Hepatic Function: hepatocytes are metabolically active in many biochemical pathways involved in synthesis, degradation, transport, and elimination of many compounds.
The major adult blood protein is albumin, and the level of albumin gives an indication of hepatocyte synthetic functional capacity (in the absence of dietary deficiencies) over weeks to months, since the half-life is about 3 weeks.
When hepatocytes are injured, they can spill their contents, including enzymes. The two most commonly measured enzymes relating to liver are aspartate aminotransferase (AST) and alanine aminotransferase (ALT). Of the two, ALT is more specific for liver.
When any part of the biliary tract is obstructed, the alkaline phosphatase (Alk Phos) increases. However, alkaline phosphatase may also come from gastrointestinal tract, bone, and placenta.
The liver clears bilirubin from the blood, conjugates it, and excretes it into bile. A high unconjugated bilirubin alone suggests hemolysis with increased amounts of bilirubin that overwhelm the ability of the liver to clear it. A predominantly high conjugated bilirubin suggests biliary tract obstruction. Elevations in both conjugated and unconjugated bilirubin suggest problems with hepatocytes, such as hepatitis.
The liver manufactures the vitamin K dependent coagulation factors II, VII, IX, and X. A decrease in synthesis of these factors is more specifically measured by the prothrombin time (PT), which is a measure of the in vitro extrinsic coagulation pathway. Thus, a prolonged (increased) PT time may be an early indication of decreased liver function.
Pancreatic function: tests of pancreatic insufficiency with a stool specimen include quantitative stool fat and pancreatic elastase. The amount of stool fat is dependent upon diet, but also upon digestive function, particularly biliary tract and pancreatic function. The best test for steatorrhea is a 24 hour stool collection. For persons 6 years of age and older there are normally 0.0 - 6.0 g/d of fecal fat, assuming the patient is not on a low fat diet. However, a qualitative stool fat on a single sample may give relevant results.
Pancreatic elastase is measured with a single random stool sample. Normally there are 201-500 micrograms of elastase per gram of stool, only 100 - 200 with mild to moderate pancreatic exocrine dysfunction, and <100 with severe exocrine pancreatic disease.
Tests of pancreatic injury, typically pancreatitis, include lipase, which can be measured in serum and urine. Of the two, lipase is the better test.
Gastrointestinal tract: the GI tract is involved with digestion of food and absorption of nutrients.
Electrolytes and acid-base status: in general, vomiting leads to loss of hydrogen ion and chloride, resulting in hypochloremic metabolic alkalosis. In general, a severe watery diarrhea leads to loss of bicarbonate with metabolic acidosis.
D-Xylose absorption is a test for small intestinal function using carbohydrate absorption. The patient is given measured amount of d-xylose and urine is collected for the next 5 hours. Normally, 16 to 40% of the d-xylose dose should be excreted in the urine.
The hydrogen breath test can measure carbohydrate absorption, and specifically lactose. The patient is given an oral solution of lactose. Normally, colonic bacteria digest any unabsorbed lactose, resulting in elevated hydrogen content in exhaled air. If lactase deficiency is present, then the hydrogen in the breath increases.