Neutrophil alkaline phosphatase (NAP) score in the diagnosis of hypophosphatasia

Hypophosphatasia is an inherited disease characterised by low tissue non-specific alkaline phosphatase (TNSALP) levels and skeletal defects. Diagnosis is usually made by measurement of serum total alkaline phosphatase (TALP, reference range 40-130 iu/l) and pyridoxal-5'-phosphate (PLP), and urine phosphoethanolamine (PEA). Neutrophil alkaline phosphatase (NAP) scores (reference range 20-150) have been reported to be low in isolated cases, but no comparison has been made of the diagnostic value of NAP, TALP, PEA and PLP in hypophosphatasia. We undertook such a comparison in six families with hypophosphatasia. In four families (Families 1, 2, 5, 6) with the adult type of hypophosphatasia, inherited as autosomal dominant, the NAP score and TALP (<40 iu/l), were low, <20 and <40 iu/l respectively, in all affected subjects, though the PEA and PLP were not consistently abnormal. In one of the two families (Family 3) with the autosomal recessive type of hypophosphatasia an affected subject had low NAP as well as low TALP, PLP and PEA. In another family (Family 4) one of the heterozygotes had a low NAP while the other had a normal NAP score (45). A child in this family had a normal TALP level. Her low NAP score (15) supported her to be a possible heterozygote. NAP score is readily available from most laboratories and may be diagnostically helpful in hypophosphatasia.