What is the difference between direct and indirect bilirubin?

Bilirubin is a breakdown product of heme

conjogated ...direct 

unconjogated ...indirect 

 Direct Bilirubin are the Bilirubin Conjugated &water soluble(bound to Glucuronic Acid ) and reacts quickly when dyes (diazo reagent) are added to the blood specimen to produce

 azobilirubin .

Indirect Bilirubin are the Bilirubin Unconjugated& fat soluble and reacts more slowly. Still produces azobilirubin. Ethanol makes all bilirubin react promptly then calc: Indirect bilirubin = Total bilirubin – Direct bilirubin

Bilirubin is a breakdown product of heme (which, in turn is part of the hemoglobin molecule that is in red blood cells). It is a yellow pigment that is responsible for the yellow color of bruises, and the yellowish discoloration of jaundice.

When old red cells pass through the spleen, macrophages eat them up and break down the heme into unconjugated bilirubin (which is not water soluble). The unconjugated bilirubin is then sent to the liver, which conjugates the bilirubin with glucuronic acid, making it soluble in water. Most of this conjugated bilirubin goes into the bile and out into the small intestine. (An interesting aside: some of the conjugated bilirubin remains in the large intestine and is metabolized into urobilinogen, then sterobilinogen, which gives the feces its brown color! Now you know.)

So: if you have an increase in serum bilirubin, it could be either because you’re making too much bilirubin (usually due to an increase in red cell breakdown) or because you are having a hard time properly removing bilirubin from the system (either your bile ducts are blocked, or there is a liver problem, like cirrhosis, hepatitis, or an inherited problem with bilirubin processing).

The lab reports the total bilirubin, and also the percent that is conjugated vs. unconjugated. If you have a lot of bilirubin around and it is mostly unconjugated, that means that it hasn’t been through the liver yet – so either you’ve got a situation where you’ve got a ton of heme being broken down (and it’s exceeding the pace of liver conjugation), or there’s something wrong with the conjugating capacity of the liver (like a congenital disorder where you’re missing an enzyme necessary for conjugation – for example, Gilbert syndrome). If you’ve got a lot of bilirubin around and it’s mostly conjugated, that means it’s been through the conjugation process in the liver – so there’s something preventing the secretion of bilirubin into the bile (like hepatitis, or biliary obstruction), and the bilirubin is backing up into the blood.

Unconjugated ("Indirect

Erythrocytes generated in the bone marrow are disposed of in the spleen when they get old or damaged. This releaseshemoglobin, which is broken down to heme as the globin parts are turned into amino acids. The heme is then turned into unconjugated bilirubin in the reticuloendothelial cells of the spleen. This unconjugated bilirubin is not soluble in water, due to intramolecular hydrogen bonding. It is then bound to albumin and sent to the liver.

The measurement of direct bilirubin depends on its reaction with diazosulfanilic acid to create azobilirubin. However, unconjugated bilirubin also reacts slowly with diazosulfanilic acid, so that the measured indirect bilirubin is an underestimate of the true unconjugated concentration.

Conjugated ("Direct")

In the liver bilirubin is conjugated with glucuronic acid by the enzyme glucuronyltransferase, making it soluble in water. Much of it goes into the bile and thus out into the small intestine. However,95% of the secreted bile is reabsorbed by the intestines(Terminal Ileum) and reaches the liver by portal circulation and then resecreted by the liver into the small intestine. This process is known as enterohepatic circulation.

About half of the conjugated bilirubin remaining in the large intestine (about5% of what was originally secreted) is metabolised by colonic bacteria to form urobilinogen, which may be further oxidized to urobilin and stercobilin. Urobilin, stercobilin and their degradation products give feces its brown color. However, just like bile, some of the urobilinogen is reabsorbed and95% of what is reabsorbed is resecreted in the bile which is also part of enterohepatic circulation. A small amount of the reabsorbed urobilinogen (about5%) is excreted in the urine following further oxidation tourobilin which gives urine its characteristic yellow color. This whole process results in only1–20% of secreted bile being lost in the feces. The amount lost depends on the secretion rate of bile.

Although the terms direct and indirect bilirubin are used equivalently with conjugated and unconjugated bilirubin, this is not quantitatively correct, because the direct fraction includes both conjugated bilirubin and δ bilirubin (bilirubin covalently bound to albumin, which appears in serum when hepatic excretion of conjugated bilirubin is impaired in patients with hepatobiliary disease).Furthermore, direct bilirubin tends to overestimate conjugated bilirubin levels due to unconjugated bilirubin that has reacted with diazosulfanilic acid, leading to increased azobilirubin levels (and increased direct bilirubin).

Bilirubin is a brownish-yellow pigment that is found in bile, which is a liquid secreted by the liver that allows the body to digest fats. There are two forms: direct and indirect. Direct bilirubin is often referred to as conjugated bilirubin, while indirect bilirubin is known as unconjugated bilirubin. The difference between them is that direct bilirubin is water-soluble, meaning that it can be dissolved in water, and indirect bilirubin is not.

• Unconjugated bilirubin—when heme is released from hemoglobin, it is converted to unconjugated bilirubin. It is carried by proteins to the liver. Small amounts may be present in the blood.
• Conjugated bilirubin—formed in the liver when sugars are attached (conjugated) to bilirubin. It enters the bile and passes from the liver to the small intestines and is eventually eliminated in the stool. Normally, no conjugated bilirubin is present in the blood.

Usually, a chemical test is used to first measure the total bilirubin level (unconjugated plus conjugated bilirubin). If the total bilirubin level is increased, the laboratory can use a second chemical test to detect water-soluble forms of bilirubin, called "direct" bilirubin. The direct bilirubin test provides an estimate of the amount of conjugated bilirubin present. Subtracting direct bilirubin level from the total bilirubin level helps estimate the "indirect" level of unconjugated bilirubin. The pattern of bilirubin test results can give the healthcare provider information regarding the condition that may be present.

In adults and older children, bilirubin is measured to:

• Diagnose and/or monitor diseases of the liver and bile duct (e.g., cirrhosis, hepatitis, or gallstones)
• Evaluate people with sickle cell disease or other causes of hemolytic anemia; these people may have episodes called crises when excessive RBC destruction increases bilirubin levels.

In newborns with jaundice, bilirubin is used to distinguish the causes of jaundice.

• In both physiologic jaundice of the newborn and hemolytic disease of the newborn, only unconjugated (indirect) bilirubin is increased.
• In much less common cases, damage to the newborn's liver from neonatal hepatitis and biliary atresia will increase conjugated (direct) bilirubin concentrations as well, often providing the first evidence that one of these less common conditions is present.