My 6 yr old daughter got a routine cbc done and found that her hemoglobin levels were high. 15.5mg/dl..anything to worry about? she is a known asthma

Probably OK: For her age, normal is 12-14.5. Her level of 15.5 will not cause her any health issues. One trivial explanation for a slightly higher HGB would be if she was a little dehydrated when the blood test was drawn. If she is otherwise doing well (rest of CBC ok and asthma under good control), i would nothing further at this point. Perhaps recheck in 6-12 months to make sure it hasn't risen more.

Repeat again : First is to repeat again .Dehydration commonly can give you a high number. Also will depend on how significant her asthma is. If she has severe asthma and has frequent severe attacks with multiple visits to er /admission- then perhaps oxygen deprivation related to the asthma attacks trigger her body to make more red blood cells to compensate with her low oxygen level -that can increase hb level.