Isoenzyme Testing for Elevated Alkaline Phosphatase
ALP isoenzyme fractionation is a useful second-line test when GGT is unavailable or equivocal, as it can directly determine the percentage of ALP derived from liver versus bone sources. 1
When to Order Isoenzyme Testing
- Measure GGT first as the primary test to confirm hepatobiliary origin of elevated ALP—elevated GGT confirms liver source while normal GGT suggests bone disease. 1, 2
- Order ALP isoenzyme fractionation specifically when GGT results are unavailable or equivocal, as this directly quantifies the percentage contribution from liver versus bone. 1, 2
- Isoenzyme testing is particularly valuable when total ALP is elevated but the clinical picture doesn't clearly point to either hepatobiliary or bone pathology. 1
Technical Considerations and Limitations
- Bone-specific ALP measurement is less useful in chronic liver disease because it is difficult to measure accurately when liver ALP is simultaneously elevated, creating technical interference. 1
- The two major circulating ALP isoenzymes (bone and liver) are products of a single gene and differ only by posttranslational glycosylation, making them challenging to distinguish. 3
- Modern immunoassays for bone-specific ALP demonstrate 3-8% cross-reactivity with liver ALP and 16% liver cross-reactivity in some assays, which can affect accuracy. 3, 4
Clinical Applications
- For suspected bone origin: Bone-specific ALP is a sensitive marker for bone turnover and bone metastases, particularly useful in postmenopausal women, Paget's disease, osteomalacia, and primary hyperparathyroidism. 1, 3, 4
- For cholestatic liver disease: Electrophoresis can detect large molecular weight liver cell membrane fragments containing ALP activity that occur at the origin of the electrophoretogram in cholestatic conditions. 5
- Intestinal isoenzyme may be elevated in certain liver diseases such as cirrhosis, and can be detected through isoenzyme analysis. 5
Practical Algorithm
- Step 1: Measure GGT concurrently with ALP as the first-line discriminatory test. 1, 2
- Step 2: If GGT is elevated, proceed with hepatobiliary workup (ultrasound, then MRI/MRCP if needed). 1, 2
- Step 3: If GGT is normal, consider bone-specific ALP or bone imaging based on clinical symptoms. 1
- Step 4: Reserve isoenzyme fractionation for cases where GGT is borderline, unavailable, or when results don't match clinical suspicion. 1, 2
Important Caveats
- Avoid relying on bone-specific ALP alone when significant liver disease is present, as technical limitations reduce accuracy. 1
- Treatments like bisphosphonates and denosumab can alter ALP levels despite underlying bone pathology, potentially confounding interpretation. 1
- In patients with Paget's disease, bone-specific ALP correlates highly with total ALP (r² = 0.94) and shows larger decreases than total ALP after bisphosphonate treatment. 4