Laboratory tests provide valuable information necessary to evaluate a patient's condition and to monitor recommended treatment. Chemistry and hematology test results are compared with those of healthy individuals or those undergoing similar therapeutic treatment to determine clinical status and progress. In the past, the term normal ranges relayed some ambiguity because statistically, the term normal also implied a specific (Gaussian or normal) distribution and epidemiologically it implied the state of the majority, which is not necessarily the desirable or targeted population. This is most apparent in cholesterol levels, where values greater than 200 mg/dL are common, but not desirable. Use of the term reference range or reference interval is therefore recommended by the International Federation of Clinical Chemistry (IFCC) and the Clinical and Laboratory Standards Institute (CLSI, formerly the National Committee for Clinical Laboratory Standards [NCCLS]) to indicate that the values relate to a reference population and clinical condition.
Reference ranges are established for a specific age (eg, alpha-fetoprotein), sex, and sexual maturity (eg, luteinizing hormone and testosterone); they are also defined for a specific pharmacologic status (eg, taking cyclosporine), dietary restrictions (eg, phenylalanine), and stimulation protocol (eg, growth hormone). Similarly, diurnal variation is a factor (eg, cortisol), as is degree of obesity (eg, insulin). Some reference ranges are particularly meaningful when combined with other results (eg, parathyroid hormone and calcium), or when an entire set of analytes is evaluated (eg, lipid profile: triglyceride, cholesterol, high-density lipoprotein, and low-density lipoprotein).
Laboratory tests are becoming more specific and measure much lower concentrations than ever before. Therefore, reference ranges should reflect the analytical procedure as well as reagents and instrumentation used for a specific analysis. As test methodology continues to evolve, reference ranges are modified and updated.
CHALLENGES IN DETERMINING & INTERPRETING PEDIATRIC REFERENCE INTERVALS
The pediatric environment is particularly challenging for the determination of reference intervals since growth and developmental stages do not have a distinct and finite boundary by which test results can be tabulated. Reference ranges may overlap and, in many cases, complicate diagnosis and treatment. Collection and allocation of test results by age for the purpose of establishing a reference range is a convenient and manageable way to report them, but caution is needed in their interpretation and clinical correlation.
A particular difficulty lies in establishing reference ranges for analytes whose levels are changed under scheduled stimulation conditions. The common glucose tolerance test is such an example, but more complex endocrinology tests (eg, stimulation by clonidine and cosyntropin) require skill and extensive experience to interpret. Reference ranges for these serial tests are established over a long period of time and are not easily transferable between test methodologies. Changing analytical technologies add a new dimension to the challenges of establishing pediatric reference ranges.
K: Special Issue on Laboratory Reference Intervals. eJIFCC. September 2008. http://www.ifcc.org/
C28A3: Defining, Establishing, and ...