The complete blood count (CBC) provides a high-level assessment of a patient's immunologic state and guides the diagnosis and treatment of almost all diseases. Hematology analyzers evaluate CBCs by making high-dimensional single-cell measurements of size and cytoplasmic and nuclear morphology in high throughput, but only the final cell counts are commonly used for clinical decisions. Here, we utilize the underlying single-cell measurements from conventional clinical instruments to develop a mathematical model guided by cellular mechanisms that quantifies the population dynamics of neutrophil, lymphocyte, and monocyte characteristics. The dynamic model tracks the evolution of the morphology of WBC subpopulations as a patient transitions from a healthy to a diseased state. We show how healthy individuals and hospitalized patients with similar WBC counts can be robustly classified based on their WBC population dynamics. We combine the model with supervised learning techniques to riskstratify patients under evaluation for acute coronary syndrome.In particular, the model can identify more than 70% of patients in our study population with initially negative screening tests who will be diagnosed with acute coronary syndrome in the subsequent 48 hours. More generally, our study shows how mechanistic modeling of existing clinical data can help realize the vision of precision medicine.acute coronary syndrome | mathematical modeling | white blood cells | disease prognosis | population dynamics C irculating blood cells continuously interrogate almost all tissues in high throughput, and their collective states of maturation, activation, proliferation, and senescence reflect current pathophysiologic conditions: healthy quiescence, acute response to pathology, chronic compensation for disease, and ultimately, decompensation. Routine complete blood counts (CBCs) involve measurements of single-cell characteristics for tens of thousands of blood cells and provide a high-level view of these pathophysiologic states. Each routine CBC measures high-dimensional single-cell information, but clinical decisions are currently based on only a few derived statistics. The vast potential of the complete set of CBC measurements has been well-appreciated, with previous efforts attempting early detection of infection by identifying immature granulocytes or prognosis for some malignancies by counting the number of WBCs with atypical characteristics (1-3). These efforts have had limited impact but hint at the potential for enhanced clinical decision support (4, 5).Here, we develop mathematical models of the population dynamics of neutrophils, lymphocytes, and monocytes using these CBC measurements. To test the hypothesis that qualitative aspects of WBC population dynamics are altered in disease, we first compare WBC population dynamics in healthy individuals with those in patients with an acute illness requiring treatment in a hospital. We ensure that overall WBC counts are normal to investigate the effect of acute illness on WBC dynamics independ...