Definition
- The urine dipstick is a relatively insensitive marker for proteinuria, not becoming positive until protein excretion exceeds 300 " “500 mg/day. The normal rate of albumin excretion is <20 mg/day (15 Ž ¼g/minute); persistent albumin excretion between 30 and 300 mg/day (20 and 200 Ž ¼g/minute) is called microalbuminuria. Albumin excretion >300 mg/day (200 Ž ¼g/minute) is considered to represent overt or dipstick positive proteinuria (also called macroalbuminuria).
- In type 1 and 2 DM, the presence of microalbuminuria on repeat specimens collected in the basal state may signify early diabetic nephropathy. It is a marker, in patients with or without diabetes, for cardiovascular mortality. For a definition of microalbuminuria, see Table 16.56.
- Measurement of the urine albumin-to-creatinine ratio in an untimed urinary sample is the preferred screening strategy for microalbuminuria. This test has several advantages: it does not require early morning or timed collections, it gives a quantitative result that correlates with the 24-hour urine values over a wide range of protein excretion, it is simple to perform and inexpensive, and repeat values can be easily obtained to ascertain that microalbuminuria, if present, is persistent.
- Other name: albumin/creatinine ratio.
- Normal range:
- Albumin/creatinine ratio (random urine): <30.0 Ž ¼g/mg creatinine
- Microalbumin excretion (24-hour urine): 0 " “29.9 mg/day
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TABLE 16 " “56American Diabetes Association Definition of MicroalbuminuriaView LargeTABLE 16 " “56American Diabetes Association Definition of Microalbuminuria Category 24-Hour Collection Timed Collection Spot Collection Normal <30 mg/24 h <20 Ž ¼g/min <30 Ž ¼g/mg creatinine Microalbuminuria 30 " “300 mg/24 h 20 " “200 Ž ¼g/min 30 " “300 Ž ¼g/mg creatinine Clinical albuminuria >300 mg/24 h >200 Ž ¼g/min >300 Ž ¼g/mg creatinine
Use
- Diagnosis of kidney dysfunction.
- Recommended by the American Diabetes Association to screen for microalbuminuria.
- Medications that act on the renin " “angiotensin system may delay onset of renal and cardiovascular disease, making screening for microalbumin important in the care of diabetic patients.
Interpretation
- Increased excretion of albumin (microalbuminuria) is a predictor of future development of clinical renal disease in patients with hypertension or DM.
Limitations
- Microalbuminuria may be seen transiently during pregnancy, after exercise, and with protein loading, hyperglycemia, fever, and urinary tract infections. There is also day-to-day, as well as diurnal, variation in albumin excretion. Hence, it is important to base treatment on the results of several tests.
- Vigorous exercise can cause a transient increase in albumin excretion. Patients should refrain from vigorous exercise in the 24 hours prior to the test.
- The optimal time to measure the urine albumin-to-creatinine ratio is not clearly defined. The first-morning void specimen is preferred.
- The accuracy of the urine albumin-to-creatinine ratio will be diminished if creatinine excretion is substantially different from the expected value; this is particularly important in patients with borderline values. Albumin excretion will be underestimated in a muscular man with a high rate of creatinine excretion and overestimated in a cachectic patient in whom muscle mass and creatinine excretion are markedly reduced.
Suggested Reading
1American Diabetes Association. Standards of medical care in diabetes. Diabetes Care. 2004;27(Suppl 1):S79.