99.99% Accuracy Explained: What the Number Really Means
Published February 12, 2026 · Medically reviewed by the partner laboratory's medical director, AABB-accredited relationship-testing laboratory
Paternity test results are stated as 99.99%+ probability of paternity (inclusion) or 0% (exclusion). The 0.01% is a mathematical convention because 100% would require ruling out every human on earth — statistically impossible. In practice, 99.99% at 34 STR markers means the odds of a non-father matching by chance are less than 1 in a trillion.
Why not 100%?
To state 100%, you'd need to test every human alive against the child and rule each out. Because that's impossible, labs report the Combined Paternity Index (CPI) — usually stated as 99.99% or higher when the alleged father is included.
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Exclusion is definitive
When a man is excluded, the result is stated as 0%. This IS mathematically definitive — a single marker mismatch at multiple loci rules him out completely, regardless of population statistics.
What 99.99% really means
At 34 markers, the probability that an unrelated man matches by chance is on the order of 1 in 100 trillion. In practical terms, 99.99% is as certain as anything gets in modern science.