When are Alpha-Fetoprotein (AFP) levels checked during pregnancy?

Alpha-Fetoprotein (AFP) Screening During Pregnancy

AFP levels are optimally checked between 16 and 18 weeks of gestation for maternal serum screening, with the reliable interpretation window for amniotic fluid AFP extending from 13 to 22 weeks of gestation. 1

Timing of AFP Screening

Maternal Serum AFP (MSAFP) Screening

• Optimal timing: 16-18 weeks gestation 1
• This timing provides the best balance of detection and time for follow-up interventions
• Screening before 16 weeks may yield less reliable results
• Screening after 18 weeks reduces time available for additional diagnostic testing if needed

Amniotic Fluid AFP (AFAFP) Testing

• Can be reported in MoM (multiples of median) levels from 11-25 weeks gestation 1
• Optimal detection window for open neural tube defects: 13-22 weeks gestation 1
• Testing before 13 weeks has poorly defined diagnostic performance 1
• Results should be reported with appropriate warnings if performed outside optimal windows

Clinical Protocol for AFP Screening

Standard MSAFP Screening Protocol

1. First serum specimen evaluation (16-18 weeks)
2. Second serum specimen evaluation (if indicated by borderline results)
3. Counseling when abnormal results are found
4. Ultrasonography for abnormal results
5. Amniocentesis with AFP and AChE testing if indicated
6. Karyotyping when appropriate 1

Important Considerations for Interpretation

• AFP values must be adjusted for:
  • Gestational age (calculated in completed weeks)
  • Maternal weight
  • Maternal race
  • Presence of insulin-dependent diabetes mellitus
  • Multiple gestation 1

Technical Aspects of AFP Testing

Amniotic Fluid Testing

• Abnormal amniotic fluid AFP results are typically defined as elevations ≥2.0 MoM 1
• Amniotic fluid AFP concentrations decrease 13-15% per week from 15-22 weeks 1
• Each laboratory must establish its own normative data for accurate interpretation 1

Maternal Serum Testing

• Results are reported as multiples of the median (MoM) for specific gestational age
• Patient-specific risks should be calculated based on AFP values and other relevant clinical data 1

Limitations and Pitfalls

Common Causes of False Results

• Blood contamination: Can cause false elevations in amniotic fluid AFP 1

  • Unspun amniotic fluid samples should be submitted for analysis
  • Reflex testing for fetal hemoglobin should be performed on elevated samples

• Incorrect gestational dating: Most common cause of misleading results 1

  • Ultrasound confirmation of gestational age is essential for accurate interpretation

• Fetal calf serum contamination: Can cause falsely positive AChE results in amniotic fluid samples 1

  • Occurs when laboratory equipment has residue of culture medium

Neural Tube Defect Detection Limitations

• First trimester maternal serum AFP is not effective for neural tube defect screening
• Research shows that AFP levels are not reliably elevated in first trimester (8-13 weeks) for neural tube defects, while the same pregnancies show significant elevations at 16-18 weeks 2

Patient Education and Consent

• Patients should be fully informed about AFP screening before testing
• Written informed consent should document the patient's decision to participate 1
• Educational materials in appropriate language should be provided 1

By following these guidelines for AFP screening timing and interpretation, healthcare providers can optimize the detection of neural tube defects and other conditions while allowing sufficient time for follow-up diagnostic procedures when indicated.