Table 1.

Comparison of differences in ⁵¹Cr and biotin RBC labeling for use in RBC population kinetic studies

Methodological features	51Cr	Biotin	Comments
1) Artifact from hemo-concentration or hemo-dilution	Yes	No	Both capillary and venous (or arterial) blood can be used with BioRBC54
2) Duration of accurate measurement of labeled RBC after reinfusion	30 d	>120+ d	Problem of variable 51Cr elution4
3) Ability to use multicolor flow cytometry to track transfused BioRBC changes by recovering these for ex vivo analysis	No	Yes	Useful for investigation of RBC senescence and age-dependent changes of normal and pathological RBC3
4) Ability to study >1 RBC population concurrently	No	Yes	Direct comparison to eliminate subject-to-subject variability
5) Use in vulnerable patient populations	No	Yes	Ethical considerations prohibit radiation exposure for research in vulnerable populations, e.g., fetuses, infants, children and pregnant women55
6) Volume of blood required by assay (mL)	0.1-1.0	0.01	BioRBC are suitable for fetuses and infants53
7) Radioactive waste disposal	Yes	No	51Cr disposal is a hazard and expense
8) Complexity of labeling procedure	1 wash	4-6 washes	4 h for biotin versus 1.5 h for 51Cr
9) Development of antibodies	No	Infrequent	There is no evidence of harm from anti-BioRBC antibodies25,56
10) Requirement for FDA IND (in U.S.A.)	No	Yes	Immunogenicity testing (https://www.federalregister.gov/documents/2016/04/25/2016-09449/assay-development-and-validation-for-immunogenicity-testing-of-therapeutic-protein-products-revised)