Abreast of Pneumatic Maturity: Lamellar Body Count Predicts Lungs in Good Shape

SUMMARY

The ability to quickly, cost-effectively and accurately predict fetal pulmonary maturity would be a helpful adjunct in the management of problem pregnancies when either delivery or administration of glucocorticoid are management options. It would also be beneficial for timing elective term delivery when dates are uncertain and awaiting the onset of labor is impractical or contraindicated.

If the ideal test is instantaneously available at all hours, free, and certain in its predictive power, then the lamellar body count comes close to this ideal test in all criteria. In modern days of cost management it deserves a role as the primary test in a stratified testing scheme for lung maturity, and would be the only test necessary in the majority of cases.

INTRODUCTION

In the days before early and accurate sonographic pregnancy dating, and when the dictum "once a cesarean, always a cesarean" represented modern practice, one of the leading causes of prematurity and respiratory distress in the newborn was iatrogenic prematurity by repeat cesarean section. To reduce the appalling rate of neonatal morbidity and mortality from this problem, Gluck¹ introduced the lecithin sphingomyelin ratio (L/S) test on amniotic fluid to predict fetal pulmonary maturity. Used in this context, the decision point for the ratio was highly conservative. It was set high enough to virtually guarantee that it would not falsely predict lung maturity (In practice 1-2% false mature rate).

The L/S is based on the principle that surfactant is rich in phospholipid, and that mature surfactant contains high concentrations of lecithin.

Unfortunately the L/S ratio is not an easy test. It involves centrifugation, extraction, thin layer chromatography on fresh activated glass plates, development by heat charring and reading by absorbtiometry. The test is labor intensive, tedious, and slow. Reproducibility is low (± 15%), even in high quality labs.

Routine amniocentesis for fetal lung maturity became a necessary preliminary in the ritual of repeat cesarean section, neonatal and serious respiratory sequelae were largely avoided. The search continued for a better test.

It was observed that the surfactant in a mature amniotic fluid promoted the stability of a foam layer when the specimen was shaken. Standardization of kits for bedside use occurred², but the method was operator dependent and results were not as reliable as the L/S.

Other methods were proposed, such as the decrease in fluorescence polarization of amniotic fluid with maturity³. Many of these were based on the observation that the actual pulmonary surfactant was present in the amniotic fluid packaged in lamellar bodies. These tiny (1-5 um diameter) bodies are made of layer upon layer of membrane rich in surfactant protein and phospholipid. They are the secretory granule of the Type II pneumatocyte. Their presence in mature amniotic fluid was measured by light scattering (OD 650)⁴, fluorometrically and high speed centrifugation during the 1980's but no method was clearly superior to the L/S in results.

Meanwhile, other developments occurred which have reduced the need for such a task. The development of early, accurate sonographic dating became common. We recognized that at thirty-nine completed weeks of pregnancy the risk of respiratory distress is minimal, even in the presence of diabetes has occurred. The modern practice has been recognized by the American College of Obstetricians and Gynecologists.⁵

This trend away from routine amniocentesis has diminished the need for accurate testing, and cause the closure of many laboratories surviving on the LS test.

It was recognized that the presence of Phosphatydial Glycerol is a highly specific, albeit poorly sensitive, measure of fetal pulmonary maturity.⁶

On the other side, increasing the need for a reliable fetal lung maturity test, is the recent increase in the use of Betamethasone and Dexamethasone to induce fetal pulmonary maturation, and reduce the risk for respiratory distress. The ACOG - AAP consensus opinion advocating use with possible early delivery advise only when fetal pulmonary is not present. The actual risks and benefits of assessment of amniotic fluid maturity and therapy dependent on those results has not been established.

THE TEST

During the last decade there has been dramatic development of equipment that is capable of counting, and separating by size and type, cells, and subcellular particles, such as platelets. These techniques have developed to the point where extremely accurate platelet counts, even at low counts, can be made reproducibly on very small aliquots of blood.

In 1989,⁷ Duben described a simple method of running amniotic fluid lamellar body counts with a blood counter to determine the lamellar body count of the fluid, corresponding to the platelet count result. These counts have been shown to be more reproducible than the LS test, with coefficients of variation of 3 to 10% across a wide variation in counts.

Unfortunately, numerous authors have had different techniques, and methods, primarily involving centrifugation of the sample before counting. It is not clear why this step is necessary, however, it is common before calculation of LS ratio.⁸

In 1993, Greenspoon evaluated unspun amniotic fluid, and a critical determination of 46,000, and showed sensitivity and specificity that were better at predicting seven cases of respiratory distress in a sample size of 62 than absorbents A650, LS, or PG.

Other studies show comparable results with larger numbers.^{9,10,11,12,13}

It is quite clear that lamellar body count is a simple, rapid, inexpensive, and reliable predictor of pulmonary maturity, as demonstrated by several authors. It is necessary to standardize the technique in a particular institution that is used and the cut offs are determined by that technique. With undiluted samples the cut off for pulmonary maturity should be 46,000 based on Greenspoon's data.

CONCLUSION

Lamellar body count is a simple rapid inexpensive, and very reliable test of pulmonary maturity. The draw back of the method is that it needs to be standardized by institution.

References:

BIBLIOGRAPHY

1. Gluck L, Kulovich MV, Borer RC, Brenner PH, Anderson GG, Spellacy WN. Diagnosis of the respiratory distress syndrome by amniocentesis. Am J Obstet Gynecol 1971;109:440.

2. Sher G Statland BE. Assessment of fetal pulmonary maturity by the Lumadex Foam Stability Index Test. Obstet Gynecol 1983;61:444-9.

3. Tait JF, Foerder CA, Ashwood ER, Benedetti TJ. Prospective clinical evaluation of an improved fluorescence polarization assay for predicting fetal lung maturity. Clin Chem 1987;33:554-8.

4. Turner RJ, Read JA. Practical use and efficiency of amniotic fluid OD 650 as a predictor of fetal pulmonary maturity. Obstet Gynecol 1983;61:551-5.

5. American College of Obstetricians and Gynecologists: Fetal Maturity Assessment Prior To Elective Repeat Cesarean Delivery: ACOG Committee Opinion 98. Washington, DC, American College of Obstetricians and Gynecologists, 1991

6. Spellman TA, Cotton DB, Golonsky E, E. Detection frequency by thin layer chromatography of Phosphatydial Glycerol in amniotic fluid with clinically functional pulmonary surfactant. Clin Chem 1988; 34: 1976-82.

7. Duben SB: Characterization of amniotic fluid Lamellar bodies by resistive-pulse counting: Relationship to measures of fetal lung maturity. Clin Chem 1989; 35: 612-616

8. Ashwood ER, et al measuring the Number of Lamellar Body Particles in Amniotic Fluid: Obstet Gynecol 1990; 75:289.

9. Dalence CR, et al. Amniotic fluid lamellar body count: A rapid and reliable fetal lung maturity test. Obstet Gynecol 1995;86:235-9.

10. Fakhoury G, et al. Lamellar body concentrations and the prediction of fetal pulmonary maturity. Am J Obstet Gynecol 1994;170:72-6.

11. Pearlman ES et al. Utility of a rapid lamellar body count in the assessment of fetal maturity. J Clin Path 1991;95:778-780.

12. Ashwood ER, Oldroyd RG, Palmer SE. Measuring the number of lamellar body particles in amniotic fluid. Obstet Gynecol 1990;75:289.

13. Ashwood ER, Palmer SE, Taylor JS, Pingree SS. Lamellar body counts for rapid fetal lung maturity testing. Obstet Gynecol 1993;81:619-24.