Measurement of glomerular filtration rate with chromium-51 ethylene diamino tetraacetic acid in the presence of gallium-67 citrate: a novel method for the solution of the problem.

Chromium-51 (⁵¹Cr)-ethylene diamino tetraacetic acid plasma clearance is commonly used in glomerular filtration rate measurements. In the presence of additional radionuclides in plasma samples, glomerular filtration rate may be seriously underestimated and a correction of the crosstalk interference may be necessary. This type of correction is problematic in the case of gallium-67 (⁶⁷Ga) mainly because of the close vicinity of its 300 keV photopeak with that of the 320 keV photopeak of ⁵¹Cr. A novel method of calculating and removing the interfering ⁶⁷Ga counts within the ⁵¹Cr counting window, based on the different decay rates of the two radionuclides, is presented, requiring two series of sample counting in the ⁵¹Cr counting window only, separated by a 3-day interval. This method was developed to solve a clinical problem and then it was validated by a controlled ⁶⁷Ga contamination of plasma samples with known counts from ⁵¹Cr-ethylene diamino tetraacetic acid.