Children develop in different ways. Systems and organs can mature at different rates, and the ratio of blood cells, levels of biochemical markers, trace elements, hormones, and vitamins will be constantly changing. Therefore, it is impossible to display average values or analyze data automatically. There are too many indicators that change over time—often abruptly. To suggest a pathological process in time or to avoid false diagnoses, it is necessary to take into account the peculiarities of the current age of the child. Here are some examples: ## Physiological crossover In adults, the normal ratio of [neutrophils](https://ul.orna.me/KOge/librarybiomarker?id=93) to [lymphocytes](https://ul.orna.me/KOge/librarybiomarker?id=135) is approximately 2:1 (around 60 % and 30 %). The ratio is the same in newborns and adolescents. At the age of five days and five years, there is a "physiological cross"—the number of lymphocytes and neutrophils is the same. In the period from five days to five years, the ratio gradually changes: first, the number of neutrophils to the number of lymphocytes becomes 1:2 (around 30 % and 60 %, respectively), then the percentage of lymphocytes decreases, and the percentage of neutrophils increases. If misinterpreted, normal values are seen as a sign of an inflammatory process. Through this, obvious signs of the disease may go unnoticed. ## Passive immunity In the baby's blood, antibodies transferred from the mother during fetal development or with milk can circulate, creating passive immunity. These immune defenses can persist for weeks or months after birth. A positive antibody test in adults indicates a current or previous illness or preventive vaccinations. When interpreting the test results of infants, it is necessary to take into account the likelihood of maintaining passive immunity—in this case, a positive test does not indicate disease. ## Hormones The loads and stress that the body experiences during puberty are so great that they can be compared to the loads experienced at birth, which is why this period is sometimes called the "second birth." Puberty in girls occurs in the period from 11 to 18 years (the range is from 9 to 21 years), and in boys from 13 to 20 years (the range is from 10 to 25 years). Hormonal changes in adolescence can proceed violently and quickly, or vice versa, gradually and for a long time, influencing all organs and systems. During this period, hereditary pathologies, and autoimmune processes may appear. ## Physical indicators Test results can also depend on individual parameters such as weight, muscle mass, mobility, nutrition, and more. For example, the level of creatinine depends on muscle mass and physical activity, and the composition of the microbiota depends on nutrition (breastfeeding or artificial feeding, the introduction of complementary foods, etc.). ## Proper preparation Most blood tests are done in the morning on an empty stomach (for those children who understand the need to postpone breakfast) or no earlier than 2 hours after feeding the baby. You also need to try to reduce the child's anxiety and fear of the upcoming procedure because stress may cause some indicators to change, which can lead to an incorrect assessment of the baby's condition.