What is the S-CDT (Serum Carbohydrate-Deficient Transferrin) lab test?

Serum Carbohydrate-Deficient Transferrin (S-CDT) Lab Test

S-CDT is a biomarker used primarily to detect chronic excessive alcohol consumption, with high specificity but limited sensitivity, requiring alcohol intake of 50-80g daily for 1-2 weeks to become positive. 1

Definition and Mechanism

• S-CDT refers to specific isoforms of transferrin (the second most abundant glycosylated protein in human serum) that are deficient in sialic acid groups, primarily asialo, monosialo, and disialo transferrin 1
• Chronic alcohol consumption inhibits transferrin glycosylation, leading to increased carbohydrate-deficient isoforms that can be measured in serum 1
• After alcohol cessation, CDT levels normalize within approximately 2-3 weeks 1

Clinical Applications

• Primary use is to detect and monitor chronic excessive alcohol consumption, particularly in:

  • Patients with suspected alcohol-related liver disease 1
  • Monitoring abstinence in alcohol addiction programs 1
  • Evaluating patients prior to liver transplantation 1
  • License reapplication after driving under alcohol influence 2
  • Differentiating alcohol-induced liver disease from other causes 2

• Secondary use is in screening for congenital disorders of glycosylation (CDGs), where it serves as a common screening test to detect aberrant glycosylation 1

Testing Methodology

• Modern laboratories typically use one of these methods:

  • High-performance liquid chromatography (preferred method) 1
  • Capillary zone electrophoresis 3, 4
  • Mass spectrometry techniques:
    • Liquid chromatography-electrospray ionization 1
    • Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) 1

• Results are typically expressed as percentage of CDT per total transferrin (%CDT) to account for individual variations in transferrin levels 1

Interpretation and Clinical Significance

• Positive results indicate chronic excessive alcohol consumption (typically >50-80g ethanol daily for at least 1-2 weeks) 1, 2
• CDT has higher specificity for chronic alcohol abuse compared to other markers like GGT, AST, ALT, and MCV 1, 5
• Normal values do not exclude significant alcohol use due to limited sensitivity (25-50% in some studies) 1

Limitations and Confounding Factors

• Limited sensitivity (25-50%) compared to its high specificity 1

• False-positive results can occur in:

  • Patients with severe liver disease even without alcohol use 1
  • Congenital disorders of glycosylation 2
  • Patients treated for galactosemia 2

• False-negative results are more common in:

  • Patients with cirrhosis 1
  • Women (who generally show less elevated CDT values after alcohol intake) 2
  • Immunosuppressed patients 2

Comparison with Other Alcohol Biomarkers

• CDT is more specific but less sensitive than GGT for detecting alcohol consumption 1
• Direct alcohol markers (ethyl glucuronide, ethyl sulfate, phosphatidylethanol) have higher specificity than CDT 1
• Combining CDT with other markers (GGT, MCV) improves diagnostic accuracy 5, 2

Best Practice Recommendations

• CDT should be expressed as percent of total transferrin to improve reliability 1
• Specifically measure the disialotransferrin glycoform using high-performance liquid chromatography for best results 1
• Use in combination with other biomarkers and clinical assessment rather than as a standalone test 1, 5
• Consider that CDT is a better marker for chronic alcohol misuse than for recent high alcohol consumption alone 6