Is there an analytical or diagnostic advantage from including trisialo transferrin into the fraction of carbohydrate-deficient transferrin? Lessons from a comparison of two commercial turbidimetric immunoassays with the carbohydrate-deficient transferrin determination by high-performance liquid chromatography.

OBJECTIVES

Carbohydrate-deficient transferrin CDT has originally been defined as the sum of isotransferrins exhibiting isoelectric point values > or = 5.7 asialo, monosialo, and disialo transferrin but may also include at least in part trisialo transferrin when measured by modern commercial immunoassays. To examine the effects of divergently defining the analyte CDT, we compared two commercial assays yielding differently composed CDT fractions with a high-performance liquid chromatography HPLC assay commonly regarded as a reference method of CDT determination.

METHODS

Relative CDT levels (CDT concentrations expressed as percent of total transferrin) were determined in 142 sera by (i) a turbidimetric immunoassay (ChronAlco I.D.) reportedly detecting asialo to disialo transferrin as CDT, (ii) an analogous assay (CDT Turbidimetric ImmunoAssay [TIA]) said to additionally include part of trisialo transferrin into the CDT fraction measured, and (iii) an anion-exchange HPLC method. Isotransferrins separated by the two commercial assays were also investigated by isoelectric focusing.

RESULTS

Data from HPLC and isoelectric focusing indicate that the ChronAlco assay detects major parts of asialo, monosialo, and disialo transferrin as CDT while the CDT TIA yields CDT as the total of asialo, monosialo, disialo, and trisialo transferrin. When relative CDT concentrations obtained by both assays were classified as either normal or elevated according to reference ranges cited by the manufacturer and then were compared to analogously classified HPLC data, there were clearly more discrepancies between corresponding results from CDT TIA and HPLC (22%) than between ChronAlco and HPLC results (9%).

CONCLUSION

Including trisialo transferrin into the CDT fraction enlarges the analytical signal and therefore slightly improves assay precision but also results in a significant number of pathologic results in samples exhibiting physiologic levels of the classical CDT components asialo to disialo transferrin. As long as the diagnostic information of the trisialo transferrin concentration is largely unknown, we strongly recommend not to include this isotransferrin into the determination of CDT.