It is seen from FIGURE that a solid support having affixed thereto an antibody specific for an antigen is contacted with a liquid suspected of containing the antigen, and conditions conducive to binding the antigen to the antibody are provided (Step 1). A second antibody (labeled A E b in the drawing), specific for a different determinant of the antigen and having enzyme conjugated thereto, is contacted under binding conditions to provide an antibody-antigen-antibody sandwich on the support (Ste 2). After a conventional separation of the support from the liquid, the sandwich on the support is contacted with a delta 5-3-ketosteroid substrate (Step 3). The enzyme component of the second antibody on support converts the substrate to a delta 4-3-ketosteroid, the extent of the conversion being proportional to the quantity of bound enzyme, which in turn is proportional to the quantity of antigen in the liquid. In Step 4, a tetrazolium salt is added. Any remaining delta 5 substrate reacts with the dye to give a color, but the delta 4-3-ketosteroid formed by the enzymatic reaction does not react with the dye and thus does not cause color formation. The color formed, or its rate of formation, may be measured to indicate the presence of antigen in the liquid.
The fetal adrenal cortex lacks expression of the enzyme early on, thus mineralocorticoids (. aldosterone ) and glucocorticoids (. cortisol ) cannot be synthesized. This is significant because cortisol induces type II pneumocytes of the lungs to synthesize and secrete pulmonary surfactant ; without pulmonary surfactant to reduce the alveolar surface tension , premature neonates may die of neonatal respiratory distress syndrome . If delivery is unavoidable (. because of placental abruption , or pre-eclampsia / HELLP syndrome ), then glucocorticoids (. cortisol) can be administered.