Spurious testosterone laboratory results in a patient taking synthetic alkaline phosphatase (asfotase alfa)

Objectives: We report a case of discordant total and free testosterone values in a patient with hypogonadism and juvenile hypophosphatasia after he initiated treatment with asfotase alfa, recombinant tissue non-specific alkaline phosphatase. Methods: Total testosterone was evaluated using immunoassay pre and post initiation of therapy with asfotase alfa, and free testosterone was evaluated using radioimmunoassay and LC-MS/MS while on asfotase alfa therapy. Results: Total testosterone measured by immunoassay was normal prior to therapy with asfotase alfa, and was low post initiation of therapy. During the same time frame, free testosterone measured using RAI and total testosterone measured using LC-MS/MS were normal on asfotase alfa therapy. This suggests assay interference with the total testosterone immunoassay. Conclusion: When laboratory results are discordant or do not match the clinical impression, the possibility of assay interference should be considered. Alternative laboratory methods free of the interference should be selected to evaluate these patients. Human genes discussed in the paper: ALPL gene, Approved name: Alkaline phosphatase, liver/bone/kidney, Synonym: Tissue non-specific alkaline phosphatase (TNSAP).