Nuclear Medicine
CLINICAL DECISION SUPPORT
[125I]I-HSA is not licensed in all countries. [131I]I-HSA and [99mTc]Tc-HSA are alternatives. The former results in a higher radiation dose, and the Technetium-99m label may be unstable in vivo, which might result in inaccurate quantitative results.
Following i.v. injection, radiolabelled RBC are mixed in circulating blood as in a closed compartment, and mixing is homogeneous after 5-10 min. The concentration of labelled RBC in a blood sample is, therefore, inversely proportional to the red cell mass (also called volume). Following i.v. injection, radiolabelled HSA is mixed with circulating plasma and a minor component of the extracellular fluid. Circulating radioactivity decays very slowly and linearly after 10 min of equilibration.
The direct determination of both the RBC volume and plasma volume are necessary for the diagnosis of true polycythaemia according to the International Committee on standardization in Haematology [173].
The suggested activities to administer are
The effective dose for [51Cr]Cr-RBCis 170 µSv/MBq [109].
The effective dose for [125I]I-HSAis 220 µSv/MBq [109].
The range in effective dose for [51Cr]Cr-RBCis: 0.09-0.17 mSv per procedure.
The range in effective dose for [125I]I-HSAis: 0.02-0.04 mSv per procedure
Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
The standard way of expressing results is as mL/kg BW, i.e., 30 ± 5 mL/kg for males and 25 ± 5 mL/kg for females (RBC volume), whereas plasma volume is in the range of 40 mL/kg in both genders. The ICSH93 panel committee proposed formulas that allow the calculation of the theoretical volumes according to weight, height and gender [173]. Measurements above or below 125% of the theoretical value are considered abnormal. The body haematocrit is on average smaller than the venous haematocrit (ratio 0.93). It raises in case of polycythaemia vera especially when the spleen is enlarged.
The major pitfall is with an unstable patient or inappropriate technical considerations (cross-contamination of samples, inadequate counting method, or inaccurate measurement of the venous haematocrit).
The patient must not be fasting and should be normally hydrated.
The patient must be lying down for at least 15 min before reinjection and while the samples are taken in order to ensure stabilization of body fluids.
If possible, the injection and sampling should be performed through separate veins.
At least two samples must be taken, for instance, at 10- and 20-min post-injection. In case very high venous haematocrit, delayed samples (e.g. 30-60 min) are advised to account for the slow mixing of RBC in the spleen.
An aliquot of the injected syringes must be set apart after appropriate homogenization.
Blood volume is calculated from the mean of the two samples; plasma volume is measured by extrapolation to time 0 of the two samples.
Samples of the standard aliquot and whole EDTA-anticoagulated blood are counted, and the RBC and plasma volumes, V, are calculated as the quotient of the injected activity, Q, and blood samples, [q], according to the formula V=Q/[q]. The body haematocrit is calculated as RBC Volume/ (RBC volume + plasma volume).