Asymptomatic elevation of parathyroid hormone levels by antibodies against reagent alkaline phosphatase

Context: Although immunoassay interference is a well-known phenomenon, its detection in routine clinical practice remains challenging. Most immunoassay interference can be attributed to the presence of heterophilic or anti-hormone antibodies. However, reports on immunoassay interference specifically related to parathyroid hormone (PTH) are scarce. Case description: A 77-year-old woman with hypertension, nephrotic syndrome, and high PTH levels for one year was admitted to our Surgical Department for treatment. The patient had no specific symptoms and normal calcium and alkaline phosphatase (ALP) levels but markedly elevated PTH levels. PTH was 2172 pg/mL using the Beckman Coulter system, whereas the Roche, Abbot, and Siemens systems yielded normal results. PTH concentration decreased to 63.8 pg/mL after pretreatment with polyethylene glycol 6000 and did not decrease to normal levels following pretreatment with heterophilic blocking tube-50 (HBT-50), heterophilic blocking reagent (HBR)-21, or HBR-25. When the HBR-21 concentration was increased, serum PTH decreased to 99.0 pg/mL. After treatment with scavenger bovine alkaline phosphatase (inactive), the concentration of PTH decreased to a normal value (51.3 pg/mL). Additionally, PTH (1-84) concentration was 17.6 pg/mL using LC-MS/MS. Conclusion: PTH was falsely evaluated due to anti-bovine ALP antibodies (antibodies against reagent ALP). Antibovine ALP antibodies should be considered in assays that use ALP as a signal generator.