Determining Quantitative Diagnostic Test Cut-off in G6PD Heterozygous Women for Tafenoquine Therapy

Background Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common enzyme disorder in the world. Its main function is to generate NADPH that is required for anti-oxidative pathway in the cells especially in red blood cells (RBC). G6PD deficiency is X-linked and thus subject to random X-chromosome inactivation in women giving them mosaic expression of G6PD activities in their individual cells. This phenomenon makes it difficult for diagnosis with the currently available G6PD qualitative diagnostic tests. With the rolling out of newly marketed anti-malarial drug tafenoquine which has a long half-life, screening for G6PD deficiency becomes a necessity where those with <70% G6PD activity cannot receive this drug. Thus, evidence for a quantitative cut-off for G6PD activity is needed to ensure safe drug administration.Methods RBC models were developed to analyze the effect of oxidant on RBC oxidative markers namely total glutathione (GSH) and malondialdehyde (MDA). G6PD activity was measured using quantitative assay from Trinity Biotech and was correlated with cytofluorometric assay. RBC from G6PD heterozygous women with different G6PD activities were also analyzed for comparison.Results There was a negative correlation between G6PD activity and CuCl concentration and a strong association between G6PD activities and proportion of G6PD normal RBC in CuCl-treated models and in ex vivo RBC. However, in terms of oxidative stress markers analyses, unlike the hypothesis where the lower G6PD activity, the higher MDA and the lower GSH level, our CuCl RBC model showed that in low G6PD activities (10-30%) cells, the MDA level is lower compared to the rest of the models (p<0.05). Our ex vivo model however were in line with the hypothesis, although the result was not significant (p=0.5). There was a significant difference between RBC with <60% and those with >80% G6PD activities in CuCl RBC model but not in ex vivo RBC (p=0.5). Genotyping heterozygous subjects showed G6PD Viangchan variant with 2.97 U/g Hb (33% activity) and 6.58 U/g Hb (74% activity).Conclusions The MDA and GSH analyses have pointed to the 60% G6PD activity cut-off. This provides an evidence of possible cut-off for tafenoquine administration in G6PD heterozygous women.