I do not think that what you are describing should be called
breastmilk jaundice.  It is *normal* for exclusively breastfed well
gaining babies to be jaundiced, sometimes into the third month.
Usually, this is very mild and not noticed unless one looks for it.
But, as breastfeeding is the physiologic mode of feeding, and
breastfed babies are almost always jaundiced, even if only a little,
it is normal to be jaundiced.  What is not normal and thus deserves a
name is the absence of jaundice in artificially fed babies.  This
could be called artifical feeding anicterus, probably a benign
syndrome, but there is no proof yet that it is.

If the baby is thriving and healthy, there really should not be any
tests done at all.  Leave the little blighter alone.  However, since
most lactneters live in litigation heaven (USA), you might consider:

congenital hypothyroidism:  Usually picked up in the screen, but
everyone messes up occasionally.  It is rare (1:6000 live births).
Babies tend to be constipated, sometimes sleepy, but may present with
jaundice only.

urinary tract infection:  I have checked the urine on nearly every
baby I have ever seen in the clinic in 14 years (about 10000), and
picked up 4 urinary tract infections, only one having anything to do
with jaundice.

Make sure this is not direct hyperbilirubinemia, which is *always*
abnormal, and often serious.  The baby's urine will be brownish.

Jack Newman, MD, FRCPC