Determination of lamellar body size, number density, and concentration by differential light scattering from amniotic fluid: physical significance of A650.

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Abstract
A new method for characterizing amniotic fluid is presented. The difference in optical absorbance between amniotic fluid diluted with distilled water or glycerol is measured. This differential approach is shown to determine the total light scattered by the surfactant-containing lamellar bodies and not to be affected by interfering chromogens. The lamellar body size, number density, and concentration are in turn quantified from the wavelength dependence of the absorbance difference, which is shown to be accurately predicted by light-scattering theory in the domain of "anomalous diffraction." The concentration of the lamellar bodies is demonstrated to be directly related to the absorbance difference at 650 nm. A maturity criterion, based on that generally accepted for the direct measurement of absorbance at 650 nm (that is, A650 greater than or equal to 0.15), is determined for the differential approach. Hence, measurement of the absorbance difference at 650 nm can be used to assess amniotic fluid both for its optical absorbance and its lamellar body concentration.