Amniotic Reviewer (HY)
AMNIOTIC FLUID — High Yield Exam Reviewer
QUICK FACTS
Product of fetal metabolism
Amnion = membranous sac of cuboidal cells
Most important function = protective cushion
Clinical lab use = assess fetal maturity
Cytogenetic lab use = detect genetic/cellular abnormalities
VOLUME — HIGH YIELD NUMBERS
Value | What it means |
|---|---|
60 mL | 12 weeks AOG (normal) |
800–1200 mL | 3rd trimester (normal) |
>1200 mL | Polyhydramnios |
<800 mL | Oligohydramnios |
30 mL | Max collected via amniocentesis |
2–3 mL | First portion DISCARDED |
4 Mechanisms:
Production → fetal urine + lung fluid
Absorption → fetal swallowing + intramembranous flow
Late pregnancy → fetal urine = major component; regulated by fetal swallowing
POLYHYDRAMNIOS vs. OLIGOHYDRAMNIOS
Polyhydramnios | Oligohydramnios | |
|---|---|---|
Volume | >1200 mL | <800 mL |
Cause | Failure to swallow | ↑ swallowing, urinary tract deformities, membrane leakage |
Associated with | Neural tube defects (CSF leaks into AF) | PROM, umbilical cord compression, congenital malformations |
PROM = Premature Rupture of Membranes → fatal condition, must be monitored
CHEMICAL COMPOSITION
Early pregnancy → mirrors maternal plasma (placenta is source) Late pregnancy → mirrors fetal urine
When fetal urine dominates:
↑ Creatinine, urea, uric acid
↓ Glucose, protein
Creatinine & AOG
Creatinine Level | AOG |
|---|---|
1.5–2.0 mg/dL | <36 weeks |
>2.0 mg/dL | >36 weeks |
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 | Negative |
TESTS FOR PROM
Test | Key Info |
|---|---|
Fern Test | Vaginal fluid air-dried on slide → fernlike crystals = AF (due to protein + NaCl) |
Nitrazine Test | pH >6.0 = blue = ruptured membranes; unreliable (false + with urine, blood, semen) |
pH | AF = 7.1–7.3; Vaginal = 4.5–6.0; ↑ vaginal pH = ruptured membranes |
Biochemical markers | Most reliable |
Biomarkers for PROM
Marker | Key Info |
|---|---|
PAMG-1 | 1,000–10,000x higher in AF than vaginal fluid; detected by AmniSure ROM |
IGFBP-1 | Also called PP12; detected by Actim PROM |
ROM Plus | Detects both AFP & IGFBP-1 |
COLLECTION — AMNIOCENTESIS
Type | Key Info |
|---|---|
Transabdominal | Most common; ultrasound-guided; safe after 14th week; relatively safer |
Vaginal | Greater risk of infection |
Indications: abnormal AFP, genetic disorders, ultrasound abnormalities, fetal lung maturity assessment
SPECIMEN HANDLING
Test | Handling |
|---|---|
Fetal Lung Maturity | Ice; refrigerated |
Bilirubin/HDFN | Protect from light |
Cytologic & Microbial | Room/body temp; aseptic |
Chemical testing | Centrifuge ASAP |
APPEARANCE
Color | Meaning |
|---|---|
Clear + slight turbidity | Normal (sloughed fetal cells) |
Blood-streaked | Traumatic tap, abdominal trauma, intraamniotic hemorrhage |
Yellow | Bilirubin; HDFN |
Dark green | Meconium = fetal stress |
Dark brown | Fetal death |
Kleihauer-Betke Test
Acid elution test
Used to identify blood source + determine RhoGAM dose
Fetal Hgb → resistant to acid → bright red
Adult Hgb → eluted by acid → pale
HDFN — BILIRUBIN TESTING
Bilirubin measured at OD 450 nm → spike = bilirubin present
Fluid measured at 350–550 nm; normal OD peak at 350 nm
ΔOD450 plotted on Liley graph
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; covers 14–40 weeks AOG; 4 zones; earlier hemolytic crisis prediction
Spectrophotometric Interferences
Contaminant | Effect |
|---|---|
Light (30 min) | Markedly ↓ values |
Meconium | Falsely LOW at 450 nm; unacceptable |
Blood (oxyhemoglobin) | Absorbs at 410 nm; interferes; solve with chloroform extraction |
Cells/debris | Centrifuge immediately |
NEURAL TUBE DEFECTS (NTDs)
Most common birth defect in USA
Types: Spina bifida + Anencephaly
Skin fails to close → fetal elements leak into AF
Markers
Marker | Key Info |
|---|---|
AFP | Major fetal liver protein; peaks 12–15 weeks then declines; NTD = ↑ in maternal serum AND AF; reported in MoM; >2x median = abnormal |
AChE | Follows positive AFP; more specific than AFP; invalid if maternal blood present |
FETAL LUNG MATURITY (FLM)
RDS = most frequent complication of preterm delivery
Caused by insufficient surfactant + structural lung immaturity
Surfactants → keep alveoli open by decreasing surface tension
Absence/↓ surfactant → collapsed alveoli → RDS
1. L/S RATIO ← MOST COMMON METHOD
Component | Key Info |
|---|---|
Lecithin | Major lung surfactant; low until 35th week then ↑ |
Sphingomyelin | Produced at constant rate after ~26 weeks; acts as reference |
L/S Ratio | Interpretation |
|---|---|
<1.6 | Immature (before 35th week) |
≥2.0 | Mature; safe for preterm delivery |
Method: Thin Layer Chromatography (TLC) — expensive; largely replaced
Falsely ELEVATED by blood and meconium — DO NOT process contaminated specimens
Rationale: sphingomyelin is constant → any rise in ratio = lecithin is increasing
2. PHOSPHATIDYL GLYCEROL (PG) ASSAY
Alternative to L/S ratio
Detected at 35th week AOG
Not affected by blood or meconium ← major advantage over L/S
Delayed in maternal diabetes (use even when L/S ≥2.0 if diabetic mother)
Test: Amniostat-FLM (immunologic agglutination; polyclonal anti-PG antibodies)
Negative = pulmonary immaturity; Low positive = pulmonary maturity
3. FOAM STABILITY (SHAKE) TEST
Obsolete
AF + 95% ethanol (antifoaming agent) → shaken 15 sec → sit 15 min
Continuous ring of bubbles = sufficient surfactant = FLM
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
Size similar to platelets → use platelet channels of hematology analyzers
Method | Key Info |
|---|---|
OD 650 nm | OD 0.150 = L/S ≥2.0 + PG present |
Lamellar Body Count (LBC) | Rapid, cheap, widely available, low volume needed |
LBC Interpretation
Count | Result |
|---|---|
>50,000/µL | Mature |
<15,000/µL | Immature |
15,000–50,000/µL | Indeterminate → use alternative methods |
FLM TESTS — QUICK COMPARISON
Test | Method | Advantage | Affected by Blood/Meconium? |
|---|---|---|---|
L/S Ratio | TLC | Most reliable | YES — falsely elevated |
PG Assay | Agglutination | Not affected by contaminants | NO |
Foam Stability | Mechanical | Simple | — |
Lamellar Body Count | Hematology analyzer | Rapid, cheap, easy | — |
MASTER NUMBER LIST
Number | Meaning |
|---|---|
60 mL | AF at 12 weeks |
800–1200 mL | Normal 3rd trimester |
>1200 mL | Polyhydramnios |
<800 mL | Oligohydramnios |
30 mL | Max amniocentesis collection |
2–3 mL | First portion discarded |
14th week | Safe amniocentesis cutoff |
12–15 weeks | Peak AFP production |
>2x MoM | Abnormal AFP |
24 weeks | Lamellar bodies secreted |
26 weeks | Lamellar bodies enter AF; sphingomyelin constant rate |
35th week | Lecithin ↑; PG detected |
L/S <1.6 | Immature lung |
L/S ≥2.0 | Mature lung |
Creatinine >2.0 | >36 weeks AOG |
OD 450 nm | Bilirubin (HDFN) |
OD 410 nm | Oxyhemoglobin peak |
OD 650 nm | Lamellar body analysis |
OD 0.150 @ 650 nm | = L/S ≥2.0 |
LBC >50,000/µL | Mature |
LBC <15,000/µL | Immature |
pH 7.1–7.3 | Normal AF |
pH 4.5–6.0 | Normal vaginal fluid |
pH >6.0 | Nitrazine positive = ruptured membranes |
30 minutes | Light exposure enough to ↓↓ bilirubin values |
Top 5 Most Tested Concepts:
L/S ratio ≥2.0 = mature lung; falsely elevated by blood & meconium
AFP >2x MoM = NTD; AChE is more specific
Dark green = meconium (fetal stress); dark brown = fetal death
Biochemical markers (PAMG-1, IGFBP-1) = most reliable for PROM
Bilirubin at OD 450 nm; protect from light; Liley graph zones I–III