Interference by endogenous glycerol in an enzymatic assay of phosphatidylglycerol in amniotic fluid

We investigated the possibility of interference by endogenous glycerol with the enzymatic measurement of phosphatidylglycerol in amniotic fluid. Phosphatidylglycerol is an important indicator of fetal lung maturity. The concentrations of glycerol and phosphatidylglycerol in amniotic fluid were measured by using a coupled enzymatic assay with and without phospholipase D (EC 3.1.4.4). The precision of the assay was acceptable (within-run CV = 1.2%, between-run CV = 4.8%). Endogenous glycerol content was demonstrated to be approximately 10-20 times that of phosphatidylglycerol. This high proportion of endogenous glycerol in amniotic fluid would preclude the accurate enzymatic determination of amniotic fluid phosphatidylglycerol unless the glycerol is first removed. Nor can the actual phosphatidylglycerol concentration be determined by subtracting the endogenous glycerol concentration from the total glycerol, which includes that glycerol derived from phosphatidylglycerol. With a usual range of 9 +/- 7 mumol/L, the error for a given phosphatidylglycerol measurement of +/- 6.6 mumol/L (+/- 2 SD) clearly is too high for this assay to be clinically useful. There was no correlation between concentration of endogenous glycerol or apparent phosphatidylglycerol in amniotic fluid and the lecithin/sphingomyelin ratio of the sample.