The serum-ascites albumin gradient or gap (SAAG) is a calculation used in
medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pr ...
to help determine the cause of
ascites
Ascites is the abnormal build-up of fluid in the abdomen. Technically, it is more than 25 ml of fluid in the peritoneal cavity, although volumes greater than one liter may occur. Symptoms may include increased abdominal size, increased weight, a ...
. The SAAG may be a better discriminant than the older method of classifying ascites fluid as a
transudate Transudate is extravascular fluid with low protein content and a low specific gravity (< 1.012). It has low nucleated cell counts (less than 500 to 1000 /microliter) and the primary cell types are exudate.
The formula is as follows:
:SAAG = ( serum
albumin
Albumin is a family of globular proteins, the most common of which are the serum albumins. All the proteins of the albumin family are water-soluble, moderately soluble in concentrated salt solutions, and experience heat denaturation. Albumins ...
) − (albumin level of ascitic fluid).
Ideally, the two values should be measured at the same time.
This phenomenon is the result of Starling's forces between the fluid of the circulatory system and ascitic fluid. Under normal circumstances the SAAG is < 1.1 because serum oncotic pressure (pulling fluid back into circulation) is exactly counterbalanced by the serum hydrostatic pressure (which pushes fluid out of the circulatory system). This balance is disturbed in certain diseases (such as the
Budd–Chiari syndrome
Budd–Chiari syndrome is a very rare condition, affecting one in a million adults. The condition is caused by occlusion of the hepatic veins that drain the liver. It presents with the classical triad of abdominal pain, ascites, and liver enlarge ...
, heart failure, or liver cirrhosis) that increase the hydrostatic pressure in the circulatory system. The increase in hydrostatic pressure causes more fluid to leave the circulation into the peritoneal space (ascites). The SAAG subsequently increases because there is more free fluid leaving the circulation, concentrating the serum albumin. The albumin does not move across membrane spaces easily because it is a large molecule. A rare cause of ascites, with elevated SAAG, and without change in hydrostatic/osmotic pressure is urinary bladder rupture with leakage of urine into the peritoneal space.
Differential
High gradient
A high gradient (> 1.1 g/dL, >11g/L) indicates the ascites is due to portal hypertension, either liver related or non-liver related, with approximately 97% accuracy. This is due to increased
hydrostatic pressure
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body " fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imm ...
within the blood vessels of the
hepatic portal system
In human anatomy, the hepatic portal system is the system of veins comprising the hepatic portal vein and its tributaries. It is also called the portal venous system (although it is not the only example of a portal venous system) and splanchnic ...
, which in turn forces water into the
peritoneal cavity
The peritoneal cavity is a potential space between the parietal peritoneum (the peritoneum that surrounds the abdominal wall) and visceral peritoneum (the peritoneum that surrounds the internal organs). The parietal and visceral peritonea are la ...
but leaves proteins such as albumin within the vasculature.
Important causes of high SAAG ascites (> 1.1 g/dL, >11 g/L) include:
cirrhosis of the liver
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, and end-stage liver disease, is the impaired liver function caused by the formation of scar tissue known as fibrosis due to damage caused by liver disease. Damage causes tissue repai ...
portal vein thrombosis
Portal vein thrombosis (PVT) is a vascular disease of the liver that occurs when a blood clot occurs in the hepatic portal vein, which can lead to increased pressure in the portal vein system and reduced blood supply to the liver. The mortality ...
, and idiopathic portal fibrosis.
Low gradient
A low gradient (< 1.1 g/dL, <11 g/L) indicates causes of ascites not associated with increased portal pressure such as:
tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, i ...
,
pancreatitis
Pancreatitis is a condition characterized by inflammation of the pancreas. The pancreas is a large organ behind the stomach that produces digestive enzymes and a number of hormones. There are two main types: acute pancreatitis, and chronic pancr ...
,
infections
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable di ...
,
serositis
Serositis refers to inflammation of the serous tissues of the body, the tissues lining the lungs (pleura), heart (pericardium), and the inner lining of the abdomen (peritoneum) and organs within. It is commonly found with fat wrapping or creeping ...
, various types of peritoneal
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...