blood test

One of the functions of the liver is to help bilirubin bind (conjugate) into something water-soluable so it can be excreted in the urine and stool. Bilirubin is a byproduct of the "heme" part of hemoglobin created when red blood cells are broken down. In infants, immature livers often mean this byproduct can't be conjugated and excreted quickly enough, causing a build up in the blood stream called jaundice. The point of phototherapy is to use light waves to transform the unconjugated (also called indirect) bilirubin in the blood into a water-soluable form so that it can be excreted while bypassing the liver.

So, based on that explanation, you'll see that a blood test to measure hemoglobin or RBC level wouldn't tell you whether phototherapy is indicated, because that level reflects healthy red blood cells, not those that have been broken down. That leaves you with your second choice, plasma bilirubin levels, specifically indirect/unconjugated bilirubin.

Thanks for explaining this. You're very knowledgeable!

BIG HINT: What is phototherapy for?

thanks for your help, I thought it was bilirubin but a friend of mine thought it might be the Coombs test. So I was just looking for some clarification.

thanks for your help, I thought it was bilirubin but a friend of mine thought it might be the Coombs test. So I was just looking for some clarification.

Coombs test is used to test for antibodies when the mother is Rh negative and the baby is Rh positive.