Monday, 21 April 2008

How do urine dipsticks help in jaundiced patients?

As you may have found already, jaundice is readily classified in a helpful way. Personally, my classification initially divides jaundice up into three groups:
  • Haemolysis (prehepatic destruction of erythrocytes)
  • Hepatocellular (predominant rise in AST and ALT)
  • Obstructive (predominant rise in Alk. phos and GGT)

I prefer this over the common alternative (prehepatic, intrahepatic, posthepatic) because I find it more helpful clinically. Thus, 'obstructive' could mean that the jaundice is caused by an obstructive picture either intrahepatically or extrahepatically.

Anyway, urine dipsticks usually measure both urobilinogen and bilirubin.

There is usually no bilirubin found in urine. That is because before it gets to the liver, it is unconjugated and hence not water-soluble (it has to bind to albumin). Hence, it isn't filtered by the kidneys. After it is conjugated by the liver, it is passed straight into the bile, to be excreted via the intestine. Therefore, it never normally reaches the bloodstream and so it also can't be filtered by the kidney.

Whew! OK, let's put that altogether. If the urine dipsticks come back positive for bilirubin, it means that (1) there is a pathological increase in serum bilirubin concentration and (2) the cause must be hepatocellular or obstructive (either intrahepatic or posthepatic). That's because only conjugated bilirubin is water-soluble enough to be filtered by the glomeruli. Haemolysis, for instance won't produce excessive bilirubin in the urine.

Now for urobilinogen. This substance is formed by the action of intestinal bacteria on the bilirubin in the bile - they deconjugate it and reduce it. Some urobilinogen is then reabsorbed by the intestine. If it gets past the liver (which shoves it straight back into the bile for excretion) it can be filtered by the kidney and thus appear in the urine.

So, if you get urobilinogen showing up on your urine dipsticks, it means that the haemolysis can't be obstructive (either intrahepatic or posthepatic) since bilirubin must get to the intestine for any of it to become urobilinogen.

Putting that all together then, there are three possible options, and each suggests (but isn't 100% accurate) one pathological category.

  • Bilirubin positive, urobilinogen negative - this would suggest obstructive jaundice.
  • Bilirubin negative, urobilinogen positive - this would suggest haemolysis as the cause.
  • Bilirubin positive, urobilinogen positive - this would suggest a hepatocellular cause for the jaundice.

As I said, this isn't foolproof, but is handy.

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