Amniotic Reviewer
Here's a comprehensive reviewer for Amniotic Fluid (MLS 419):
AMNIOTIC FLUID — Exam Reviewer
OVERVIEW
Product of fetal metabolism
Analysis: cytogenetic labs (genetic/cellular abnormalities) and clinical labs (fetal maturity)
Located within the amnion — a membranous sac of cuboidal cells surrounding the fetus
FUNCTIONS
Protective cushion ← MOST IMPORTANT
Allows fetal movement
Stabilizes temperature
Permits proper lung development
VOLUME REGULATION — 4 Mechanisms
Production: fetal urine (major in late gestation) + lung fluid Absorption: fetal swallowing + intramembranous flow
Normal volumes:
12 weeks AOG → 60 mL (35 mL from maternal circulation, replaced by fetal urine)
3rd trimester → 800–1200 mL
Decreases before delivery (due to fetal swallowing)
Abnormal volumes:
Condition | Volume | Cause | Associated With |
|---|---|---|---|
Polyhydramnios | >1200 mL | Failure to swallow | Neural tube defects (CSF leaks in) |
Oligohydramnios | <800 mL | ↑ swallowing, urinary tract deformities, membrane leakage | PROM, umbilical cord compression, congenital malformations |
CHEMICAL COMPOSITION
Early pregnancy → placenta is the source (mirrors maternal blood) Late pregnancy → fetal urine replaces maternal plasma
When fetal urine dominates:
↑ Creatinine, urea, uric acid
↓ Glucose, protein
Amniotic creatinine and AOG:
1.5–2.0 mg/dL → <36 weeks AOG
2.0 mg/dL → >36 weeks AOG
NTD markers in amniotic fluid: Alpha-fetoprotein (AFP) + Acetylcholinesterase (AChE)
MATERNAL URINE vs. AMNIOTIC FLUID
Amniotic Fluid | Maternal Urine | |
|---|---|---|
Creatinine | <3.5 mg/dL | 10 mg/dL |
Urea | <30 mg/dL | 300 mg/dL |
Fern Test | Positive (fernlike crystals) | Negative |
Tests for PROM:
Fern Test — vaginal fluid spread on slide, air dried, observed for fernlike crystals (+ = amniotic fluid, due to protein + NaCl)
Nitrazine Test — screening test; 1 drop fluid + nitrazine strip; pH >6.0 (blue) = ruptured membranes; unreliable (false positives with urine, blood, semen)
pH — vaginal normal: 4.5–6.0; amniotic fluid normal: 7.1–7.3; ↑ vaginal pH = ruptured membranes
Biochemical Markers (most reliable for PROM):
Biomarker | Notes |
|---|---|
PAMG-1 (Placental alpha macroglobulin-1) | 1,000–10,000x higher in amniotic fluid than vaginal fluid |
IGFBP-1 (Insulin-like growth factor binding protein-1) | Also called placental protein 12 (PP12) |
Automated tests:
AmniSure ROM → detects PAMG-1
Actim PROM → detects IGFBP-1
ROM Plus → detects both AFP & IGFBP-1
COLLECTION
Max 30 mL collected; first 2–3 mL discarded (contaminated with maternal blood/tissue)
Method: amniocentesis (needle aspiration)
Type | Notes |
|---|---|
Transabdominal | Most common; guided by continuous ultrasound; safe after 14th week (post-1st trimester); done by doctors; RELATIVELY SAFER |
Vaginal | Greater risk of infection |
Indications for amniocentesis: abnormal maternal serum AFP, suspected genetic disorders, ultrasound abnormalities, determine fetal lung maturity
SPECIMEN HANDLING
Test | Handling |
|---|---|
Fetal Lung Maturity | Place on ice, keep refrigerated |
Bilirubin analysis (HDFN) | Protect from light at all times |
Cytologic & Microbial studies | Aseptic; room or body temperature |
Chemical Testing | Centrifuge ASAP to remove cellular elements |
APPEARANCE
Color | Significance |
|---|---|
Clear with slight turbidity | Normal (sloughed fetal cells) |
Blood-streaked | Traumatic tap, abdominal trauma, intraamniotic hemorrhage |
Yellow | Presence of bilirubin; RBC destruction (HDFN) |
Dark green | Meconium (fetal bowel movement in utero = fetal stress) |
Dark brown | Fetal death |
Blood source determination: Kleihauer-Betke Test (acid elution test)
Fetal Hgb = resistant to acid → appears bright red
Adult Hgb = eluted by acid → appears pale
Also used to determine RhoGAM dose
TESTS FOR FETAL DISTRESS
HDFN (Hemolytic Disease of Fetus & Newborn)
Fetal anemia due to RBC destruction
Bilirubin measured via OD at 450 nm (spike at 450 = bilirubin present)
Spectrophotometric interferences:
Contaminant | Effect |
|---|---|
Light exposure | ↓↓ values (as little as 30 min exposure) |
Meconium | Falsely LOW absorbance at 450 nm; unacceptable |
Blood (oxyhemoglobin) | Absorbs at 410 nm; interferes with bilirubin peak; solve with chloroform extraction |
Cells/debris | Interferes; centrifuge immediately |
Liley Graph (3 zones):
Zone | Meaning |
|---|---|
Zone I | Mild hemolysis |
Zone II | Moderate; monitor closely; early delivery or exchange transfusion |
Zone III | Severe; induce labor or intrauterine exchange transfusion |
Queenan Curve — modified Liley graph; covers 14–40 weeks AOG; 4 zones; earlier prediction of hemolytic crisis
NEURAL TUBE DEFECTS (NTDs)
Most common birth defect in the USA
Most common types: Spina bifida and Anencephaly
Skin fails to close neural tissue → fetal elements leak into amniotic fluid
Markers:
AFP (Alpha-fetoprotein) — major fetal liver protein; peaks at 12–15 weeks then declines; NTDs = ↑ AFP in maternal serum AND amniotic fluid; reported as MoM; >2x median = abnormal
AChE (Acetylcholinesterase) — follows positive AFP; more specific than AFP; invalid if maternal blood present
FETAL LUNG MATURITY (FLM)
RDS (Respiratory Distress Syndrome) = most frequent complication of preterm delivery
Caused by insufficient surfactant + structural lung immaturity
Surfactants keep alveoli open by reducing surface tension
Tests for FLM:
1. Lecithin-Sphingomyelin (L/S) Ratio — MOST COMMON
Component | Key Info |
|---|---|
Lecithin | Major lung surfactant; produced at low rate until 35th week, then increases |
Sphingomyelin | Produced at constant rate after ~26 weeks; serves as reference |
Timeline | L/S Ratio | Interpretation |
|---|---|---|
Before 35th week | <1.6 | Immature |
After 35th week | ≥2.0 | Mature; preterm delivery considered safe |
Method: Thin Layer Chromatography (TLC) — expensive; largely replaced
Falsely ELEVATED by blood and meconium contamination
2. Phosphatidyl Glycerol (PG) Assay
Alternative to L/S ratio
Detected at 35th week AOG; parallels lecithin
Delayed in maternal diabetes
Not affected by meconium or blood
Test: Amniostat-FLM (immunologic agglutination using polyclonal anti-PG antibodies)
Negative = pulmonary immaturity; Low positive = pulmonary maturity
3. Foam Stability Test (Shake Test)
Obsolete method
Amniotic fluid + 95% ethanol (antifoaming agent), shaken 15 sec, sit 15 min
Presence of continuous bubbles = sufficient surfactant
Modified version: >47 = FLM
4. Lamellar Body Analysis
Surfactants (90% phospholipids, 10% proteins) packaged into lamellar bodies
Secreted by type II pneumocytes at 24 weeks AOG
Enter amniotic fluid at 26 weeks AOG
Similar size to platelets → platelet channels of hematology analyzers can be used
Measured by OD 650 nm or lamellar body count (LBC)
OD 0.150 at 650 nm = L/S ratio ≥2.0 + presence of PG
LBC Count | Interpretation |
|---|---|
>50,000/µL | Mature fetal lung |
<15,000/µL | Immature fetal lung |
15,000–50,000/µL | Indeterminate; use alternative methods |
Advantages: rapid, low cost, widely available, low technical difficulty, small volume needed
KEY NUMBERS TO MEMORIZE
Value | Meaning |
|---|---|
60 mL | AF volume at 12 weeks |
800–1200 mL | Normal 3rd trimester volume |
>1200 mL | Polyhydramnios |
<800 mL | Oligohydramnios |
30 mL | Max volume collected via amniocentesis |
2–3 mL | First portion discarded |
14th week | Earliest safe transabdominal amniocentesis |
26th week | Sphingomyelin produced at constant rate; lamellar bodies enter AF |
35th week | Lecithin production increases; PG detected |
L/S <1.6 | Immature lung |
L/S ≥2.0 | Mature lung (safe for delivery) |
AFP >2x MoM | Abnormal; suggests NTD |
Creatinine >2.0 mg/dL | >36 weeks AOG |
OD 450 nm | Bilirubin measurement (HDFN) |
OD 650 nm | Lamellar body analysis |
OD 0.150 at 650 nm | Equivalent to L/S ≥2.0 |
LBC >50,000/µL | Mature |
LBC <15,000/µL | Immature |
pH >6.0 (Nitrazine) | Ruptured membranes |
AF pH 7.1–7.3 | Normal |
Vaginal pH 4.5–6.0 | Normal |
Good luck on your exam!