What is a primary concern when using a modified in vivo method?

The primary concern when using a modified in vivo method is the risk of inadequate labeling of red blood cells. In nuclear medicine, particularly in procedures such as red blood cell tagging for imaging purposes, it is crucial that the tracer is adequately bound to the red blood cells to ensure accurate imaging and diagnostic results. If the labeling is inadequate, it can lead to false negatives or inaccurate representation of blood flow and volume, compromising the effectiveness of the study.

Additionally, while other options address various aspects of nuclear medicine procedures, they do not capture the critical impact that inadequate labeling can have on the reliability of the diagnostic information obtained. Adequate tracer binding is essential for producing clear and interpretable results, making the concern about labeling a focal point in modified in vivo methods.