In surveying some of the limitations of studies with Cr51 and Fe59 my purpose has been not to deny their usefulness but to put them in perspective. These technics have made possible many extensions of the fundamental understanding of red cell physiology and iron metabolism. They will continue to be valuable experimental tools. It is hoped that if some of the limitations of isotope technics are considered, the non-experimental use of these isotopes will be confined to situations in which otherwise unavailable information of diagnostic or therapeutic importance can be obtained. Unfortunately isotope technics are used when more conventional means would be adequate or even preferable. An extreme example is the suggestion24 that repeated Fe59 turnover studies might be used to determine the total dose of parenteral iron (as Imferon or saccharated iron oxide) to be given in iron deficiency anemia, pointing out that in so doing the possibility of iatrogenic hemochromatosis could be avoided. The usual calculations for determining dose25 however are not only safer but more accurate.

The use of Fe59 and Cr51 entails some risk, the main hazards being hepatitis, with the use of donor plasma or cells, and the possibility of untoward effects from radiation. An estimate of the risk of hepatitis can be gained from its incidence after transfusion. The radiation hazard is more difficult to assess. Leukemia has occurred after large doses of radiation but the extent of the radiation hazard is unknown from the much smaller doses of radiation employed in the usual isotopes studies. Certainly the risk is not such as to preclude the use of isotopes to obtain information essential for diagnosis. However, when such information can be obtained by other means or when results cannot be adequately interpreted, the use of isotopes in clinical medicine appears unwarranted. In considering the use of isotopes in the doubtful case, the dose of radiation to be delivered should not be thought of as an isolated event but rather as adding to a total radiation dose, which as shown by the British survey28 may be appreciable.

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