Reference interval for total alkaline phosphatase in adults

Alkaline phosphatase (ALP) is a marker of liver and bone disorders and assessment of the total ALP concentration in plasma is a commonly requested biochemical test as part of routine care. We established reference interval (RI) for total ALP in adults across routinely used methods. Cross-sectional study in the North Denmark Region including results of plasma ALP in samples from 120 blood donors (BD) and in 183,267 routinely requested samples from patients managed in general practice (GP). The total ALP concentration was measured using an enzymatic method on Alinity, Abbott Laboratories (method 1a; no correction factor, method 1b; with a correction factor of 0.91), Atellica, Siemens Healthineers (method 2), and Cobas, Roche Diagnostics (method 3). RIs were established non-parametrically (2.5 and 97.5 percentiles) stratified by population and method after exclusion of outliers using Tukey's fences. The established RIs among BD and GP samples, respectively, were for method 1a: 45-132 IU/L and 46-131 IU/L, method 1b: 41-120 IU/L and 42-119 IU/L, method 2: 42-119 IU/L and 42-121 IU/L, and method 3: 42-118 IU/L and 42-119 IU/L. When a RI of 40-120 IU/L was applied among GP samples, altogether 0.7%/8.7% (method 1a), 1.6%/5.4% (method 1b), 1.5%/5.6% (method 2), and 1.7%/5.1% (method 3) of samples were below/above the RI. The established RIs for total ALP were largely consistent across methods when a correction factor was applied for the Alinity method. Based on these results, a uniform RI of 40-120 IU/L has been implemented in our region of Denmark.