Improved determination of variant erythrocytes at the glycophorin A (GPA) locus and variant frequency in patients treated with radioiodine for thyroid cancer.

Red blood cells from individuals with the blood group MN express each form of the allelic GPA protein (GPAM and GPAN) on their cell surface. Variant cells have lost one form of the protein. Their frequency is about 10(-5) in blood from unexposed persons. The BR6 assay is currently the most widely used assay to determine variant frequency (VF) by immunolabelling and flow cytometry. The precision of the BR6 assay is mainly limited by the Poisson error because only small numbers of variant cells are detected in each assay. The BR6 assay has been improved by magnetic cell separation (MACS) of variant erythrocytes prior to their determination by this assay. This new version of the assay is named 'MACS-BR6'. It allows enumeration of variant cells from 2 x 10(8) or more blood cells instead of 5 x 10(6) in the BR6 assay with a precision which is about 5 times higher than that of the BR6 assay. The MACS-BR6 assay was used to determine the VF of GPAN/0 and GPAN/N variant cells in 12 healthy adults and 11 patients treated with radioiodine for thyroid cancer 2 to 16 years before. The average red bone marrow dose was 347 mGy. In healthy adults the mean VF of GPAN/0 and GPAN/N variant cells was 16.1 x 10(6) and 5.3 x 10(-6) respectively. In patients the corresponding mean VF was 25.4 x 10(6) and 11.9 x 10(-6), respectively. The patients GPAN/0 VF was significantly higher than that of controls. In patients VF increases linearly with the dose. The linear regression parameters of VF were 16.6 x 10(-6) (intercept), 23.7 x 10(-6) GY-1 (slope) and 6.3 x 10(-6) (intercept), 12.9 x 10(-6) Gy-1 (slope) for GPAN/0 and GPAN/N variant cells, respectively.