Albumin is a transport protein that is produced mainly in the liver and maintains osmotic pressure. Albumin has a long half-life (20 days) and a small (~5%) daily turnover. In humans, albumin levels rise from birth up to age 1 year, thereafter remaining stable at approximately 3.5 to 5.5 grams per deciliter (g/dL) throughout adult life. Albumin levels are reduced with advancing liver disease, nephrotic syndrome, protein-losing enteropathy, malnutrition, and some inflammatory diseases (Table 15-4). Elevations of serum albumin are unusual except in dehydration.
In severe acute infection, reduced albumin production combined with increased catabolism causes a reduction in serum albumin levels beginning in 12 to 36 hours and reaching a maximum nadir in about 5 days. As a marker for malnutrition, however, albumin levels decline relatively late. Albumin levels are most helpful in the evaluation of edema, liver disease, and proteinuria.
The difference between the serum albumin level and the albumin in ascites fluid, the serum-ascites albumin gradient (SAAG), can help differentiate portal hypertension from other causes of ascites. SAAG greater than 1.1 g/dL is seen with portal hypertension; SAAG less than 1.1 g/dL suggests another cause of the ascites, such as peritoneal inflammation or malignancy.
Most of the albumin filtered through the kidneys is reabsorbed, so significant urinary albumin is a sign of abnormal renal function. Large amounts (>300 mg/dL) of albumin can be detected on standard urine dipsticks. Microalbuminuria is defined as a persistent increase of urinary albumin that is below the detectable range of the standard dipstick test. Microalbuminuria is a marker for early diabetic nephropathy and also predicts macrovascular disease. Urinary albumin can be assayed from a spot urine specimen, which is corrected by the urine creatinine, or a 24-hour urine collection. A 24-hour urinary albumin excretion in mg/day equates to the same numeric value for the spot urine albumin (mg)/creatinine (g) ratio. Therefore the reference ranges for each test are normal <30, microalbuminuria 30-300, and clinical albuminuria >300. Factors that may interfere with the test accuracy include strenuous or prolonged exercise, upright posture, hematuria, menses, genital or urinary infections, congestive heart failure, uncontrolled hypertension or uncontrolled hyperglycemia, and high protein or high salt intake.
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