A diagnostic index for Down syndrome.

A simple and effective index for the diagnosis of Down syndrome is presented. It makes use of 12 characters with from two to nine states. The simple format allows persons with limited knowledge of the characters to use it. Simplicity was attained by combining character states when this did not result in a loss of discrimination and by avoiding characters that were redundant, subject or difficult to evaluate. A combination of characters that occur frequently in Down syndrome (wide applicability and characters that have a high relative frequency (high probability) was used. Additional data from comparable samples were used to calculate more representative scores for some characters. Over 82% of suspected cases may be diagnosed as having or not having Down syndrome with 99.9% confidence. Conditional probabilities for the various scores are provided. Individuals whose scores are in the Down or non-Down zones have a 98.7% probability or greater of having or not having the syndrome, respectively. The index is more effective than others as measured both by the percentage of individuals with and without Down syndrome whose scores fall into distinct zones (overlap method) and by the percentage with Down syndrome who have positive scores and controls who have negative scores (single point classification).