Liver Function Tests
Liver Enzyme Tests: Alkaline Phosphatase, 5’ Nucleotidase, and GGT
Another of the liver's key functions is the production of bile, which helps digest fat. Bile flows through the liver in a system of small tubes (ducts), and is eventually stored in the gallbladder, under the liver.
When bile flow is slow or blocked, blood levels of certain liver enzymes rise:
- Alkaline phosphatase
- 5' nucleotidase
- Gamma-glutamyl transpeptidase (GGT)
Liver tests may check for any or all of these enzymes in the blood. Alkaline phosphatase is by far the most commonly tested of the three.
If alkaline phosphatase and/or 5' nucleotidase and GGT are elevated, a problem with bile flow is most likely present. Bile flow problems can be due to a problem in the liver, the gallbladder, or the tubes connecting them.
Liver Function Tests: PT and INR
Besides its functions in metabolism, the liver makes proteins that are essential to normal blood clotting. True liver function tests check the liver's ability to make these proteins. They include:
- Prothrombin time (PT): A test of the time it takes for a blood sample to clot, under specific conditions in a lab. If low levels of clotting factors are present, the prothrombin time is longer.
- International normalized ratio (INR): Not really a test, but a standardized way for all labs to report PT, so their results can be compared accurately with each other.
PT and INR rise in people with severe liver disease because the liver fails to make normal amounts of certain clotting factors. An elevated PT can have many other causes besides liver disease, however.
PT is often checked together with PTT (partial thromboplastin time), which is not a liver function test. If PT and/or PTT are elevated, a problem with bleeding or clotting may be present.
Liver Function Tests: Albumin
The liver also makes albumin, an essential protein that circulates in blood. Albumin levels are low in people with severe chronic liver disease, because the liver does not make normal amounts of albumin. However, albumin levels may fall in a variety of medical conditions. A low albumin level is often temporary, so it is not a reliable way to diagnose liver disease.