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Comparability of a new turbidimetric digoxin test with other immunochemical tests and with HPLC—a multicenter evaluation

¹ Klinisch-Chemisches Laboratorium, Kantonsspital Basel, Switzerland.

² Abteilung Klinische Chemie, Klinikum Universität Freiburg, Germany.

³ Abteilung Klinische Chemie, Zentrum Innere Medizin, Georg-August-Universität Göttingen, Germany.

⁴ Institut für Laboratoriumsdiagnostik, Zentralkrankenhaus, Gauting, Germany.

⁵ Institut für Klinische Chemie, Klinikum Mannheim, Germany.

⁶ Abteilung Klinische Chemie und Hämatologie, Zentrum Geburtshilfe und Gynäkologie, Universität Bonn, Germany.

⁷ Hôpital Universitaire Brugmann, Bruxelles, Belgium.

⁸ Boehringer Mannheim GmbH, Mannheim, Germany.
^a Address correspondence to this author at: Klinisch-Chemisches Laboratorium, Petersgraben 4, CH-4031 Basel, Switzerland. Fax 0041-61-265-4600; e-mail scholer{at}ubaclu.unibas.ch

A new turbidimetric inhibition immunoassay for digoxin (Tina-quant® a Digoxin, Boehringer Mannheim) was evaluated in seven laboratories. It can be performed without sample pretreatment with ready-to-use reagents on nondedicated analyzers in combination with routine clinical chemistry. The studies revealed a good analytical performance: lower limit of detection 0.12 µg/L (3 SD from mean of blank); linearity up to 7.5 µg/L; median between-run CVs 8.1% (0.6 µg/L), 2.8% (1.5 µg/L), 1.9% (3 µg/L); mean analytical recovery in control sera 98–102%; slopes from 0.97 to 1.09 and intercepts from -0.28 to 0.10 µg/L in comparison with four immunoassays; and a high resistance to common interferents. The test was more resistant to digoxin-like immunoreactive factor (DLIF) interference than other methods, showing cross-reactivity only in some intensive care patient samples. Among 192 patients in whom DLIF is expected (e.g., pregnant women, patients with renal failure, newborns), 90% of results were 0.26 µg/L digoxin. Cortisol showed no cross-reactivity and digoxigenin had a low reactivity. An interlaboratory survey revealed a good comparability of the Tina-quant a test with the median of all methods (slope 0.99, intercept -0.06 µg/L). An HPLC method for digoxin based on isocratic separation of samples on an RP-18 column followed by detection by an immunoassay yielded a reasonable comparability with the immunochemical tests with noncritical samples. Divergent results of immunoassays caused by DLIFs or different cross-reactivities with digoxin metabolites or derivatives can be explained by the use of this HPLC method.

Key Words: indexing terms: turbidimetry • immunoassay • methods comparison • liquid chromatography

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