One of the advantages of IVFC compared to standard flow cytometry is that the sensitivity of the measurements, both in terms of the maximum and minimum number of cells that is detected, can be controlled to a significant extent. For example, the maximum number of cells that can be detected for a certain recording time period is determined directly by the frequency with which data is sampled. To achieve fast data processing, we analyze data that is sampled at a 5-KHz rate, which would allow detection of up to , a number that is significantly higher than the number of cells that we are typically interested in detecting. For example, only approximately 1100 white blood cells are expected to traverse an excitation slit focused on a blood vessel, when flowing at an average velocity of (typical for arterial mouse circulation). In principle, the maximum number of cells that can be detected will increase proportionately with the data sampling rate, and can exceed the total number of blood cells typically present within the sampled blood volume. Practically, some sensitivity limitations may arise when multiple cells traverse the excitation slit at exactly the same time. One of the main advantages of a noninvasive method such as IVFC for detecting such cells is that the monitoring time can be tailored to achieve the required sensitivity. In addition, as long as the cell population of interest is fluorescently labeled, either by fluorescent protein expression or antibody tagging, there is no risk of contaminating or eliminating any of the cells of interest due to processing or artifacts. So, for a 60-min data acquisition from a single blood vessel, IVFC samples of blood containing a total of approximately cells. If one of these cells is labeled, then it should be detectable by IVFC. If the cell population of interest is even scarcer, then we can increase the acquisition time or select a bigger blood vessel for data acquisition or modify the system to accommodate sampling from two or more blood vessels.