Information. Babies that don't thrive can get there two basic ways.First, some are not taking in enough nutrition to grow.Second, the babies organs are not working right and burning off fuel too fast, or losing it without digesting it properly.To decide how to fix the problem, they must get information on organ function which the blood test will tell.
Diagnostic tests. Blood tests are necessary to look for metabolic, endocrine, nutritional and some times genetic changes that may explain the failure to thrive.
Diagnosis. Failure to thrive, which is failure to meet accepted standards of development, weight and height, has many causes. Some may affect the blood picture and blood tests are done to determine both the possible diagnosis of the problem as well as the effect of the problem on body fluids which might need supplementation.