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STRENSIQ® (Asfotase Alfa) Assay Interference

Test code(s): 234, 237, 521, 5081, 7260, 7462, 7463, 7464, 7465, 8593, 94777, 10231,10256, 14966, 17569, 18944, 29477, 30278 30740, 37073, 37226, 37227, 38075, 31348, 90378, 93927

Asfotase alfa can cause interference with certain immunoassays. Certain immunoassays incorporate alkaline phosphatase (ALP) as part of the detection system, and their results may be affected by the presence of asfotase alfa. 

Asfotase alfa is a metabolic agent used to treat perinatal/infantile-and juvenile-onset hypophosphatasia (HPP).1

The recommended dosage is 6 mg/kg/week, administered subcutaneously as  

  • 2 mg/kg 3 times per week, or
  • 1 mg/kg 6 times per week

The dose may be increased to 3 mg/kg 3 times per week in the case of insufficient efficacy in patients with perinatal/infantile-onset HPP.

Steady-state exposure is achieved as early as 3 weeks after administration of the first dose.1 The elimination half-life following subcutaneous administration is approximately 5 days.1

STRENSIQ (asfotase alfa) is indicated for the treatment of perinatal/infantile- and juvenile-onset HPP,1 an inherited systemic bone disease characterized by decreased mineralization of bone.2  The causative gene, ALPL, encodes tissue-nonspecific ALP. Defects in bone mineralization in HPP patients can manifest in several ways, including rickets and osteomalacia. Patients with severe forms of HPP may suffer from respiratory failure because of hypoplastic chest. Some patients with HPP may present with seizures.

No, but the prescribing information for STRENSIQ states the following: “Studies have shown that there is analytical interference between asfotase alfa and laboratory tests that utilize an alkaline phosphatase (ALP)-conjugated test system, rendering erroneous test results in patients treated with STRENSIQ.”3

To avoid erroneous test results, clinicians should inform laboratory personnel that the patient is being treated with asfotase alfa and discuss with a laboratory medical director the use of a testing platform that does not utilize an ALP-conjugated test system.

High serum ALP measurements detected through clinical laboratory testing are expected in patients receiving asfotase alfa and reflect circulating concentrations of this drug. Clinicians should not rely on serum ALP measurements for clinical decision-making in patients treated with asfotase alfa.

The required duration of abstention from asfotase alfa before blood draw has not been determined.

The following Quest tests are subject to interference from asfotase alfa:

For the assays in which Quest has confirmation from the test manufacturer that there is known interference, we have placed a message in our collection instruction section in the Quest Test Directory. The message is as follows: 

“Administration of STRENSIQ may interfere in certain assays and may falsely elevate values. For patients receiving STRENSIQ, consideration should be given to using alternate methods”.

Currently we are not aware of STRENSIQ (asfotase alfa) interference with other assays on our standard platforms. 

In addition to immunoassays, Quest offers many tests, such as testosterone measurement, using methods such as LC-MS. There is no evidence that asfotase alfa interferes with LC-MS assays.

Yes. In March 2020, an article on this topic was published in the JALM.3    


  1. STRENSIQ®. Prescribing information. Alexion Pharmaceuticals Inc; 2020. Accessed November 29, 2022.
  2. Orimo H. Pathophysiology of hypophosphatasia and the potential role of asfotase alfa. Ther Clin Risk Manag. 2016;12:777-786. doi:10.2147/TCRM.S87956
  3. Piec ID, Tompkins B, Fraser WD. Interference of asfotase alfa in immunoassays employing alkaline phosphatase technology. J Appl Lab Med. 2020;5(2):290-299. doi:10.1093/jalm/jfz007


This FAQ is provided for informational purposes only and is not intended as medical advice. Test selection and interpretation, diagnosis, and patient management decisions should be based on the clinician’s education, clinical expertise, and assessment of the patient.


Document FAQS.290 Version: 0

Version 0: Effective 12/09/2022 to present