What test is performed to determine if vaginal fluid is amniotic fluid and how are the results interpreted?

Diagnostic Tests for Identifying Amniotic Fluid in Vaginal Secretions

The nitrazine test and ferning test are the primary diagnostic methods for determining if vaginal fluid is amniotic fluid, with newer immunoassays like placental alpha microglobulin-1 (PAMG-1) offering superior accuracy. 1

Traditional Diagnostic Tests

Nitrazine Test

• Nitrazine paper changes color when exposed to the alkaline pH of amniotic fluid (pH 7.0-7.5) compared to the normally acidic vaginal pH (4.5-6.0) 2
• Sensitivity ranges from 86.7% to 97%, with specificity between 81.3% to 87% 2, 1
• The test is positive throughout gestation (12-41 weeks) when genuine amniotic fluid is present 2
• Results are immediate and require minimal equipment 2

Ferning Test (Crystallization Test)

• Based on the microscopic detection of amniotic fluid arborization (ferning pattern) when dried on a slide 3
• Sensitivity ranges from 84% to 100%, with specificity between 78.7% to 84% 2, 1
• Optimal results require:
  • Complete air-drying of the specimen for 10 minutes 2, 4
  • Examination under microscope for characteristic fern-leaf crystallization pattern 3
• Blood contamination can inhibit ferning when concentration is greater than 1:10, requiring centrifugation to remove cellular debris 3, 4
• Amniotic fluid crystallizes on the entire slide, whereas cervical mucus (which can also fern) crystallizes in a linear fashion 3

Advanced Diagnostic Methods

Placental Alpha Microglobulin-1 (PAMG-1) Test

• Detects a fetal-specific protein in vaginal secretions when amniotic fluid is present 1
• Superior performance compared to traditional tests:
  • Sensitivity: 97.3%
  • Specificity: 98.7%
  • Positive predictive value: 98.6%
  • Negative predictive value: 97.4%
  • Overall accuracy: 98% 1

Alpha-Fetoprotein (AFP) and Acetylcholinesterase (AChE) Testing

• Primarily used for amniotic fluid analysis in amniocentesis samples rather than vaginal fluid 5
• AFP elevations (≥2.0 MoM) can indicate neural tube defects but are not typically used for diagnosing membrane rupture 5
• AChE testing is performed alongside AFP for specific clinical scenarios but not routinely for rupture of membranes 5

Interpretation of Results

Positive Diagnosis of Amniotic Fluid

• Confirmed when:
  • Nitrazine test shows color change indicating alkaline pH 2
  • Ferning test demonstrates characteristic fern pattern after proper drying 2, 3
  • PAMG-1 test (AmniSure) is positive 1

Potential False Results

• False positive nitrazine results may occur with:
  • Presence of blood, semen, or alkaline antiseptics
  • Bacterial vaginosis (which raises vaginal pH) 1
• False negative ferning results may occur with:
  • Insufficient drying time (less than 10 minutes) 2, 4
  • Blood contamination (can be resolved by centrifugation) 3, 4
  • Insufficient sample collection 3

Clinical Algorithm for Testing

1. Perform sterile speculum examination to visualize pooling of fluid 1
2. Collect sample from posterior fornix using sterile swab 1
3. Perform nitrazine test for immediate results 2
4. Prepare slide for ferning test and allow to dry completely (10 minutes) 2, 4
5. If results are equivocal or contradictory, consider PAMG-1 testing for definitive diagnosis 1
6. If blood contamination is suspected, centrifuge the sample before testing 3, 4

Special Considerations

• Gestational age does not significantly impact ferning detection in the second trimester 4
• For optimal ferning detection in second-trimester samples, ensure at least 10 minutes of drying time 4
• Centrifuging blood-contaminated samples increases ferning detection by approximately 15% 4