Archived Issues

2001, Volume 12, Number 2

Fluorescent Polarization Test (FLM): the New Test for Assessing Fetal Lung Maturity

by David F. Keren, M.D.

In medical school, I learned about gold standards. Because of the compulsive nature of medical students, myself certainly included, these gold standards have stuck with me over the years. Yet, in science and medicine, as knowledge advances, the gold standards of yesterday may become the anachronisms of today.

About 20 years ago, I taught medical students about the importance of using the sheep erythrocyte rosette test to measure the T lymphocytes in the peripheral blood—today, flow cytometry with a variety of monoclonal antibodies is the gold standard, and sheep are spared the venepuncture. The acidified urine heat precipitation test described by Dr. Henry Bence Jones for detecting monoclonal free light chains has been replaced by immunofixation of urine. These examples have in common the fact that newer methods were developed that were more sensitive, more specific and had assays which were more reproducible from one laboratory to another.

The article by Dr Bak in this issue of the Warde Report clearly demonstrates that another old saw has rusted beyond usefulness. In medical school, I was taught that the lecithin/sphingomyelin (L/S) ratio was the gold standard to determine if the fetal lung maturity is sufficiently advanced to make delivery possible. The L/S ratio is a complex manual process that requires mixing the fluid with various solvents, aspirating an infranatant, evaporating the remaining solvent to dryness, chilling the sample on ice, re-dissolving the residue with acetone in the cold, centrifuging, removing the acetone, drying the pellet, re-dissolving the pellet in chloroform and spotting it onto chromatogram sheets, drying the spot in an oven, using solvent to migrate the spot on the sheet, staining the sheet with bromothymol blue dye, and reading the density in a reflectance densitometer. Can one be surprised that such a procedure has very poor laboratory to laboratory reproducibility? Indeed, the 2000 College of American Pathologists survey of laboratories performing the test reported laboratory to laboratory coefficients of variation (CV) as high as 34.4%. The same sample was interpreted as “mature” by 95.1% of laboratories.

The new fluorescent polarization test (FLM) is a standardized assay that is performed in less than half the time for the L/S ratio and provides more precise information at the medical decision point. For instance, on the same CAP sample mentioned above, the FLM had a CV of only 4.8% and was interpreted as “mature” by 99.1% of laboratories. From this information, it is easy to understand why the time has come to relegate the L/S ratio to the shelf of emeritus methods—yes it was the gold standard, but has been replaced by the FLM test.

Warde Medical Laboratory offers the FLM on a routine and STAT basis. In cases with grossly contaminated specimens, the phosphatidylglycerol (PG) method is useful, but it appears relatively late in pregnancy. Like the FLM, PG is offered on a routine and STAT basis. Because of this information, Warde Medical Laboratory no longer recommends performing the L/S ratio. Be sure to pass this new gold standard on to your residents and students.