The use of amniotic fluid bubble stability, L/S ratio, and creatinine concentration in the assessment of fetal maturity

Abstract

Amniotic fluid bubble stability and creatinine concentration are analyzed in relation to the L/S ratio and clinical outcome. Bubble stability by the standard method1 showed 20.5 per cent (31/151) initial disagreement between two laboratories, 53.6 per cent (81/151) correlation with L/S ratio, and did not correlate well with patient outcome. However, a “foamy” test, i.e., foam noticeable at a distance, in both 1:1 and 1:2 dilutions, indicated an elevated L/S ratio and mature pulmonary outcome. Amniotic fluid creatinine levels showed an estimated 5 to 6 per cent risk for RDS at levels we formerly thought to be safe (≥ 2.0 mg. per 100 ml. with maternal serum ≤ 0.9 mg. per 100 ml.), and has many false-negative values. Fetal maturity can be estimated with clinical data and amniotic fluid analysis. Amniotic fluid sampling is necessary in all instances where the clinical data leave a reasonable doubt of actual gestational age and in all cases for elective preterm delivery. We recommend a sequential approach to amniotic fluid analysis. First a bubble stability test (a “foamy” at both 1:1 and 1:2 dilutions indicates pulmonary maturity). If the bubble stability test is not “foamy”, an L/S ratio. The risk of RDS associated with creatinine concentration makes this test unacceptable in any case of elective delivery; however, it may be useful as a weighing factor in emergency situations where phospholipid analysis is unavailable.