Hepatic synthetic function: Labs
Basic, Organ-Based and Clinical Sciences, Physiology - Renal/Urine/Electrolytes
The best lab test for measuring hepatic synthetic function is PT/INR. PT, or prothrombin time, is a laboratory measure of the extrinsic and common clotting system. INR, or international normalized ratio, is a standardized value that measures the same system but eliminates variance found in lab PT measurements. As almost all of the clotting factors in the body are produced in the liver, these tests are useful to measure hepatic synthetic function.
Factor VII is the clotting factor involved in the extrinsic clotting system that has the shortest half life (4-6 hours). This short half-life means that an acute decrease in hepatic synthetic function will be quickly reflected in worsening PT/INR values, as levels of Factor VII decrease. Elevated PT/INR can also be found in situations without worsened hepatic synthetic function, such as Vitamin K deficiency, warfarin use, presence of a clotting factor inhibitor, etc.
Albumin is another lab value to measure hepatic synthetic function, although it is not as useful as PT/INR. While albumin is made in the liver, it has a significantly longer half-life (3 weeks) than Factor VII. This means that more acute changes of liver function will not be accurately reflected in the albumin lab value. There are also many reasons to have decreased albumin that are not related to hepatic synthetic function, including malnutrition (decreased protein intake leads to decreased albumin production), volume overload (dilutional decrease in albumin levels), or kidney disease that leads to renal albumin excretion or proteinuria.
PTT, or partial thromboplastin time, is another measure of the clotting factors made in the liver. It primarily tests the common and intrinsic clotting systems. However, these clotting factors all have longer half-lives than Factor VII, making it an inferior test of hepatic synthetic function than PT/INR.
Other lab values that are commonly used to evaluate the liver and liver injury are not as useful to monitor hepatic synthetic function. Transaminases are often normal in cirrhotic patients, and are a better marker of acute hepatic injury. Indirect, or unconjugated, bilirubin levels may be increased due to liver disease, but may also be elevated due to hemolysis. Direct, or conjugated, bilirubin levels are increased with liver disease or some sort of biliary obstruction.