COMPARABILITY OF A NEW TURBIDIMETRIC DIGOXIN TEST WITH OTHER IMMUNOCHEMICAL TESTS AND WITH HPLC - A MULTICENTER EVALUATION

A new turbidimetric inhibition immunoassay for digoxin (Tina-quant(R) a Digoxin, Boehringer Mannheim) was evaluated in seven laboratories. I t can be performed without sample pretreatment with ready-to-use reage nts on nondedicated analyzers in combination with routine clinical che mistry. The studies revealed a good analytical performance: lower limi t of detection 0.12 mu g/L, (3 SD from mean of blank); linearity up to 7.5 mu g/L; median between-run CVs 8.1% (0.6 mu g/L), 2.8% (1.5 mu g/ L), 1.9% (3 mu g/L); mean analytical recovery in control sera 98-102%; slopes from 0.97 to 1.09 and intercepts from -0.28 to 0.10 mu g/L in comparison with four immunoassays; and a high resistance to common int erferents. The test was more resistant to digoxin-like immunoreactive factor (DLIF) interference than other methods, showing cross-reactivit y only in some intensive care patient samples. Among 192 patients in w hom DLIF is expected (e.g., pregnant women, patients with renal failur e, newborns), 90% of results were less than or equal to 0.26 mu g/L di goxin. Cortisol showed no cross-reactivity and digoxigenin had a low r eactivity. An interlaboratory survey revealed a good comparability of the Tina-quant a test with the median of all methods (slope 0.99, inte rcept -0.06 mu g/L). An HPLC method for digoxin based on isocratic sep aration of samples on an RP-18 column followed by detection by an immu noassay yielded a reasonable comparability with the immunochemical tes ts with noncritical samples. Divergent results of immunoassays caused by DLIFs or different cross-reactivities with digoxin metabolites or d erivatives can be explained by the use of this HPLC method.