99% Accurate: What does it really mean?

Noninvasive Prenatal Testing (NIPT) screens for a handful of genetic conditions.  The detection rate of NIPT is high for many of these conditions, especially for Down syndrome.  Many labs boast that  their NIPT is “greater than 99% accurate”.  So what does “>99% accurate” really mean?  Let’s start first with what it does not mean:

A positive NIPT result rarely ever means that there is a greater than 99% chance that the baby has a genetic condition!

In fact, if your NIPT result is positive, the chance your baby actually has the condition is usually far less than 99%. So how do we answer the question:  what is the chance my positive result is a true positive result?  For this we need to know the Positive Predictive Value (PPV).  Learn more about PPV here.

Are the claims of 99% accuracy false advertising?  Actually, they are not false at all, but it is important to keep the magnitude of these claims in perspective.  Accuracy refers to the proportion of all test results that are correct, both positive and negative results.  Since NIPT screens for relatively rare conditions, most results will be negative indicating a low chance for the conditions screened and indeed they will be true negatives. The rarity of the conditions screened makes achieving 99% accuracy quite attainable!

Interestingly, if one were to write “negative” on a piece of paper and give it as the result to every single patient, it would be a “99% accurate” test result when testing for a condition that is present in less than 1 out of 100 people.

In this case of just calling all results negative, 99% of women would have true negative results: The given test predicted that their baby was unaffected and in fact the baby is unaffected.  Of course, unlike NIPT, which has a high detection rate, a method that assigns a “negative” result to every patient across the board is not at all precise and the detection rate of such a screen is a big fat zero. This example is used merely to illustrate the point that claims of >99% accuracy aren’t necessarily anything to be impressed by.

What is more important is the sensitivity (detection rate) and specificity (false positive rate) so that personalized predictive values can be calculated for each individual. These are the numbers that will allow an individual to determine the chances that their negative result is a true negative (negative predictive value) or that their positive result is a true positive (positive predictive value).

Because most women start off with a very small chance to have a baby with any of the conditions for which NIPT screens, the negative predictive value for NIPT is high, usually >99%: most women will have negative results indicating that the chance of a having a baby with any of the screened for conditions is low and most of these women will have true negative results since the conditions are generally rare.  On the other hand, the likelihood that a positive NIPT result is a true positive result (positive predictive value) is often to be far less than the coveted 99%.  For more information about NIPT, click here.

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