Screening for Foetal Lung Maturity – Lamellar Body Count
Respiratory distress syndrome (RDS) is the most common cause of respiratory failure in neonates and is a major cause of death in the newborn period. It is caused by inadequate production of pulmonary surfactant by immature foetal lungs and the risk to develop RDS decreases with advancing gestational age (Fig.1).
The internal surface of the alveolus is covered with a thin coating of fluid. Water in this fluid has a high surface tension which promotes the collapse of the alveolus. Pulmonary surfactant coats the interior of the alveoli in the lungs to decrease the surface tension of water, which in turn increases lung compliance and prevents collapse of alveoli during expiration.
Insufficient surfactant causes smaller alveoli to collapse and larger alveoli to overinflate and become rigid resulting in diminished ventilation and hypoxia.
Pulmonary surfactant is composed of a mixture of 90% phospholipids, including phophotidylcholine and phosphotidylglycerol and 10% protein. Type II pneumocytes package surfactant into intracellular storage granules called lamellar bodies which are excreted into the alveolar space via exocytosis.
Once secreted, lamellar bodies become hydrated in the surface water layer and unravel to form tubular myelin(TM), a lattice-like structure composed of lipids and proteins that support the surfactant monolayer (Fig.2). Foetal breathing movements in utero expel pulmonary lamellar bodies and surfactant into the amniotic fluid.
Surfactant and lamellar bodies appear in the amniotic uid at 28 – 32 weeks gestation and increase exponentially as gestation advances.
Thus, measuring amniotic fluid lamellar bodies or surfactant phospholipid components will help with estimation of foetal lung maturity and risk of RDS during weeks 32 – 36 of gestation. By 37 weeks of gestation and later, the risk of RDS is so low that laboratory testing is not necessary as foetal lung maturity is rarely a problem.
LAMELLAR BODY COUNT PRINCIPLE
The intact lamellar body seen in Fig.3 has an average diameter of 1 – 5 microns and a volume of 1.7 – 7.3 fL, which is very similar to the size of platelets (2 – 5 microns or 5 to 7 fL). Automated haematology analysers accurately count platelets in whole blood using impedance or light refraction to determine particle size and distinguish them from other cellular components.
As a result of similar size of lamellar bodies and platelets, automated haematology analysers can quantify amniotic fluid lamellar bodies in the platelet channel to provide a lamellar body count
UNACCEPTABLE AMNIOTIC SAMPLES
Specimens that are grossly contaminated with meconium, mucus and/or blood.
Szallasi, A Gronowski, AM. Eby, CS Lamellar body count in amniotic uid: A comparative study of four different hematology analyzers, Clinical Chemistry 2003;49, p.994-997