Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q.
When reporting reference ranges for absolute differential counts, should the ranges be age specific or is a single reference range acceptable?

A.January 2022—Laboratories should evaluate healthy people within such categories as gender, age group, and genetic background when establishing institutional reference intervals for a given analyte. This is necessary because various intrinsic and extrinsic factors can influence RIs in a laboratory or patient population, or both. This evaluation also helps determine where certain populations may overlap and, therefore, where groups may be combined.¹ Reference intervals are usually based on Gaussian distribution and defined as the range of values into which 95 percent of healthy (nondiseased) individuals fall. Accordingly, five percent of healthy people will have laboratory results above or below the established RI.

Recruiting healthy individuals, particularly from the pediatric population, to establish institution-specific RIs can be costly and time intensive and may preclude laboratories from running robust studies. Laboratories that cannot perform institution-specific studies may use previously established RIs from a laboratory database of test results.² It takes a much smaller pool of participants to verify previously reported RIs, making these studies more palatable to most laboratories. Regardless of the method employed, the reporting laboratory should independently verify or validate the RIs and tailor them to its patient population. Independent verification is important as normal ranges vary slightly depending on the automated analyzer used.

Immune cellular constitution matures throughout childhood and adolescence and is relatively stable in adulthood (over 18 years old). The total white blood cell count and absolute differential count for cell types tend to be higher in newborns and decrease until about five to 10 years of age. Because of these variances, most published RIs are partitioned into different groups for neonates through 18 years old. These partitions vary by institution and depend on the initial studies performed or prior studies verified. Total WBC counts and differential counts show small variances between males and females, with secondary partitions provided for gender. Large studies have found no significant differences between males and females with regard to WBC counts and differential counts and no age dependency between 18 and 100 years old. Therefore, RIs for the age interval of 18 to 100 years for men and women are usually combined. Age-dependent RIs more accurately represent the patient population being examined and, therefore, provide clinicians with a better platform for diagnosing and managing various disease states.³

A common question is whether to provide WBC RIs as proportional amounts (percentages) or absolute counts, or both. According to the Clinical and Laboratory Standards Institute, providing absolute concentrations of WBCs is preferred given that these are the medically significant values.⁴ In accordance with the CLSI recommendation, the College of American Pathologists hematology and coagulation checklist item HEM.36820 states, “For WBC differential counts, the CAP recommends that laboratories report absolute cell counts, along with their corresponding reference intervals. The CAP discourages the reporting of percent cell counts without absolute counts on WBC dif­ferentials.”⁵

1. Katayev A, Balciza C, Seccombe DW. Establishing reference intervals for clinical laboratory test results: is there a better way? Am J Clin Pathol. 2010;133(2):180–186.
2. Soldin SJ, Brugnara C, Wong EC. Pediatric Reference Intervals. 6th ed. AACC Press; 2007.
3. Valiathan R, Ashman M, Asthana D. Effects of ageing on the immune system: infants to elderly. Scand J Immunol. 2016;83(4):255–266.
4. Clinical and Laboratory Standards Institute. H20-A2: Reference Leukocyte (WBC) Differential Count (Proportional) and Evaluation of Instrumental Methods; Approved Standard, 2nd ed.; 2007.
5. College of American Pathologists. HEM.36820 Reference intervals. In: Hematology and coagulation checklist. Sept. 22, 2021.

Additional references

Chabot-Richards DS, George TI. White blood cell counts: reference methodology. Clin Lab Med. 2015;35(1):11–24.

International Council for Standardization in Haematology, Writing Group; Briggs C, Culp N, et al. ICSH guidelines for the evaluation of blood cell analysers including those used for differential leucocyte and reticulocyte counting. Int J Lab Hematol. 2014;36(6):613–627.