STANDARDIZATION OF SURFACE-TENSION MEASUR EMENT IN TRACHEAL ASPIRATESOF NEWBORNS FOR DIAGNOSIS OF SURFACTANT DEFICIENCY - SECRETORY IMMUNOGLOBULIN-A AS DILUTION MARKER

The surface tension value (gamma min) of tracheal aspirate samples (TA ) from newborns depends on lung maturity and is determined by concentr ations of surfactant components (phospholipids, surfactant proteins, i ons). During tracheal aspirate collecting the aspirate is diluted with physiological saline. Information about TA dilution is necessary for a standardized surface tension measurement. The purpose of this study was to establish an ELISA for determination of secretory IgA (SIgA), t o determine the cutoff value of SIgA for surface tension measurement i n tracheal aspirates and to test SIgA as marker for tracheal aspirate dilution. Patients In group 1 (normal range determination of gamma min ) pharyngeal aspirates of 42 healthy newborns (nb) were investigated. In group 2 (determination of SIgA cutoff value) 15 TA and 8 pharyngeal aspirates of 23 pulmonary healthy nb were included. 27 TA of 19 pulmo nary healthy nb (group 3) were used for validation of SIgA cutoff valu e. In group 4 36 TA of 22 nb with respiratory distress syndrome were s tudied. Method The gamma min of group 1 were measured to establish the range of normal gamma min. The TA of group 2 were diluted stepwise an d the dependence of gamma min on SIgA concentration were depicted in a diagram. The SIgA cutoff value was estimated by these dilution curves . Below this value the TA are very diluted and a measurement of gamma min is not useful. To test the reliability of SIgA as dilution marker the gamma min and SIgA values of TA. from group 3 were determined. Aft er exclusion of TA with reduced SIgA the gamma min of group 3 and 4 we re compared. Results For the enzyme immunoassay following performance characteristics were determined: the accuracy (recovery): 95.6%. sensi tivity: 4 ng/ml, and precision (intra- and interassay coefficient of v ariation): 9.8 and 19.1%,respectively. The range of normal gamma min ( median (5th and 95th percentile)) amounts to 23.0 (14.8 and 28.7) mN/m . A SIgA cutoff value of 80 ng/ml was estimated. The gamma min from 8 of 27 TA (group 3) were above the normal range of gamma min during exa mination of the estimated SIgA cutoff value. 5 of these 8 TA had conce ntrations of SIgA below the cutoff point and could be excluded with th e help of SIgA as dilution marker. The median of gamma min was signifi cantly lower (p<0.001) in group 3 (18.3 mN/m) in comparision to the me dian (35.8 mN/m) of group 4 (nb with respiratory distress syndrome). C onclusion The performance characteristics of the SIgA enzyme immunoass ay and the tested reliability of the SIgA cutoff value demonstrate, th at a simple determination of surfactant deficiency by surface tension measurement of TA is possible using the concentration of SIgA as dilut ion marker.