Fluorescence polarization immunoassay of gentamicin or netilmicin in blood spotted on filter paper

In this improved simple method for determination of aminoglycoside antibiotics in dried-blood spots on filter paper, gentamicin or netilmicin is recovered from the blood spot most effectively by incubation for 60 min in an ultrafiltration tube containing 500 microL of 0.5 mol/L Na2HPO4 buffer. The eluates from the paper are centrifuged, then transferred to an Abbott TDx cartridge for measurement of gentamicin or netilmicin by fluorescence polarization immunoassay. The dried sample on paper is stable for about eight days at ambient temperature. Intra-assay CVs for gentamicin and netilmicin are less than 8.5% and less than 6.1%, respectively. Analytical recovery of gentamicin and netilmicin from the paper exceeded 90%. This method permits simple blood collection and monitoring of the therapeutic concentration of gentamicin or netilmicin in serum, particularly that of newborn infants and small children.