Fetal Lung Maturity - Neonatal Respiratory Distress Syndrome

Indications for Testing

Preterm labor; elective delivery <39 weeks (ACOG, 2008) but not <32 weeks (nearly all test results will be immature)

Diagnosis of Fetal Lung Immaturity

Rapid postnatal tracheal instillation of surfactant to reduce risk of RDS
Delayed birth to permit in utero maturation – mothers can be given betamethasone to induce fetal pulmonary maturation

Laboratory Testing

All testing requires amniotic fluid; rapid testing performed first with continued reflex testing until mature result achieved or all testing options exhausted
- Do not test if delivery is indicated by maternal or fetal indications and cannot be delayed
- “Cascade” testing advocated by ACOG
  - Fetal Lung Maturity Test Cascade (ACOG)
Principles of FLM testing
- Utilize rapid test method first (fluorescence polarization, lamellar body counts)
  - If mature result, stop testing
  - If immature result, consider additional FLM tests
  - Negative (mature) predictive value is high for mature results, meaning RDS unlikely if one of the tests indicates lung maturity

Available FLM tests

Fluorescence polarization (surfactant/albumin [S/A] ratio)
- Test of choice for rapid, accurate results; has widespread availability (National Academy of Clinical Biochemistry)
- Sensitivity 99%, specificity 70%; high predictive value for lung maturity
- Commercially available test (TDx FLM II) reported in units of mg surfactant/g albumin
  - S/A values increase during gestation in parallel with lung maturation
- ARUP-produced test reported in units of mPol
  - Polarization values decrease during gestation in parallel with lung maturation
- Blood contamination can affect results – mature and immature results may become transitional
- Meconium contamination produces an unpredictable effect on results

RDS risk table

RDS Risk Table
	GA (wk)
FPOL	29	30	31	32	33	34	35	36	37	38	39	40
330	>1000	>1000	>1000	>1000	>1000	828	572	394	272	188	130	89.4
320	>1000	>1000	>1000	>1000	753	519	358	247	171	118	81.2	56.0
310	>1000	>1000	991	684	472	325	225	155	107	73.7	50.9	35.1
300	>1000	900	621	428	296	204	141	97.1	67.0	46.2	31.9	22.0
290	818	564	389	269	185	128	88.2	60.9	42.0	29.0	20.0	13.8
280	512	354	244	168	116	80.1	55.3	38.1	26.3	18.2	12.5	8.64
270	321	222	153	106	72.8	50.2	34.7	23.9	16.5	11.4	7.85	5.42
260	201	139	95.8	66.1	45.6	31.5	21.7	15.0	10.3	7.13	4.92	3.40
250	126	87.1	60.1	41.4	28.6	19.7	13.6	9.39	6.48	4.47	3.08	2.13
240	79.1	54.6	37.6	26.0	17.9	12.4	8.53	5.89	4.06	2.80	1.93	1.33
230	49.6	34.2	23.6	16.3	11.2	7.75	5.35	3.69	2.55	1.76	1.21	0.836
220	31.1	21.4	14.8	10.2	7.04	4.86	3.35	2.31	1.60	1.10	0.760	0.524
210	19.5	13.4	9.27	6.40	4.41	3.04	2.10	1.45	*1.00*	0.690	0.476	0.328
200	12.2	8.42	5.81	4.01	2.77	1.91	1.32	0.908	0.627	0.432	0.298	0.206
190	7.65	5.28	3.64	2.51	1.73	1.20	0.825	0.569	0.393	0.271	0.187	0.129
180	4.79	3.31	2.28	1.57	1.09	0.750	0.517	0.357	0.246	0.170	0.117	0.081
170	3.01	2.07	1.43	0.987	0.681	0.470	0.324	0.224	0.154	0.106	0.073	0.051
The odds ratio (OR) of RDS based on the gestational age and fluorescence polarization of the amniotic fluid (FPOL) result as compared to the odds of RDS at 37 weeks and a FPOL value of 210 mPol (boldface/italics number). This reference point was selected because 37 weeks is considered “term” and 210 mPol is the median FPOL result at 37 weeks. An OR greater than 1.0 is associated with an increased risk of RDS when compared to a FPOL value of 210 mPol at 37 weeks; an OR less than 1.0 indicates a decreased risk of RDS when compared to an FPOL value of 210 mPol at 37 weeks.

Lamellar body counts
- Lamellar bodies are similar in size to blood platelets and can be enumerated using automated cell counter
- Lung maturity is predicted from the number of lamellar bodies within amniotic fluid
- Sensitivity >95%, specificity of 70%; high predictive value for lung maturity
- Analyzer-specific cutoffs are required for interpretation
- Blood contamination can affect results – usually makes result less mature
- Meconium contamination can affect results – usually makes result more mature
Phosphatidylglycerol
- Rapid, qualitative test with moderate availability
- Sensitivity >95%, specificity 70%; high predictive value for lung maturity
- Blood and meconium contamination have no effect
- Detectable in amniotic fluid in late gestation
Lecithin-sphingomyelin ratio
- Labor intensive test offered by few laboratories
- Offers no advantage over fluorescence polarization
- Sensitivity >95%, specificity 70%
- Blood contamination can affect results – mature and immature results may become transitional
- Meconium contamination produces an unpredictable effect on results

See Fetal Lung Maturity Tests table for more information

Fetal Lung Maturity Tests

Fetal Lung Maturity Tests
Test	Method	Immaturity Cutoff	Maturity Cutoff	Negative (Mature) Predictive Value	Positive (Immature) Predictive Value	Effect of Blood	Effect of Meconium
Fluorescence polarization (FPOL)	Fluorescence polarization	≥290 mPol	≤260 mPol	95-100%	15-35%	High FPOL values will decrease; low FPOL values will increase Results <260 or >290 mPol are still interpretable	Unpredictable
Lamellar body count	Automated cell count	≤14,000/µL	≥60,000/µL	95-100%	15-65%	Decreases count due to trapping of lamellar bodies in clot matrix	Increases count
Phosphatidyl- glycerol (PG)	Immune agglutination	Negative	Positive	95-100%	20-50%	None	None
Lecithin/ sphingomyelin ratio	Thin layer chromatography	≤1.5	≥2.5	85-100%	20-80%	Mature results will be decreased; immature results will be increased Mature results are still interpretable	Makes L/S ratio result unreliable

Prematurity is associated with numerous complications, including neonatal respiratory distress syndrome (RDS), a cause of infant morbidity and mortality.

Epidemiology

Incidence – 20/100,000 infant deaths due to RDS
Age – more common the younger the gestational age
Sex – M>F (minimal)

Pathophysiology

Pulmonary surfactants are synthesized by type II pneumocytes and packaged into storage granules called lamellar bodies; these function to decrease alveolar surface tension
- Lecithin – detected at week 28; surges at week 36
- Phosphatidylinositol – detected at week 28; peaks at week 35
- Sphingomyelin – detected at week 28
- Phosphatidylglycerol – detected at week 36 with increases until delivery
RDS is caused by insufficient concentrations of pulmonary surfactants, resulting in collapsed alveoli (alveoli are perfused but hypoventilated)
- Leads to hypoxia, hypercapnia and respiratory acidosis
- Conditions cause vasoconstriction of pulmonary arteries and decreased pulmonary blood flow
  - Pulmonary vasoconstriction causes epithelial cell damage, allowing plasma to leak into alveoli
    - Fibrin accumulation and necrotic cells create a hyaline membrane (RDS previously called hyaline membrane disease)
Nearly always associated with preterm birth
- Risk of RDS is inversely related to gestational age at birth
  - >60% at <30 weeks
  - 20% at 34 weeks
  - <5% at >36 weeks
- Measurement of fetal lung maturity through biochemical testing of amniotic fluid helps predict risk of RDS

Clinical Presentation

Respiratory distress that occurs within the first few hours of life – almost exclusively in preterm infants
Hypoxia, hypercapnia and acidosis ensue with respiratory failure in many neonates

Prevention

All attempts should be made to prevent preterm birth
Administration of steroids in mother at least 24 hours prior to birth decreases risk of RDS
Instillation of exogenous surfactant intratracheally immediately after birth reduces risk of RDS

Tests generally appear in the order most useful for common clinical situations

Test name: Fluorescence Polarization (Fetal Lung Maturity)

ARUP #: 0020486

Methodology: Quantitative Fluorescence Polarization

Use:

Assess fetal lung maturity (FLM) to time delivery of fetus for optimal outcome

Test of choice for urgent clinical situations

Limitations:

Blood contamination can change results, but immature result will not be classified erroneously as mature

Evaluate reports of FLM with caution since no gold standard exists

Follow-up:

For immature or transitional results, consider another FLM test (cascade testing)

Test name: Lamellar Body Counts

ARUP #: 0080940

Methodology: Quantitative Automated Cell Count

Use:

Assess FLM to time delivery of fetus for optimal outcome

Limitations:

Do not apply reference values for test to other instruments without performing comparison studies

Evaluate reports of FLM with caution since no gold standard exists

Follow-up: For immature or transitional results, consider another FLM test (cascade testing)

Test name: Phosphatidylglycerol

ARUP #: 0080275

Methodology: Qualitative Immune Agglutination

Use: Assess FLM to time delivery of fetus for optimal outcome

Limitations:

If gestational age of fetus is <35 weeks, results are likely to be negative

10% of healthy term infants have no measurable PG

Not affected by presence of blood or meconium in amniotic fluid

Evaluate reports of FLM with caution since no gold standard exists

Follow-up:

For immature or transitional results, consider another FLM test (cascade testing)

Test name: Lecithin-Sphingomyelin Ratio

ARUP #: 0080200

Methodology: Quantitative Chromatography

Use:

Assess FLM to time delivery of fetus for optimal outcome

Least-utilized FLM test due to labor intensiveness

Limitations:

Blood contamination introduces plasma lipids, which increases low values and decreases high values

Requires a minimum of 4 hours for results

L/S ratio takes more time to perform and is less precise than fluorescence polarization

Follow-up:

For immature or transitional results, consider another FLM test (cascade testing)

Additional Tests Available

Test name: Fetal Fibronectin

ARUP #: 0082024

Methodology: Semi-Quantitative Solid Phase/Immunosorbent

Comments:

Assess risk of preterm delivery

Guidelines

ACOG Practice Bulletin No. 97: Fetal lung maturity. Obstet Gynecol. 2008; 112 (3) :717-726.PubMed

General

ARUP Institute for Clinical and Experimental Pathology®

Chapman JF, Ashwood ER, Feld R, Wu AH. Evaluation of two-dimensional cytometric lamellar body counts on the ADVIA 120 hematology system for estimation of fetal lung maturation. Clin Chim Acta. 2004; 340 (1-2) :85-92.PubMed

Grenache, David G., PhD. Medical Director, Special Chemistry at ARUP Laboratories; Associate Professor of Pathology (Clinical), University of Utah

Comprehensive Review: November 2011
Last Update: November 2011

Prenatal Screening and Diagnosis