Aim: Determine reference ranges of lymphocyte subsets in healthy Qatari adults. Patients & Methods: Blood samples (n = 150) were investigated using four colors flow cytometery. Results: The mean percentage and absolute values of the lymphocyte subsets were: CD3: 73.6 ± 6.1%; 2,072 ± 644 cells/μl; CD4: 41.1 ± 7.9%; 1,167 ± 396 cells/ μl; CD8: 33.8 ± 7.1%; 967 ± 364 cells/μl; B cells (CD19+): 11.3 ± 3.9%; 318 ± 144 cells/μl and natural killer (NK) cells (CD16/56+): 16.5 ± 6.3%, 440 ± 401 cells/μl, respectively. The CD4/CD8 ratio was 1.3 ± 0.5. The ratio, CD3+ and CD4+ percentage and CD4+ cell counts were significantly higher in females; CD8+ and NK cell count and CD8+ percentage were significantly higher in males. The impact of age was less certain. Conclusion:The reference values of lymphocyte subsets appear to be specific to this population, demonstrating the importance of establishing local reference ranges. Keywords: B cells • CD3 • CD4 • CD8 • flow cytometry • lymphocyte subsets • natural killer cells • reference ranges, IntroductionLymphocyte immunophenotyping by flow cytometry is an important tool in differential diagnosis of congenital and acquired immune deficiencies, lymphoproliferative disorders and acute leukemias [1]. In addition, it is used for monitoring treatment response and disease progression with anti-retroviral and immunosuppressive therapy. Various studies suggest that lymphocyte subsets reference ranges are influenced by age, sex, ethnicity, and environment [2][3][4][5]. The variations in the values of the lymphocyte subsets emphasize the importance of establishing local reference ranges to allow for meaningful comparisons and accurate interpretation of immunophenotyping data across different cultures and ethnic groups. Lifestyle factors, e.g. smoking, have also been shown to influence lymphocyte subset values [6,7].Indeed, reference range for peripheral blood lymphocyte subsets have been established in many studies across different countries, revealing variations in the normal range for lymphocyte subsets according to age, sex, race, and environmental factors [3,4,[8][9][10][11][12][13][14][15][16]. Establishment of an internal normal range of lymphocyte subsets is a important for each population considering the effect of environmental factors, age, ethnical diversity on the immune system. Hence, this study aims to determine the laboratory reference ranges for lymphocyte subsets in healthy Qatari adults using flow cytometry.This article reports the local reference range for peripheral blood lymphocyte subsets, both percentages and absolute counts, in a representative sample of 150 healthy adult Qatari population.The results have been stratified by age, sex, and smoking status. The population selected is largely homogenous (100% Qatari Arabs) so effect of race was not investigated. The results are briefly compared with other population data from published studies. Materials & methods SubjectsA total of 150 healthy Qatari volunteers (18-55 yr old; all Qatari Arabs) were recruited for the stud...