Comparison of the use of 6-thioguanine riboside versus 6-thioguanine as calibration standard to monitor 6-thioguanine nucleotides in red blood cells

The analytical methods reported to evaluate 6-Thioguanine nucleotides (6-TGNs) level in red blood cells (RBC) are based on the conversion of 6-TGNs to 6-Thioguanine (6-TG) using 6-thioguanine as calibration standard.

Using the LC-DAD method previously reported by Dervieux and Boulieu in 1998 to determine 6-TGNs, we evaluated the use of 6-Thioguanine Riboside (6-TGR) as standard instead of the base 6-TG for the monitoring of 6-TGNs in RBC from patients with IBD.

Our results show that 6-TGN values measured in RBC from 30 patients were significantly (p < 0,00001) higher when 6-TGR was used as calibration standard compared to 6-TG. The difference observed may be explained by the presence of ribose in the chemical structure of 6-TGR contrary to 6-TG. This difference in 6-TGN values was also observed in external quality control assay using 6-TGR versus 6-TG calibration standard.

The use of the nucleoside 6-TGR as calibrator constitutes a better reflection of the chemical reaction which occurs in RBC compared to 6-TG. This preliminary observation suggests that the choice of calibration standard to monitor 6-TGNs may have a significant impact on the values measured in patients and laboratory should be aware of this potential pitfall.