Understanding the Difference Between Accuracy of DNA Testing and Probability of Paternity

DNA testing is widely regarded as the gold standard for determining biological relationships. However, there are two critical concepts often misunderstood when discussing DNA testing for parentage: accuracy of the test and probability of paternity. While both are essential, they serve different purposes and are calculated in entirely different ways. Let’s delve into what each term means and why they’re important.

Accuracy of DNA Testing

The accuracy of DNA testing refers to the test’s ability to reliably and correctly analyse genetic markers to determine whether two individuals share a biological relationship. It is a measure of how well the laboratory performs the testing process and interprets the results.

Factors Affecting Accuracy:

• Laboratory processes: Accredited laboratories adhere to strict quality management systems and standards (e.g., ISO/IEC 17025), ensuring that results are highly reliable.
• Technology: The use of state-of-the-art genetic analysers and robust testing kits reduces the chances of errors.
• Sample quality: Poor-quality samples (e.g., degraded or contaminated DNA) can affect test performance.

Accuracy in Numbers: In a well-run lab, the accuracy of DNA testing should be extremely close to 100%. This means the likelihood of a technical error influencing the result is exceptionally low. In short, accuracy reflects the reliability of the test itself, independent of the biological relationship being tested.

Probability of Paternity

The probability of paternity (POP) is a statistical measure that expresses the likelihood that a tested man is the biological father of a child, compared to an unrelated random man from the population. It is calculated after analysing DNA test results. The result is expressed as a percentage and is often extremely high for inclusions (e.g., 99.9999%) or zero for exclusions.

How It’s Calculated: The probability of paternity is derived from:

• The genetic markers shared between the alleged father and the child.
• The frequency of these markers in the general population (population genetics data).
• The assumption of prior odds, which refers to the initial probability assigned to whether the tested man is the biological father of a child before considering the DNA evidence. In most DNA paternity testing scenarios, the prior odds are set at 50:50, reflecting equal likelihoods that the man is or is not the biological father.

Why It’s Not “Accuracy”: Probability of paternity is not a measure of test accuracy but a reflection of the likelihood that the tested individual is the parent, based on the DNA evidence. Even a perfectly accurate test could produce a POP of 0% (indicating the tested man is not the biological father) or 99.99% (indicating he almost certainly is).

Key Differences

An Analogy

Think of accuracy as the performance of a car (e.g., how reliably it starts and runs), while probability of paternity is the journey’s outcome (e.g., whether you reach your intended destination). Both are critical, but they answer entirely different questions.

Why Understanding the Difference Matters

Misinterpreting these concepts can lead to confusion, especially in sensitive cases involving legal, emotional, or familial disputes. Understanding the difference ensures:

• Confidence in the testing process.
• Clarity about the statistical conclusions drawn from the results.

When choosing a DNA testing service, always opt for an accredited laboratory that provides clear explanations of both accuracy and probability of paternity to ensure you get results you can trust.

If you have more questions about DNA testing or the science behind it, feel free to reach out on 1300 114 294 or email us at [email protected]. We’re here to help!