Abstract Background Historically, [ 131 I]I has been a common isotope in radionuclide therapy, with [ 177 Lu]Lu‐labelled radiopharmaceuticals now seeing a surge in use. These can include no‐carrier‐added or carrier‐added [ 177 Lu]Lu with slight impurities of [ 177m Lu]Lu with a significantly longer half‐life than [ 131 I]I. Wastewater from therapy wards can contain a mixture of these radioisotopes. In some countries, national regulations require wastewater to be stored in dedicated systems before it is discharged into the public sewage system. To fulfill legal requirements, the nuclide specific activity concentration must be verified. Purpose We evaluate a method for determining the activity concentration of [ 177m Lu]Lu /[ 177 Lu]Lu at equilibrium and [ 131 I]I in pure and mixed samples in order to prove that the determined values are reliably below the limits for release. Methods We analysed the emitted energy spectrum of 1 L samples with a wastewater counter using an energy window‐based approach by evaluating measurements from two different time points. Based on the law of decay and the time and energy‐dependent measured values, equation systems were set up to calculate the count rates for [ 131 I]I and [ 177m Lu]Lu, which were converted into activity concentration using calibration factors. Results There is strong linear correlation between the nominal and determined activity concentrations (correlation coefficients R = 0.99; coefficient of determinations R 2 = 0.99). We underestimate the actual activity concentration by a median of −1.4% for [ 177m Lu]Lu and overestimate the activity concentration for [ 131 I]I by a median of 7.1%. Conclusion We show that an undercut of the clearance levels for material release is measurable. We analyse and determine activity concentrations of mixed samples consisting of [ 131 I]I and [ 177m Lu]Lu/[ 177 Lu]Lu in equilibrium. The method is simple to implement using a conventional wastewater counter, however with a slightly increased effort, as two samples and measurements are required. The methodology can be adapted for the analysis of other nuclide mixtures.

Technical note: Quantifying radionuclide residues in hospital wastewater: A case study on [131I]I and [177mLu]Lu/[177Lu]Lu