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4.7: Radioisotopes

  • Page ID
    122100
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    A medical technologist performs a competitive binding radioimmunoassay (RIA) with an 125I tracer. The controls for this assay are not within established limits. While reviewing the data, the technologist notes that the raw counts per minute (cpm) for standards and controls are considerably lower (30%) than those obtained when the assay was run eight days before. The technologist is concerned that the natural decay of the 125I has caused the amount of radioactivity to fall below the acceptable limit for running the RIA.

    QUESTIONS

    1. Has the cpm of the 125I label decreased more than can be expected from natural decay?
    2. The technologist reviews the assay with a laboratory supervisor who agrees that the total counts per assay tube, 7,000 counts, is 30% lower than the counts available the previous week. When the technologist tells the supervisor the amount of radioactive decay that should have occurred, the supervisor asks the technologist to check out the accuracy and precison of the micropipettes and the scintillation counter used in the assay. How can the technologist determine the accuracy and precision of the scintillation counter and micropipettes?

    Questions to be Considered

    1. What is the half-life of a radioactive isotope?
    2. What is the decay factor for a radioactive isotope?
    3. What percent of the 125I should decay in eight days?
    4. What is the expected precision for counting 7,000 counts? What is the expected precision and accuracy for micropipettes?
    Answer
    1. The counts available have decreased more than theoretically expected. The reduction in counts should have been 8.8%, not the 30% observed. Since there cannot be a loss of radioactivity greater than expected from natural decay, there must be something wrong with the scintillation counter or with the delivery of the radioactive component of the reaction into the reaction mixture.
    2. The technologist can assess the accuracy and precision of the scintillation counter by counting a known standard that is usually available with each instrument. By comparing the cpm and the precision for the counts to what is listed, the technologist will be able to determine if the instrument is working properly.

    Answers to Questions to be Considered

    1. The half-life of a radioisotope is that time required for half of the radioactive atoms in a sample to decay into nonradioactive atoms (p 191). Thus, after this time passes, half of the radioactivity (disintegrations per minute, dpm) will remain .
    2. The decay factor is the fraction of radioactive atoms remaining after a certain amount of time has elapsed. The decay factor is related to the number of half-lives that have elapsed for an isotope.. If the number of half-lives that have elapsed is known, then the decay factor for any isotope can be determined (see p 191-192,). Table 9-1 gives the decay factors for 125I.
    3. According to Table 9-1 (p 192), 91.2% of the 125I. radioactivity should remain eight days after the initial experiment.
    4. The expected precision (as percent coefficient of variation,%CV) can be calculated from the equation found on p.198. The %CV for 7,000 counts is 1.2%. A check on the accuracy and precision of the micropipettes employed in the assay can be accomplished using gravimetric or spectrophotometric techniques (see p 19). In this case, the technologist found that the micropipette used to deliver 125I. tracer was inaccurate by 15% and very imprecise (%CV for replicate analyses of 20%). The micropipette was replaced and the assay again worked properly.

    This page titled 4.7: Radioisotopes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Lawrence Kaplan & Amadeo Pesce.

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