During the last 16 years an increasing number of studies have indicated a new diagnostic marker of alcohol abuse, unrelated to any of the conventional markers of alcoholism. This marker, now called carbohydrate-deficient transferrin, consists mainly of one or two isoforms of transferrin that are deficient in their terminal trisaccharides. Such isoforms have so far been detected by methods based on charge, i.e., isoelectric focusing, chromatofocusing, and anion-exchange chromatography of various designs combined with immunological detection techniques. This transferrin abnormality measures an accumulated effect of alcohol consumption, appearing after regular intake of 50-80 g of ethanol/day for at least one week and normalizing slowly during abstinence (half-life = about 15 days). To summarize all studies to date, approximately ˜2500 individuals have been examined, with a total clinical sensitivity of 82% and a specificity of 97%. False-positive results have only occasionally been reported: in a few patients with severe liver disease, usually primary biliary cirrhosis and chronic active hepatitis; in patients with genetic D variants of transferrin; and in patients with (and some carriers of) a recently identified inborn error of glycoprotein metabolism. The mechanism behind the transferrin abnormality is unknown but an acetaldehyde-mediated inhibition of glycosyl transfer has been suggested. Carbohydrate-deficient transferrin may thus offer a new possibility of diagnosing alcohol-related disorders. Its measurement is little affected by other conditions and, contrary to conventional markers of alcohol abuse, is apparently largely independent of concomitant liver disease.
A new simplified and rapid method for detection and quantitation of "carbohydrate-deficient transferrin" in serum is described. The method is based on isocratic anion exchange chromatography of isotransferrins in disposable microcolumns followed by a double antibody transferrin radioimmune assay. This technique, which separates all transferrin components isoelectric above pH 5.65, showed a very good reproducibility and accuracy with a coefficient of variation between 5 and 9%. 77 alcoholic patients could be clearly separated from 80 healthy "normal consumers" and 33 total abstainers with a specificity of 100% and a sensitivity of 91%. The values were significantly correlated to the amount of alcohol consumed during the latest month, and declined in abstaining alcoholics with a mean biological half-life of 17 days. Elevated levels occasionally appeared in healthy individuals after daily consumption of 60 g of ethanol during a 10-day period. In a sample of 187 patients with nonalcohol-related conditions only 2% false-positive values were found. This method is suggested as a potential tool for detecting and monitoring alcohol abuse.
The carbohydrate-deficient glycoprotein syndromes are a recently delineated group of genetic, multisystemic diseases with major nervous system involvement. Three distinct variants have been recognized and there are probably many more. They are characterized by a deficiency of the carbohydrate moiety of secretory glycoproteins, lysosomal enzymes and probably also membranous glycoproteins. The biochemical changes are most readily observed in serum transferrin and the diagnosis is usually made by isoelectric focusing of this glycoprotein. The deficiency of sialic acid, in particular, results in a cathodal shift and hence the presence of abnormal isoforms of transferrin with higher isoelectric points than normal. The basic defects are probably in the processing and synthesis of the carbohydrate moiety of glycoproteins; there is indirect evidence for a deficiency of asparagine-N-linked oligosaccharide transfer in type I (endoplasmic reticulum defect) and for a deficiency of N-acetylglucosaminyltransferase II in type II (Golgi defect). From the large number of patients detected in only a few years, it is expected that these diseases will become as important as, for example, the lysosomal, peroxisomal or mitochondrial disorders. Their study will undoubtedly yield a wealth of new information on the function of glycoproteins and of their carbohydrate moiety.
Background: Isoforms of transferrin interfere with measurement of carbohydrate-deficient transferrin (CDT) as a marker of heavy alcohol consumption. We evaluated the rate of inaccurate CDT results by immunoassays. Methods: We studied 2360 consecutive sera (1614 individuals) submitted for CDT assay without clinical information as well as samples from 1 patient with a congenital disorder of glycosylation (CDG Ia) and from 6 healthy carriers of CDG Ia. The CDTect, %CDT-TIA, and new %CDT immunoassays were compared with HPLC (%CDT-HPLC). Transferrin isoform pattern were evaluated by isoelectric focusing (IEF). Results: Transferrin BC and CD heterozygotes were found at frequencies of ∼0.7% and ∼0.2%, respectively. Another transferrin C subtype, where di- and trisialotransferrin partly coeluted (tentatively identified as C2C3), was observed in ∼0.6%. Compared with the %CDT-HPLC method, the immunoassays often produced low results for transferrin BC and high results for transferrin CD and “C2C3”. A very high trisialotransferrin value (frequency ∼1%) often produced high CDT immunoassay results. In four of six healthy carriers of CDG Ia, a- and disialotransferrin were highly increased and the HPLC and IEF isoform patterns were indistinguishable from those in alcohol abuse. Conclusions: Rare transferrin isoform types and abnormal amounts of trisialotransferrin (total frequency ∼2–3%) may cause incorrect determination of CDT with immunoassays. The observed variants were readily identified by HPLC and IEF, which can be recommended for verification of CDT immunoassay results in doubtful cases. In healthy carriers of CDG Ia, CDT is high by all assays.
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