CB = conjugated bilirubin, and "CB%<20%" means that less than 20% of the bilirubin in the blood is conjugated. Normally, unconjugated bilirubin (UCB) is formed from senescent (old and messed up) RBCs that are destroyed by splenic macrophages; UCB is the product of heme degradation. Once it's inside the liver, UDP-glucoronyl transferase adds a UDP-glucoronic acid to UCB to make it CB. Under normal circumstances, this process should commit CB to be secreted into the bile ducts and not go back in the blood. Thus, all normal bilirubin in the blood should be UCB.
In hyperbilirubinemia, something is out of whack and that can cause CB to leak back into the blood. There are a few arbitrary cutoffs based on the percentage of CB that's in the blood"
- CB<20% (unconjugated hyperbilirubinemia): extravascular hemolytic anemias, ineffective/absent UDP-glucoronyl transferase; basically, there's either an overproduction of underprocessing of UCB
- CB 20-50% (mixed hyperbilirubinemia): hepatitis and cirrhosis. A mix of impaired function and distorted architecture can cause underprocessing and leakage of CB back into the blood, respectively.
- CB>50%: (conjugated hyperbilirubinemia): obstructive liver diseases, biliary tract disease, etc. Basically, there's no problem with processing, but there's a problem with getting the CB excreted out of the liver. All the CB backs up and out into the liver like an overflowing toilet.
Hope that helps.
