Analytical and clinical evaluation of refractive index-matched anomalous diffraction (RIMAD) for assessment of fetal lung maturation.

We have evaluated refractive index-matched anomalous defraction (RIMAD) (Dubin SB, Clin Chem 1988;34:938-43) as a potential method for assessment of fetal lung maturity (FLM). This method determines the total light scattered by the surfactant-containing lamellar bodies by subtraction of the A650 from amniotic fluid diluted in glycerol from that of amniotic fluid diluted in distilled water. It is not significantly affected by such contaminating chromogens as hemoglobin and bilirubin up to 2.0 g/L and 11.0 mg/L, respectively. However, the addition of as little as 2.5 microL of erythrocytes as whole blood resulted in significant interference. RIMADs for normal respiratory outcomes (n = 78) ranged from 0.018 to 0.471. RIMADs for respiratory distress syndrome (RDS) outcomes (n = 8) ranged from 0.004 to 0.036. Use of a RIMAD referent value of > 0.040 to indicate maturity yielded sensitivity, specificity, predictive value (PV)RDS, and PVmaturity of 100%, 96.2%, 72.2%, and 100%, respectively. The areas under the receiver-operating characteristic curves were 0.997 for the RIMAD assay, 0.993 (P = 0.3) for the TDx-FLM assay, 0.89 (P = 0.017) for the lecithin/sphingomyelin ratio, and 0.87 (P = 0.023) for the foam stability index.