Quad Screen Risk Estimate

How it is calculated

The tool starts from a built-in table of age-related risk of trisomy 21 at term, given as 1 in N for each whole year from 20 to 49. Between whole years it interpolates (decimal ages work), and for ages 15 to 19 it uses a flat 1 in 1,580. That gives the baseline risk. It converts risk to odds, multiplies by the likelihood ratio (LR) you enter, and converts back: odds = risk ÷ (1 − risk), adjusted odds = odds × LR, adjusted risk = adjusted odds ÷ (1 + adjusted odds).

Age must be from 15 up to, but not including, 50, and the LR must be above zero; otherwise there is no result. An LR above 1 raises the risk and one below 1 lowers it. The LR itself is not calculated here: it comes from your lab report. The output shows the adjusted risk as 1 in N, the baseline as 1 in N and the adjusted risk as a percentage to four decimals. This is an educational estimate, not a diagnosis, and it does not replace the lab's own report.

Common mistakes

• Entering a marker value or a MoM (such as AFP 1.2) in the LR field. The tool needs the combined likelihood ratio, not an individual marker.
• Entering the age at the test date when your report quotes the age-based risk at another age (for example the expected age at delivery). The table is a term-risk table by maternal age, so use the age your lab used.
• Treating a result like 1 in 385 as if it meant the pregnancy is or is not affected. It is only a probability and does not diagnose.
• Comparing the number with a lab's cut-off without knowing how the lab computed its own risk. Different labs use different methods and the result here will not always match.
• Using an LR from a different test (for example a first-trimester or cell-free DNA result) in a model built for the quad screen, or entering an age of 50 or more, which gives no result.