How to Interpret DNA Paternity Test Results: A Step-by-Step Guide

Understanding Your Paternity Report

Accredited labs analyze 20–24 autosomal STR loci. Results hinge on statistical comparison of the alleged father's alleles with the child's (and mother's, if tested).

Key Metrics

• Paternity Index (PI) per locus: Likelihood ratio: probability of observed genotypes if alleged father is biological father vs. random man. PI = 0 → exclusion at that locus.
• Combined Paternity Index (CPI): Product of all locus PIs. CPI > 100 (often >1,000,000) supports paternity.
• Probability of Paternity (PoP): Derived from CPI: PoP = CPI / (CPI + 1) × 100%. ≥99% (often 99.99%+) is considered conclusive inclusion.
• Exclusion: ≥2–3 locus mismatches (accounting for mutation rate) → "Excluded as biological father" (PoP = 0%).

Additional Considerations

• Maternal testing: Strengthens CPI by identifying obligate paternal alleles.
• Mutations: Single-locus mismatch may reflect germline mutation; labs use mutation-rate-adjusted calculations.
• Multiple alleged fathers: Test all candidates; if related (e.g., brothers), extended panels or Y-STRs may be needed.
• Legal vs. peace-of-mind: Chain-of-custody collection required for legal admissibility (child support, inheritance, immigration).

Next Steps

Review results with a genetic counselor or the lab director. For legal matters, consult a family law attorney.