Establishment of reference values of five amniotic fluid enzymes. Analytical performances of the Hitachi 911. Application to complicated pregnancies.

Objectives: to evaluate the analytical performances of the Hitachi 911 analyzer (Roche Diagnostics, F) for gamma-glutamyl-transferase, 5'nucleotidase, amylase, aspartate-amino-transferase and total-alkaline-phosphatase assay in amniotic fluid. To establish reference intervals for these five enzymes throughout gestation. To determine their antenatal diagnostic value.

Design And Methods: amniotic fluid samples were collected between 14 and 35 weeks of gestation. Weekly numbers ranged from 31 to 92. Techniques developed for serum enzyme assays were applied to amniotic fluid. Two pools of amniotic fluid containing low and high marker levels were used. Within-day and between-day variations were calculated, together with the limits of detection and linearity. Reference ranges were established on 508 amniotic fluid samples, including 23 samples from pregnancies with chromosomal aberrations, 14 with gastrointestinal tract defects and 5 with gastroschisis.

Results: the assay technique for total-alkaline-phosphatase assay had to be adapted to amniotic fluid, but no adaptation was necessary for the other markers. The within-day CV ranged between 2.2 and 11.2% for low-level samples and from 1.1 to 3.4% for high-level samples. The between-day CV ranged from 6.3 to 13.3% for low-level samples and 1.2 to 4.7% for high-level samples. Total-alkaline-phosphatase activity fluctuated throughout gestation. Amylase levels and aspartate-amino-transferase levels increased whereas gamma-glutamyl-transferase and 5'nucleotidase levels fell until delivery. All trisomy 18 and trisomy 13 pregnancies, 65% of Down's syndrome pregnancies and all pregnancies with digestive tract defects were associated with marked changes in the level of at least one enzyme.

Conclusion: The Hitachi 911 is suited to rapid, reliable quantification of amniotic fluid enzymes with only minor adaptation. Useful reference intervals can be obtained throughout gestation. Gamma-glutamyl-transferase, 5'nucleotidase, total-alkaline-phosphatase and amylase assay can help to confirm echographic evidence of bowel disorders and thereby improve patient management especially in case of gastroschisis.