Prenatal diagnosis of cystic fibrosis using a monoclonal antibody specific for intestinal alkaline phosphatase

A monoclonal antibody (AAP-1), specific for the intestinal isoenzyme of alkaline phosphatase (ALP), has been used to develop an immunoassay for amniotic fluid samples. Values in the immunoassay correlated closely with those obtained by direct determination of phenylalanine-inhibitable ALP. A panel of 124 control second-trimester amniotic fluids and 21 fluids with a 1 in 4 risk of a cystic fibrosis fetus were examined in the immunoassay. Eight of 10 affected cases had values below an arbitrary cut-off of one third median, while all the non-affected cases were above this level. Almost identical results were obtained by enzymatic determination of phenylalanine-inhibitable ALP. However, in both systems the false positive rate (control fluids with values below one third median), was unacceptably high. It is pointed out that at present the most effective system for the prenatal diagnosis of cystic fibrosis is achieved by measuring the ratio of intestinal to total ALP in amniotic fluid supernatants. This is probably best effected by enzymatic assay in the presence of phenylalanine and homoarginine inhibition.