Particle counting immunoassay (PACIA). I. A general method for the determination of antibodies, antigens, and haptens.

By using a device designed for counting blood cells, it is possible to measure the agglutination of polystyrene beads (0.8 mu) with accuracy and great sensitivity, the agglutination resulting in a reduction in the number of particles. The latter coated with antigen can be used for determining IgM or IgG antibodies e.g. human rheumatoid factor or rabbit anti-bovine serum albumin antibodies. Macromolecules with multiple antigenic determinants agglutinate particles carrying specific antibodies. This system has been applied for determining HPL and alpha1-fetoprotein with a threshold of sensitivity of about 10 microgram/1. However the agglutination was decreased by serum factors which led to a 10-fold loss of sensitivity. The interference of rheumatoid factor which agglutinated the particles coated with rabbit or goat immunoglobulins could be avoided by reduction of the serum to be analyzed with 5mM dithiothreitol for 5 min. Haptens, i.e. DNP-lysine and T4, were determined by their inhibitory activities toward their specific antibodies, the agglutinator being a hapten-macromolecule conjugate or the antibodies themselves.

Levels of alpha-fetoprotein in maternal blood as a screening test for fetal neural-tube defect.

A range has been established for maternal plasma-alpha-fetoprotein (A.F.P.) levels in normal pregnancies (930 women). A.F.P. levels between 10 and 40 weeks gestation were examined in 51 pregnancies associated with fetal neural-tube defect. In 96% of cases (20 anencephalus, 6 spina bifida) examined between 16 and 26 weeks of gestation A.F.P. levels were above the 95th centile of the normal range. It is suggested that measurement of A.F.P. in maternal blood should become a screening test in all pregnancies, and a scheme for the futher investigation of patients with abnormal results is described.

Maternal plasma alphafetoprotein levels in the second half of normal pregnancy: relationship to fetal weight, and maternal age and parity.

A semi-automated radioimmunoassay for plasma alphafetoprotein has been developed, suitable for routine clinical use. Several hundred samples can be assayed in a working week, with results available within 24 hours. The use of extensive quality controls ensures that good precision is maintained both within and between assays. The range of plasma concentrations of alphafetoprotein in the second half of pregnancy has been established in 100 normal subjects. The levels rise progressively to reach a peak at 32 weeks, and thereafter fall until term. No relationship between circulating alphafetoprotein levels and birth weight was observed.