Complete blood count and white blood cell differential are different in children and in adults.

The number of red blood cells in newborns is 6-7×10¹²/l; in 10-day-old babies it is 4.5-5.5×10¹²/l; at the age of 1 month – 4.0-5.0×10¹²/l; since the age of 1 year until 10 years old – 4.0-4.5×10¹²/l; in 15-years-old teenagers it becomes 4.0-5.5×10¹²/l.

The number of platelets in newborns is 180-490×10⁹/l, whereas it ranges from 160 to 390×10⁹/l depending on the age (for children of 1-12 years old).

The number of white blood cells in newborns is 10-30×10⁹/l, while it is 6-17×10⁹/l at the age of 1-3 years, and then becomes 6-11×10⁹/l for those aged 6-10.

Neutrophil to lymphocyte ratio changes during the first years of life (see the figure below):

For example, after azure II and eosin staining of blood smear of a two-year-old toddler, lymphocytes #1, #2, #3, #4, #5 are found to be more common than neutrophils #1, #2, #3. The smear also shows some other white blood cells, e.g. eosinophils #1, #2 and monocytes #1, #2.