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Biochemistry
This test measures albumin directly in a body fluid, most often ascitic fluid drawn from the abdomen, rather than in blood. On its own the number means little; it's paired with a blood albumin level taken the same day to calculate the serum-ascites albumin gradient, or SAAG, found by subtracting the fluid's albumin from the blood's. That gradient, not the fluid albumin figure alone, tells your doctor whether abdominal fluid buildup is linked to raised pressure in the portal vein system, as in cirrhosis or heart failure, or to something else entirely, such as cancer or tuberculosis. One widely cited estimate puts SAAG's accuracy for spotting portal hypertension at around 97 percent, though other published studies report more modest figures.
The lab reports albumin concentration in the fluid sample, usually in grams per decilitre, matched against a blood albumin drawn on the same day. Subtracting the fluid value from the blood value gives the serum-ascites albumin gradient. A gradient of 1.1 g/dL or higher points toward portal hypertension as the cause of the fluid buildup; a gradient below that points away from it, toward causes such as cancer spread to the abdominal lining, pancreatitis, or tuberculous peritonitis.
Ascites, fluid collecting in the abdominal cavity, used to be classified mainly as a transudate or an exudate, based on the fluid's total protein content, a method that turned out to be a fairly rough guide in practice. Comparing albumin in the fluid against albumin in the blood, rather than looking at total protein alone, replaced it because it tracks something more specific: how much pressure is being pushed through the portal vein system that carries blood from the gut to the liver.
Fluid albumin by itself is affected by how much albumin is in your blood to begin with, so a low blood albumin level, common in liver disease and malnutrition, can make fluid albumin look low even without a specific local cause. Comparing the two side by side cancels that out, which is why the gradient, not the raw fluid number, is the figure doctors actually interpret.
Because the calculation depends on both numbers reflecting the same moment, the blood sample for albumin needs to be drawn on the same day as the fluid tap; a mismatch between the two undermines the whole point of the comparison. Published accuracy for the resulting gradient varies by study, with estimates for correctly identifying portal hypertension ranging from roughly two-thirds to as high as 97 percent, so it's read alongside the fluid's other findings, not as a stand-alone verdict.
The fluid albumin number only becomes useful once it's compared against a same-day blood albumin level, producing the serum-ascites albumin gradient your doctor actually reads.
It points toward portal hypertension, raised pressure in the portal vein system, as the likely cause of the fluid buildup, as seen in cirrhosis or heart failure.
It measures the same protein, but in a fluid sample rather than blood, and it's interpreted together with a same-day blood albumin rather than on its own.
Published estimates vary by study, ranging from roughly two-thirds accuracy up to around 97 percent for correctly identifying portal hypertension, so results are read alongside other findings.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
Not directly — both tuberculous peritonitis and cancer-related ascites tend to fall on the same side of the albumin gradient, which is why a separate test, ascitic fluid ADA, is often used to tell them apart.
The gradient calculation assumes both values reflect the same moment in time. A mismatch between them would make the comparison unreliable.