What test is performed to determine if vaginal fluid is amniotic fluid?

Tests for Detecting Amniotic Fluid in Vaginal Secretions

The nitrazine test and ferning test are the primary methods used to determine if vaginal fluid is amniotic fluid, with newer immunoassay tests like AmniSure (detecting placental alpha microglobulin-1) offering higher accuracy for diagnosing rupture of membranes. 1, 2

Traditional Tests

Nitrazine Test

• Involves using nitrazine paper that changes color when exposed to the alkaline pH of amniotic fluid (pH 7.0-7.5) compared to the acidic pH of normal vaginal secretions 1
• Shows 100% sensitivity for pure amniotic fluid samples and 81-87% specificity in clinical settings 1, 3
• Results are immediately available at bedside 1

Ferning Test

• Based on the characteristic crystallization pattern ("ferning") that forms when amniotic fluid dries on a microscope slide due to the interaction of proteins and electrolytes 1
• Optimal results require allowing the specimen to air-dry completely for 10 minutes 1
• Sensitivity ranges from 84-100% when performed correctly 1, 3
• Can be performed reliably throughout pregnancy from 12 to 41 weeks gestation 1

Advanced Immunoassay Tests

Placental Alpha Microglobulin-1 (PAMG-1) Test (AmniSure)

• Detects PAMG-1, a protein found in high concentrations in amniotic fluid but very low levels in blood or vaginal secretions 2
• Shows superior performance with 97.3% sensitivity and 98.7% specificity 2
• Higher accuracy (98%) compared to traditional nitrazine (84%) and ferning (81.3%) tests 2
• Less influenced by contamination with blood or other bodily fluids 2, 3

Fetal Fibronectin (fFN) Test

• Detects fetal isoform of fibronectin present in amniotic fluid 4, 3
• Shows good sensitivity (94.5%) and specificity (89.1%) but less accurate than PAMG-1 3
• May yield false-positive results in women with preterm labor, after vaginal examinations, or after sexual intercourse 3

Result Interpretation

Positive Results

• Nitrazine test: Paper turns blue-green to deep blue (pH >6.5) 1
• Ferning test: Characteristic fern-like crystallization pattern visible under microscope 1
• Immunoassay tests: Visible test line indicates presence of target protein (PAMG-1 or fFN) 2, 3

Negative Results

• Nitrazine test: Paper remains yellow (pH <6.0) 1
• Ferning test: No crystallization pattern observed 1
• Immunoassay tests: No test line visible (only control line) 2

Potential Pitfalls and Considerations

• Blood contamination can cause false-positive nitrazine results and false-negative ferning results 2
• Semen, alkaline antiseptics, and some vaginal infections can cause false-positive nitrazine results 3
• Inadequate drying time can lead to false-negative ferning results 1
• For optimal accuracy, samples should be collected before digital examination or use of lubricants 2, 3
• Home-based testing devices are available with 95.7% sensitivity and 84.5% specificity for patient self-testing 5

Clinical Algorithm for Testing

1. Collect fluid sample from posterior vaginal fornix using sterile speculum examination 2, 3
2. Perform nitrazine test for rapid initial assessment 1
3. Prepare slide for ferning test and allow to dry completely (10 minutes) 1
4. If results are equivocal or contradictory, proceed with immunoassay testing (preferably PAMG-1/AmniSure) for highest accuracy 2, 3
5. Interpret results in conjunction with clinical presentation and ultrasound findings (amniotic fluid index ≤5 cm supports diagnosis of rupture of membranes) 2, 3