Evaluation of an immunoturbidimetric assay of serum digoxin without sample pretreatment

We have evaluated a new turbidimetric immunoassay test (On Line, Roche) for the quantification of digoxin in human serum without sample pretreatment. The assay, automated on a Cobas-Mira clinical analyser, is based on the measurement of the changes in absorbance that occur when digoxin-coated microparticles aggregate with digoxin antibody; the aggregation reaction is partially inhibited by digoxin in the sample. The calibration curve extends up to 6.4 nmol/l, with a wide measuring range, and was stable at least for 10 days during the studied period. The intra- and inter-assay coefficients of variation were in all cases lower than 7%. The detection limit of the assay was 0.256 nmol/l and the linearity, assessed by measuring serial dilutions of poisoned patient sera, showed correlation coefficients higher than 0.998. There is no interference from bilirubin (up to 340 mumol/l) or haemoglobin (up to 7900 mg/l), although a positive interference by lipids was found, which can be avoided by previous delipidation of these sera. No digoxin-like factors were identified affecting digoxin values in a concentration higher than that of the sensitivity of the assay in samples of premature infants, pregnant women, or patients with renal or hepatic failure. Correlation coefficients between the turbidimetric assay and two immunoassays (fluorescence polarization and RIA) were 0.983 and 0.955 respectively, calculated from the assay of 102 and 98 samples. Nevertheless the values obtained by the evaluated assay were proportionally 20% higher, a fact probably related to differences in calibrators.