Sulfadoxine assay using capillary blood samples dried on filter paper--suitable for monitoring of blood concentrations in the field.

A high performance liquid chromatographic method for determination of sulfadoxine in whole blood obtained by finger prick and dried on filter paper is presented. The technique was validated by comparing the sulfadoxine concentration of simultaneously collected capillary blood dried on filter paper and conventional venous whole blood samples. Agreement between capillary blood dried on filter paper and venous whole blood was satisfactory. Limit of determination using 100 microliter of capillary blood was found to be 25 mumol/L (7.8 micrograms/ml). Sulfadoxine was stable in the dried filter papers for at least 15 weeks at +37 degrees C. The concentration ratio between plasma and whole blood of sulfadoxine was found to be approximately 1.8. The sampling technique on filter paper offers a convenient method for overcoming many of the practical problems of blood sampling in the field.