Standardization of surface tension measurement in tracheal aspirates of 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 concentrations of surfactant components (phospholipids, surfactant proteins, ions). 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), to determine the cutoff value of SIgA for surface tension measurement in 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 pulmonary healthy nb (group 3) were used for validation of SIgA cutoff value. In group 4 36 TA of 22 nb with respiratory distress syndrome were studied. 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 and 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. After exclusion of TA with reduced SIgA the gamma min of group 3 and 4 were compared. Results For the enzyme immunoassay following performance characteristics were determined: the accuracy (recovery): 95.6%. sensitivity: 4 ng/ml, and precision (intra- and interassay coefficient of variation): 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 examination of the estimated SIgA cutoff value. 5 of these 8 TA had concentrations of SIgA below the cutoff point and could be excluded with the help of SIgA as dilution marker. The median of gamma min was significantly lower (p<0.001) in group 3 (18.3 mN/m) in comparision to the median (35.8 mN/m) of group 4 (nb with respiratory distress syndrome). Conclusion The performance characteristics of the SIgA enzyme immunoassay and the tested reliability of the SIgA cutoff value demonstrate, that a simple determination of surfactant deficiency by surface tension measurement of TA is possible using the concentration of SIgA as dilution marker.