Pediatric Reference Intervals

All laboratory professionals are aware of the importance of reference intervals (RI). Without appropriate reference intervals, a test result is just a number. A numeric result for any given analyte cannot be used to diagnose or monitor treatment of disease unless there is an accompanying reference interval indicating what amount of that analyte should normally be present. In pediatrics, that’s even more important because as an infant develops through childhood and into adulthood, his or her biochemistry changes, adapts and develops with him or her. Using an adult RI to interpret a test result from an infant or child is likely to result in misinterpretation of the test, including misdiagnosis of disease states. A good example would be using adult alkaline phosphatase RI to interpret the results of a teenage boy’s test during accelerated bone growth in puberty.  His result will look pathological if interpreted using adult RI. Pediatric reference intervals (PRI) are age-related and often also gender-related intervals that must be used to interpret testing in the pediatric population.

Establishing reference intervals is problematic at the best of times because of the need to use 120 healthy individuals to establish them. In the pediatric population, especially in infancy, obtaining 120 healthy infants at each necessary time interval can be a daunting task. There are references available that present methods which allow the establishment of RI with smaller sample numbers (1,2). Also the CLSI document (2) allows the “validation” and “transfer” of current or literature RI, rather than complete “establishment” of RI in some cases.  Validating a current reference interval can be done with as few as 20 samples in a correlation study. “Transferring” a reference interval also involves using a correlation study and bias evaluation to adapt or adjust a current RI for use with a new assay. Transferring can also be performed with 20 – 60 patient samples.

These techniques especially come in handy with the hardest PRI to establish, the hormones during puberty. During this time, the RI are not really related to the child’s age, but related to the child’s phase of development, or Tanner Stage. To establish a PRI for these hormones, the healthy child donating the blood sample must also have his/her Tanner Stage determined, usually by an Endocrinologist.

Another consideration when dealing with PRI is that although all pediatric institutions use PRI, not all PRI are the same, even when the same instrument is used. An informal poll of 9 pediatric institutions using the same instrument resulted in 9 different PRI for common analytes such as electrolytes. There is a need to harmonize PRI, as we harmonize test results, in order to allow non-pediatric institutions a set of PRI that they can use also.

1. Horn PS, Pesce AJ. Reference Intervals: a User’s Guide. AACC Press, Washington DC, 2005
2. Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory: Approved Guideline – Third ed. CLSI C28-A3

-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.