Use of Dried Plasma Spots for the Quantification of Iothalamate in Clinical Studies

Background and objectives Although iothalamate clearances have been widely used to measure GFR, the need for transportation of plasma samples under refrigerated conditions obviates its use in resource-poor situations. Spots of blood or plasma dried on filter paper may provide a solution. Design, setting, participants, & measurements Using a validated HPLC technique, iothalamate in dried blood spots of different hematocrits was measured. GFR was measured over 5 hours in 10 subjects with CKD using dried plasma spots and standard methods. Results Lower hematocrit produced greater area of blood spreading and lowered the recovery of iothalamate from dried blood spots. However, the relationship between iothalamate concentrations in dried plasma spots and plasma showed a regression slope of 0.95 (95% confidence interval=0.92-0.98, P<0.001). Bland-Altman plot of paired sample points (n=116) showed a bias of 4 mu g/ml and limits of agreement of -38 to +30 mu g/ml. The relationship between GFRs using dried plasma spots and plasma methods also showed an excellent relationship (slope of 0.95, 95% confidence interval=0.82-1.17). Bland Altman plot of paired GFRs showed a bias of 2 ml/min, with limits of agreement of -6 to +10 ml/min. Precision was generally between 5% and 10%, and accuracy was within 5%. Conclusions Although dried blood spots are unsuitable for studies among those patients with very low hematocrit, dried plasma spots correct for this limitation, and this small pilot study shows that it is a reasonably reliable method for quantifying iothalamate and subsequently, determining GFR.