1. Use

• Diagnosis/Screening

Diabetes is the leading cause of end-stage renal disease in the US and Europe (1). Early detection of diabetic nephropathy relies upon tests for urinary excretion of albumin. The detection limits of conventional qualitative tests (chemical strips or "dipsticks") for albuminuria are above that needed to detect early disease. For this purpose, tests for "microalbuminuria" are used. Microalbuminuria is defined (1) as excretion of 30 – 300 mg of albumin /24 h (or 20 – 200 ug/min or 30 – 300 ug/mg creatinine). [footnote: Although the term microalbuminuria is recognized as a misnomer (the albumin is not small), the term is well entrenched and not likely to be replaced by alternatives (e.g., paucialbuminuria).]

The ADA recommends periodic qualitative ("dipstick") testing for urine albumin in adults with diabetes (1). Positive tests represent "clinical albuminuria" or "overt nephropathy" in the ADA recommendations and correspond to protein excretion > 300mg/24 hours ( > 200 ug/min or > 300 ug/mg creatinine). In these patients, the ADA guidelines indicate that quantitative measurement of urine protein excretion may be useful for planning of treatment. Evidence for or against this conclusion is not available. Negative "dipstick" tests for "clinical proteinuria" (albumin excretion < 300 mg/day) should be followed with a test for microalbuminuria. Testing for microalbuminuria is recommended for children with type 1 diabetes after puberty and after 5 years’ duration of diabetes.

In the ADA algorithm for urine protein testing (1), the diagnosis of microalbuminuria requires the demonstration of increased albumin excretion (as defined above) on 2 of 3 tests repeated at intervals of 3 – 6 months, and exclusion of conditions that "invalidate" the test. (See "Interferences" below.)

• Prognosis

In 80 % of people with type 1 diabetes and microalbuminuria, urinary albumin excretion increases at a rate of 10 – 20 % per year, with development of clinical proteinuria (> 300 mg albumin/day) in 10 –15 years (1). End-stage renal disease develops within 10 years in half of the type 1 individuals with overt nephropathy. In type 2 diabetes, 20 – 40 % of patients with microalbuminuria progress to overt nephropathy, but by 20 years after overt nephropathy only ~ 20 % develop end-stage renal disease.

• Monitoring

The roles of routine urinalysis and microalbumin measurements are less clear in patients with a diagnosis of microalbuminuria. Some have advocated urine protein testing to monitor treatment, which may include increased glycemic control, control of hypertension, protein restriction and therapy with angiotensin inhibitors (1). Therapy (e.g., with angiotensin converting enzyme inhibitors) has been shown to slow the rate of increase of urinary albumin excretion rate or to prevent it in short-term studies, and progression is associated with poorer glycemic control (for a recent study, see (2)).

1. Rationale

Early detection of microalbuminuria allows early intervention with a goal of delaying the onset of overt diabetic nephropathy. Moreover, microalbuminuria is a marker of increased risk of cardiovascular morbidity and mortality in both type 1 and type 2 diabetes. Thus, it is a signal for efforts to reduce cardiovascular risk factors.

Microalbuminuria rarely occurs with short duration of Type 1 diabetes or before puberty. Thus testing is less urgent in these situations. By contrast, the difficulty in precisely dating the onset of Type 2 diabetes warrants initiation of annual testing at the time of diagnosis of diabetes.

2. Analytical Considerations

• Analytical goals

Recommendation: The analytical CV of methods to measure urinary albumin should be < 15%.

The within-person variation of microalbumin excretion is large in people without diabetes and even higher in patients with diabetes. Howey et al (3) studied day-to-day variation, over 3-4 weeks, of the 24-hour albumin excretion as well as both the concentration of albumin and the albumin: creatinine ratio. The latter two were measured in the 24-hour urine sample and also in (a) the first morning void and (b) random untimed urine. In healthy volunteers, the lowest within-person CVs were found for the concentration of albumin in the first morning void (36 %) and for the albumin:creatinine ratio in that sample (31 %). They recommended use of the urine albumin concentration in the first morning void rather than 24-hour urinary excretion of albumin which had a higher within-person CV.

Based on the widely-expressed view that analytical CV should be less than half the biological CV, Howey (3) proposed an analytical goal of 18 % CV. Alternatively, if the albumin: creatinine ratio is to be used, one may calculate the need for somewhat lower imprecision (to accommodate the lower biological CV for the ratio and the imprecision contributed by the creatinine measurement). Assuming a CV of 5 % for the measurement of creatinine, we calculate a goal of 14.7% for the analytical CV for albumin when it is used to estimate the albumin:creatinine ratio. A goal of 15% appears reasonable to accommodate either use of the measurement of albumin.

In subjects with diabetes, the within-person variation (CV) was 61 % for albumin concentration in the first morning void and 39 % for the albumin:creatinine ratio. Thus the goals above appear more than adequate for use in subjects with diabetes.

• Premeasurement

Recommendation: Acceptable samples to monitor urinary albumin excretion are first morning void or 24-hour (timed) collections for measurement of albumin concentration (or albumin excretion rate in the 24-hour sample) and untimed samples for measurement of the albumin:creatinine ratio, preferably on a first morning sample.

Collection of 24-hour samples appears to not be necessary as the albumin:creatinine ratio appears to be an acceptable alternative. The ratio has a low within-person, biological variation and correlates well with timed excretion as well as with albumin concentration in a first morning void of urine (3). A first-morning void sample is to be preferred for the ratio as the ratio in a first morning sample had a lower within-person variation than did the ratio in a random sample of urine during the day (3).

Albumin is stable in untreated urine stored at 4 C for at least 4 weeks (4). Neither centrifugation nor filtration appears necessary before storage at – 20 C or – 80 C (5). Whether centrifuged, filtered or not treated, albumin concentration decreased by 0.27 % per day at – 20 C, but showed no decrease over 160 days at – 80 C (5).

Urinary albumin excretion rate reportedly has no marked diurnal variation in diabetes, but does in essential hypertension (6).

• Measurement: Detection limit, imprecision

Commercially available quantitative methods for microalbuminuria have documented detection limits of ~ 20 ug/L or less. Within-run imprecision and day-to-day (total) imprecision are well within the analytical goal of ~ 15 %, and often much less. A recent study showed that most methods, but not all, agree well with each other and support a reference interval of 2 – 20 ug albumin / mg creatinine (7).

Recommendation: Semiquantitative or qualitative for microalbuminuria are acceptable for screening when used as intended, that is, with laboratory confirmation of increased results.

Qualitative (or semiquantitative) tests for microalbuminuria are available from several manufacturers. They are intended as screening tests, that is, tests with high clinical sensitivity (low false negative rates with albumin > 20 ug/L) and an acceptable false positive rate. Positive results thus need to be confirmed by a quantitative method. Further studies are needed before the "dipstick" tests for microalbuminuria can be recommended as replacements for the quantitative tests. The use of the qualitative tests at the point of care is reasonable when it can avoid quantitative testing in a sizeable proportion of the population in a patient care area.

Transient increases of urinary albumin excretion have been reported with short-term hyperglycemia, exercise, urinary tract infections, marked hypertension, heart failure and acute febrile illness (1).

The ADA recommends annual measurement in patients with negative ("dipstick") results for overt proteinuria. After the documentation of a diagnosis of microalbuminuria (i.e., with results as defined above on 2 of 3 tests performed within a period of 3 – 6 months), repeated testing is reasonable to determine whether a chosen therapy is effective. It may also be useful in determining the rate of progression of disease and thus support planning for care of end-stage renal disease. Although the ADA recommendations suggest that testing is not needed before puberty, a more recent study makes a case for such testing (2). We believe that there is insufficient evidence to conclude that such testing is inappropriate in every case.

1. American Diabetes Association. Diabetic Nephropathy. Diabetes Care 1999;22:S66-S69.

2. Holl RW, Grabert M, Thon A, Heinze E. Urinary excretion of albumin in adolescents with type 1 diabetes: persistent versus intermittent microalbuminuria and relationship to duration of diabetes, sex, and metabolic control. Diabetes Care 1999;22:1555-60.

3. Howey JE, Browning MC, Fraser CG. Biologic variation of urinary albumin: consequences for analysis, specimen collection, interpretation of results, and screening programs. Am J Kidney Dis 1989;13:35-7.

4. Gonzalez Castro ML, Sagredo PJ, Cebrecos TR, Enriquez DO, de Baranda RA, Cordero GJ. [The stability of the microalbuminuria numbers in relation to their reading time and mode of preservation]. Aten Primaria 1999;23:533-6.

5. MacNeil ML, Mueller PW, Caudill SP, Steinberg KK. Considerations when measuring urinary albumin: precision, substances that may interfere, and conditions for sample storage. Clin Chem 1991;37:2120-3.

6. Hishiki S, Tochikubo O, Miyajima E, Ishii M. Circadian variation of urinary microalbumin excretion and ambulatory blood pressure in patients with essential hypertension. J Hypertens 1998;16:2101-8.

7. Roberts WL, Calcote CB, Cook CB, Gordon DL, Moore ML, Moore S et al. Comparison of four commercial urinary albumin (microalbumin) methods: implications for detecting diabetic nephropathy using random urine specimens. Clin Chim Acta 1998;273:21-33.