Age- and gender-specific reference intervals are pivotal to ensure appropriate interpretation of plasma alkaline phosphatase activities in the lower range Hypophosphatasia (HPP) is an inborn error of metabolism caused by loss-of-function mutations of the ALPL gene that mainly express alkaline phosphatase (ALP) in bone and liver. The clinical expression of HPP is highly variable and is classified into six different forms mainly affecting bone and tooth mineralization. The prognosis for each of these HPP forms depends upon the severity of the skeletal disease which reflects the age at presentation. The biochemical hallmark of HPP is low plasma ALP activity (hypophosphatasemia); however, HPP is often misdiagnosed because of low awareness and sometimes absence of age- and gender-specific ALP reference intervals. Children and adolescents have higher ALP levels in comparison with adults. Reliable reference intervals are pivotal for any clinical laboratory test. Harmonized age- and gender-specific plasma ALP reference intervals ought to be used to ensure appropriate interpretation of plasma ALP activities in the lower range.