Inter-method variability in bone alkaline phosphatase measurement : clinical impact on the management of dialysis patients

BACKGROUND:

Bone-specific alkaline phosphatase (BAP) is now recommended to assess bone turnover in hemodialysis (HD) patients. However, little is known about potential variability between methods available to measure BAP.

METHODS:

We measured BAP in 76 HD patients with six different assays (Beckman-Coulter Ostase IRMA, Beckman-Coulter Ostase Access, IDS iSYS Ostase, IDS Ostase enzyme immunoassay, DiaSorin Liaison Ostase and Quidel MicroVue BAP).

RESULTS:

We observed a high correlation between all the assays ranging from 0.9948 (IDS iSYS vs. IDS EIA) to 0.9215 (DiaSorin Liaison vs. Quidel MicroVue). However, using the regression equations, the equivalent concentration of a Beckman-Coulter Access value of 10μg/L can range from 7.7 to 14.4μg/L and of 20μg/L can range from 16.9 to 27.9μg/L with other assays. According to Beckman-Coulter Access, 13%, 50% and 37% of the patients presented BAP values ≤10, between 10 and 20 and ≥20μg/L, respectively. Discrepancies are observed when other assays are used (concordance from 10 to 100%).

CONCLUSIONS:

Analytical problems leading to inter-method variation should be overcome to improve the usefulness of this marker in clinical practice. According to correlation results, recalibration of BAP assays is necessary but should not be a major issue.