**5. Conclusions**

In this work, the aggregation of red blood cells was studied using different optical in vivo and in vitro measurement techniques. The aggregation for patients with coronary heart disease was statistically significantly enhanced compared to the control group. In vivo and in vitro methods yielded correlated results: The faster the cells moved in the capillaries, the less cells aggregated in vitro. Type 2 diabetes mellitus had an additional significant effect on the aggregation properties of coronary heart disease patients. These findings are prominent for diagnosing and monitoring the state of patients with pathologies that affect blood properties.

**Author Contributions:** This paper is a result of the cooperation and contribution of all authors. Conceptualization, A.P. (Alexander Priezzhev), and Y.G.; investigation, P.E., A.M., and Y.G.; methodology, M.S., supervision, A.P. (Alexandra Pigurenko) and A.L. All authors have read and approved the published version of the manuscript.

**Funding:** This research was funded by RFBR, gran<sup>t</sup> number 19-52-51015.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of the medical research and educational center of M.V. Lomonosov Moscow State University, Moscow, Russia (protocol code: 1/19, date of approval: 18 February 2019).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the ethical and privacy issues.

**Acknowledgments:** The authors would like to extend thanks for the financial support provided to this study by RFBR (19-52-51015).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
