First trimester diagnosis and screening for fetal aneuploidy

Maternal serum screening for neural tube defects and fetal aneuploidy in the second trimester has been incorporated into obstetrical practice over the past two decades. Now, as a result of several multicenter trials, first trimester screening between 11 and 14 weeks has been shown to be an effective and reliable screening test for Down syndrome and trisomy 18. Benefits of first trimester screening include earlier identification of the pregnancy at risk for fetal aneuploidy and anatomic defects, in particular, cardiac anomalies, and the option of earlier diagnosis by chorionic villus sampling, if available. This policy updates the American College of Medical Genetics policy statement entitled Second Trimester Maternal Serum Screening for Fetal Open Neural Tube Defects and Aneuploidy (2004) and complements the sections of American College of Medical Genetic’s Standards and Guidelines for Clinical Genetics Laboratories entitled “Prenatal screening for Down syndrome that includes first trimester biochemistry and/or ultrasound measurements.

Optical trapping and fluorescence detection in laminar flow streams

An optical laser trap with fluorescence excitation/emission capability has been integrated in a flow cytometric geometry for the study of microparticle confinement and off-axis fluorescence detection in laminar flow streams. Measurements of particle escape velocity, trapping efficiency, and fluorescence intensity are presented for 2 μm diameter dye-tagged latex microspheres in laminar flow streams having velocities of up to 12 mm/s. Experimental results are compared with theoretical values for flow velocity and fluorescence intensity and found to be in excellent agreement. © 1995 American Institute of Physics

Radiation trapping forces on microspheres with optical tweezers

Axial trapping forces exerted on microspheres are predicted using a Gaussian beam electromagnetic field model and a ray-optics model, and compared with experimental measurements. Ray-optics predicts a maximum trapping efficiency Q= -0.14 for optically trapped polystyrene microspheres in water, compared to a measured value of -0.12 ± 0.014 for 10 μm diam microspheres. When the microspheres are composed of amorphous silica, the predicted ray-optics Q decreases to -0.11, compared to a Q = -0.034 predicted by the electromagnetic field model, and a measured value of -0.012 ± 0.001 for 1 μm diam microspheres. These results indicate that the two models have applicability in two different size regimes, and thus, are complementary