Gamma-Glutamyl Transpeptidase (GGT)
Gamma-glutamyl transpeptidase (GGT) serves as the "poor man's alkaline phosphatase isoenzyme" to confirm that elevated alkaline phosphatase originates from the hepatobiliary tract rather than bone or other sources. 1
Why GGT Works as a Surrogate
The key principle is straightforward: GGT is found in the liver, kidneys, intestine, prostate, and pancreas, but critically, it is NOT found in bone 1. This makes it an ideal discriminator when alkaline phosphatase is elevated.
The Clinical Algorithm
When you encounter an elevated alkaline phosphatase:
- If GGT is concomitantly elevated → The ALP originates from the liver and indicates cholestasis 1
- If GGT is normal → The ALP likely originates from bone (Paget's disease, bony metastases, fracture, or physiologic elevation in children/pregnancy) 2
Why This Matters Clinically
Alkaline phosphatase elevation is nonspecific and occurs in 4-8% of hospital admissions 3. Without determining the source, you cannot appropriately direct further workup. The combination of elevated ALP with elevated GGT confirms hepatobiliary origin with >95% accuracy when combined with clinical context 3.
Alternative Approach
ALP isoenzyme fractionation is the more definitive test but is more expensive and less readily available 4. This is why GGT earned its nickname as the "poor man's" alternative—it provides similar clinical utility at lower cost and with faster turnaround.
Important Caveats
- 5'-nucleotidase can also be used for the same purpose (elevated in liver disease, normal in bone disease) but is less commonly available 4
- In cholestatic liver diseases like primary biliary cholangitis or primary sclerosing cholangitis, baseline GGT and/or ALP fractionation should be established before starting clinical trials or monitoring, as this confirms the hepatobiliary origin of any future elevations 4
- Once hepatobiliary origin is confirmed with elevated GGT, imaging of the biliary tree (ultrasound first-line, then MRCP if needed) becomes necessary to determine whether cholestasis is extrahepatic (obstruction) or intrahepatic (parenchymal disease) 1
Common Pitfalls to Avoid
Don't assume all elevated ALP is from the liver—bone sources are common, particularly in:
- Post-menopausal women with osteoporosis 4
- Children (physiologic bone growth) 1
- Pregnant women (placental production) 1
- Patients with known bone metastases or Paget's disease 1
Always check GGT when ALP is elevated to avoid unnecessary hepatobiliary workup in patients with bone disease.