A high-sensitivity assay for amniotic fluid insulin at 14-20 weeks' gestation

Objective: To examine sensitivity, precision, and sample stability of an immunochemiluminomimetric insulin assay in 14-20 week amniotic fluid (AF) and fetal age distribution of second-trimester AF insulin concentrations. Methods: We assayed fresh specimens from 576 gravidas who had amniocentesis at 14-20 weeks' gestation because of maternal age. In a preliminary study, samples were divided into aliquots and stored at 4C and -20C for 24 hours to assess freezing effect. Some samples stored at 4C were assayed repeatedly during a 14-day period and others, stored at -20C, were assayed after a 70-day period, Results: This assay reliably measured AF insulin to a detection limit of 0.03 mu IU/mL. Insulin could be measured in all amniotic fluid samples and demonstrated a log,, Gaussian distribution, ranging from 0.24 to 7.41 mu IU/mL. Interassay coefficients of variation ranged from 4.4 to 8.9% at concentrations of 0.4-2.0 mu IU/mL. Linearity of dilution from 1.5 to 10 times was 99.2 +/- 8.6%, Spike recovery of 10 mu IU/mL was from 92-109%. Recovery after freezing to -20C for 24 hours (101%) and 70 days (97%) and after storage at 4C for 14 days (97%) demonstrated no significant loss. Conclusion: A two-site, dual monoclonal, immunochemiluminomimetric insulin assay was sufficiently sensitive and precise within the lower range of measured AP insulin concentrations to investigate clinical associations ol: 14-20 week AF insulin with maternal and fetal conditions, The insulin stability in this matrix suggests that assays can be reliable on specimens stored up to 70 days. (C) 1999 by The American College of Obstetricians and Gynecologists.