Obstruction may be complete or incomplete, extrahepatic or intrahepatic. Extrahepatic obstruction is most often produced by gallstones in the common bile duct or by carcinoma of the head of the pancreas. Intrahepatic obstruction is most often found in the obstructive phase of acute hepatocellular damage, as seen in “active” alcoholic cirrhosis, hepatitis virus hepatitis, in liver reaction to certain drugs such as chlorpromazine (Thorazine), and occasionally in patients with other conditions such as metastatic carcinoma. In one series, 12% of patients with liver metastases had a total bilirubin level more than 10 mg/100 ml (170 µmol/L). Serum bilirubin levels may become markedly elevated with either intrahepatic or extrahepatic obstruction. Although extrahepatic obstruction typically is associated with considerable bilirubin elevation, bilirubin values may be only mildly elevated in the early phases of obstruction, in persons with incomplete or intermittent obstruction, and in some persons with common duct obstruction by stones. In addition, occasional patients with intrahepatic cholestasis have considerably elevated serum bilirubin levels. Thus, the degree of bilirubinemia is not a completely reliable diagnostic point. Patients with extrahepatic obstruction typically have ALP elevation more than 3 times the upper reference limit, normal or minimally elevated AST levels, and moderately or considerably elevated total bilirubin levels, with 75% or more consisting of the conjugated fraction. In contrast, intrahepatic obstruction due to hepatocellular injury usually is associated with a considerably elevated AST level and a conjugated bilirubin/nonconjugated bilirubin ratio close to 1:1. Unfortunately, as time goes on, the serum bilirubin level in extrahepatic obstruction demonstrates a progressive decline in the conjugated bilirubin/nonconjugated bilirubin ratio, until a ratio not far from 1:1 is reached. Also, the AST level may increase somewhat as liver cells are damaged by distended biliary ductules. Therefore, differentiation of long-standing extrahepatic obstruction from intrahepatic obstruction may not always be easy. Metastatic tumor to the liver has the typical laboratory picture of extrahepatic obstruction without jaundice. Occasionally, however, it is accompanied by jaundice. Finally, some drugs such as chlorpromazine, anabolic steroids, or oral contraceptives may occasionally produce liver dysfunction that has a predominantly cholestatic-type biochemical pattern.

Some Etiologies for Aspartate Aminotransferase Values Over 1000 IU/ml
Liver origin
Acute hepatitis virus hepatitis
Chronic active hepatitis (occasional patients; 16% in one study)
Reye’s syndrome
Severe liver passive congestion or hypoxia (with or without acute MI, shock, or sepsis)
Drug-induced (e.g., acetaminophen)
HELLP syndrome of pregnancy (some patients)
Other
First 2-3 days of acute common bile duct obstruction
Acute myocardial infarct (occasional patients)
Severe rhabdomyolysis

If extrahepatic obstruction is a possibility, it can be investigated with ultrasound on the biliary tract. If results of the ultrasound study are normal or equivocal and extrahepatic obstruction is strongly suspected, one of the other studies discussed previously can be attempted.