Systematic review: differences in complete blood count component rhythms

Study Objectives: The complete blood count (CBC) is one of the most commonly ordered blood tests with a large range of reference values that does not consider time of day for interpretation. Our objective was to systematically review this topic to report on peak and trough timing of CBC values. Methods: A systematic search was performed for studies evaluating any component of the CBC with at least three collections over 24 hours. The studies were screened based on the predetermined eligibility criteria. Meta-analysis of aggregated data was analyzed with polynomial functions and forest plots. Results: In total, 164 full-text articles were screened and 32 included in the final analysis with 548 total patients considering either leukocytes (n = 13), erythrocytes (n = 7), hemoglobin (n = 5), hematocrit (n = 5), platelets (n = 12), neutrophils (n = 11), lymphocytes (n = 13), monocytes (n = 8), eosinophils (n = 15), or basophils (n = 9). CBC components were analyzed by polynomial and forest plot analysis. Lymphocytes fitted best to a third-degree polynomial function (p = .010) with peak at 2264.87 cells/µL at 23:54 (CI: 1783.44 to 2746.31) with a trough of 1598.91 cells/µL at 10:47 (CI: 1230.12 to 1967.71). Lymphocytes and eosinophils peaked overnight, while erythrocytes, hemoglobin, and hematocrit peaked in the morning, and platelets, neutrophils, monocytes, and basophils peaked in late afternoon. Limitations include small sample size and significant study heterogeneity. Conclusion: We identified a limited scope of studies characterizing CBC component rhythms. However, we still noted significant differences, particularly with lymphocytes. Future work should evaluate larger datasets to inform time-dependent interpretation of the CBC as we move toward precision medicine.