Background: The purpose of this study was to investigate the degree of DeltaOD450 method variability and its effect on DeltaOD450 measurements between selected clinical laboratories in the U.S.
Method: Four amniotic fluid specimens were sent to 7 clinical laboratories in the U.S. for DeltaOD450 analysis. In addition, scanning spectrophotometric data from 152 amniotic fluid samples were used to compare DeltaOD450 values calculated using both log and linear OD scales.
Results: We found that no 2 laboratories used exactly the same method and no laboratory used precisely the same method as originally described by Liley. Despite the varied methods, the DeltaOD450 measurements were remarkably similar. The exception was 1 sample that was subjected to chloroform extraction. The DeltaOD450 measurement on this sample was 82% lower than the mean. On average, DeltaOD450 results determined from a linear OD scale were 37% lower than those determined from a log scale.
Conclusion: Although there is no standard method for performing the DeltaOD450, inter-laboratory variation of DeltaOD450 results is remarkably small. As our and other previous studies have reported, these data suggest that both chloroform extraction and use of a linear scale have the potential to result in lower DeltaOD450 results.