Albumin is the most abundant protein circulating in the blood plasma. The albumin assay is a common laboratory test that measures the concentration of this protein in a patient’s blood or urine sample. This measurement provides significant insight into a person’s metabolic capacity, fluid balance, and overall organ function. The test is frequently included in routine comprehensive metabolic panels and liver function tests.
The Role of Albumin in the Body
Albumin is synthesized by hepatocytes in the liver and then released directly into the bloodstream. Albumin performs several distinct functions that are fundamental to maintaining homeostasis.
The protein is the primary contributor to colloid oncotic pressure, the force that prevents fluid from leaking out of the blood vessels into surrounding tissues. Albumin attracts water, effectively keeping blood volume stable. If albumin levels drop, this pressure decreases, and fluid can leak out, leading to swelling, known as edema, in the extremities or abdomen.
Albumin also serves a transport role, acting as a carrier for many substances that are not easily dissolved in water. These transported substances include hormones, fatty acids, bilirubin, calcium ions, and many therapeutic drugs. Albumin also has a buffering capacity, helping to maintain the blood’s proper pH balance, and functions as a scavenger of free radicals.
Measuring Albumin: Serum and Urine Assays
The albumin assay is performed using either a serum or a urine sample. The serum albumin assay measures the total concentration of the protein circulating in the blood. This test requires a standard blood draw, and the concentration is measured in the laboratory using colorimetric methods.
Many modern laboratory analyzers use a dye-binding technique, such as the Bromocresol Green (BCG) or Bromocresol Purple (BCP) method, to quantify the protein. In the BCG method, the dye selectively binds to albumin, forming a complex that changes color. The intensity of the resulting color is then measured by a spectrophotometer, which is directly proportional to the amount of albumin present.
The urine albumin assay, often called a microalbumin test, measures the amount of albumin being excreted by the kidneys. In healthy individuals, the kidneys’ filtration barrier prevents almost all albumin from passing into the urine. Finding small, persistent amounts of albumin in the urine is an early indicator of kidney damage.
This assay is frequently quantified using the Albumin-to-Creatinine Ratio (ACR) from a random or first-morning urine sample. Creatinine is a waste product excreted by the kidneys at a relatively constant rate, and measuring the ratio of albumin to creatinine corrects for variations in urine concentration due to hydration levels. ACR values are categorized, with a result between 30 and 300 milligrams of albumin per gram of creatinine often indicating moderately increased albuminuria.
Diagnostic Applications of Albumin Testing
Since the liver is the sole site of albumin production, a persistently low serum albumin level is a measure of the liver’s synthetic function. Conditions such as hepatitis, cirrhosis, or severe fatty liver disease can impair the liver’s ability to produce adequate albumin.
Albumin testing is foundational in assessing kidney health, particularly in individuals with chronic conditions like diabetes and hypertension. The presence of albumin in the urine (albuminuria) signals that the kidney’s filtration system, the glomerulus, is damaged and allowing the protein to leak out. Regular urine albumin testing is used to screen for and monitor diabetic nephropathy, a complication that can lead to progressive kidney disease.
Serum albumin levels are also an important indicator of a person’s nutritional status and overall inflammatory state. Conditions causing malabsorption, such as Crohn’s disease or celiac disease, or chronic malnutrition can limit the building blocks for albumin synthesis. Systemic inflammation, severe burns, or infections can cause albumin to shift out of the blood vessels or increase its breakdown, leading to lower measured serum levels.
Interpreting Albumin Assay Results
A serum albumin concentration below the normal range, called hypoalbuminemia, is the most common clinically significant finding. The typical normal range for serum albumin is approximately 3.4 to 5.4 grams per deciliter (g/dL). A low level often points to one of three underlying issues: decreased production, increased loss, or increased use/breakdown.
Decreased production is linked to chronic liver disease or severe protein malnutrition, where the liver lacks the capacity or raw materials to synthesize the protein. Increased loss can occur through damaged kidneys, as seen in nephrotic syndrome, or through the gastrointestinal tract in certain malabsorption disorders. Systemic inflammation, common in critical illness or severe infection, also causes albumin to leak out of the blood vessels, leading to a drop in the measured serum level.
A high albumin level, known as hyperalbuminemia, is far less common and is rarely due to overproduction. When hyperalbuminemia is detected, it is usually a sign of acute dehydration, as the loss of water concentrates the protein remaining in the blood.
Interpreting an abnormal albumin assay result requires considering the patient’s entire clinical picture, including other laboratory tests and symptoms. For hypoalbuminemia, the goal of management is to identify and treat the underlying cause, whether that is liver failure, kidney disease, or a source of chronic inflammation. Monitoring the albumin level over time is a way to track the progression of the underlying disease or the effectiveness of treatment.