and estimation of GFR) is recommended annually in all subjects with microalbuminuria. MANAGEMENT: In patients with microalbuminuria, weight reduction, sodium restriction (<6 g per day), smoking cessation, strict glucose control in diabetic subjects, strict arterial pressure control are necessary; in diabetic subjects: use of maximal doses of angiotensin-converting enzyme inhibitors (ACEI) or angiotensin receptor blockers (ARB) are recommended; ACEI/ARB and thiazides have synergistic actions on arterial pressure and reduction of UAE; in non-diabetic subjects, any of the five classes of anti-hypertensive medications (ACEI, ARB, thiazides, calcium channel blockers or beta-blockers) can be used.

Diabetes Care February 2008 vol. 31 no. Supplement 2 S190-S193

Diabetes is a growing disease with a potentially devastating outcome. Diabetic patients run a great risk of developing multiple organ dysfunction and ultimately organ failure. The current approach of patients with diabetes is first to assess their risk profile by measuring risk factors such as glucose level, systemic blood pressure, blood lipids, body weight, and smoking. Second, to reduce the risk, the patient is advised to make a lifestyle change (lose weight and stop smoking) and to take medication that regulates glucose and lowers blood pressure and cholesterol. This approach has indeed resulted in a slowing of progressive organ dysfunction and has substantially prolonged life.

However, the residual risk of diabetic patients, despite “optimal” treatment of these risk factors, is still extremely high, and the number of patients is dramatically growing. This has urged the medical profession to improve risk profiling and design new therapeutic strategies to further reduce existing risk. In addition, the search for early disease markers was intensified with the goal to apply preventive therapeutic measures in early stages of disease, instead of waiting until the disease had fully developed.

The next paragraphs will address the status of a “new” cardiovascular and renal risk marker: increased levels of albumin in the urine. This so-called albuminuria not only marks risk in advanced stages of diabetic disease, but also indicates risk in the very early stages of the disease. Moreover, new antihypertensive therapies not only lower blood pressure, but also reduce albuminuria. We will address the need of not only measuring the risk marker, but also targeting therapies to lower albuminuria. Finally, the individual response to such therapies appears to be highly variable, offering us opportunities to optimize organ protection by individualizing therapies with the goal to overcome therapy resistance.

Clearly, diabetes constitutes a multifactorial disease in its organ damage (and maybe even in its cause). This forms a sound reason to look for multiple targets (next to optimization of treating existing targets).