In the field of pediatric laboratory medicine (PLM), one of the truisms that you hear over and over is that “children are not small adults.” This saying is a truism for a reason. Metabolically and biochemically children are often as different from adults as it is possible to be. The basic chemical and hematological components of blood, urine, cerebral spinal fluid, and other body fluids are often more diverse and almost always more dynamic than comparable components in adults. A pediatric patient is rarely static in any way. Even when physically well, an infant and child is constantly growing and changing, and analytes that we measure in blood and other body fluids reflect those changes. In contrast to infants and children, a chemical component in an adult may not change much over multiple decades unless the person has a pathophysiological problem occurring; however, in an infant these parameters may change hourly. Total bilirubin measurements are a good example of this type of rapid changes in infancy.
In addition, especially in infancy, the body can change much more rapidly from a critical state to a well state and vice versa. Infants have very little in the way of reserves, such as stored fats and/or glycogen, and a higher percentage of their body is water. Thus, for example, infants can easily become critically hypoglycemic with even short periods of fasting. Critical electrolyte imbalances can occur with a relatively brief intake of improperly made formula, and dehydration is common after fairly short periods without fluid intake, or of vomiting and diarrhea. At the same time, infants possess a great deal of plasticity – an enormous ability to deal with rapid internal as well as environmental changes. For example, infants can survive brief periods of blood pH values in the 6.8 range with no lasting physical harm. All of these scenarios are uncommon in the adult population.
Other aspects of PLM also cause this part of laboratory medicine to be special. Infants and children are subject to some disease processes not found routinely in adults, and conversely some diseases are found only in adults. These differences in disorders require that different tests be offered in pediatric labs and in adult labs. Thus serum protein electrophoresis for diagnosing multiple myeloma is not found in pediatric laboratories, as multiple myeloma is not a disease of childhood. On the other hand catecholamines, such as vanillylmandelic acid and homovanillic acid are rarely analyzed in adult labs, but not uncommon in pediatric ones, as neuroblastoma is a childhood cancer. While most basic testing such as that included in a basic or comprehensive metabolic panel is essentially identical between children and adults, some of the tests found in those panels have different significance in the pediatric population. Thus test menu differences will be found between the two types of labs and test interpretation may differ as well.
Along with the different disorders found between the two populations, some diseases have begun crossing over between the age-groups in recent years. Disorders that used to be considered pediatric are now being found in the adult population as improving medical treatment for pediatric diseases allows these children to survive to adulthood. For example, cystic fibrosis and many other inborn errors of metabolism (IEM) now require clinics to treat adults with these disorders. Pediatric labs commonly are asked to provide IEM testing for some adult patients now, and pregnant women who themselves were born with an inborn error of metabolism are requiring close monitoring of their own disease and prenatal testing for their unborn child.
Conversely, some disorders that were at one time considered adult-only are now being increasingly seen in the pediatric population, like the spectrum associated with obesity and type 2 diabetes. The obesity epidemic in children will cause more and more testing once considered part of the adult realm, such as lipid analyses, to be necessary in children.
Thus laboratory medicine practiced at a pediatric institution has unique characteristics specific to pediatrics. In addition, infants and children have a limited blood supply. This particular issue affects many of the basic day-to-day mechanics of running a pediatric laboratory, as will be seen in the chapters of this book. Many aspects of laboratory medicine are affected by the differences seen in the pediatric population, all the way from basic, day- to-day operational issues through test selection for pediatric-specific disorders, and on up to age-related reference intervals and changes in basic tests during growth and changing metabolism. Specialty areas also abound in PLM, including such areas as testing for inborn errors of metabolism, testing of hormones during puberty, and prenatal and maternal-fetal testing. Training a new practitioner in PLM must incorporate various aspects specific to children. This book is intended as a resource for the combined knowledge needed for basic PLM training.