PREDICTING FETAL CHROMOSOME-ANOMALIES IN THE 1ST-TRIMESTER USING PREGNANCY-ASSOCIATED PLASMA PROTEIN-A - A COMPARISON OF STATISTICAL-METHODS

The analysis of the clinical efficiency of a biochemical parameter in the prediction of chromosome anomalies is described, using a database of 475 cases including 30 abnormalities. A comparison was made of two different approaches to the statistical analysis: the use of Gaussian frequency distributions and likelihood ratios, and logistic regression . Both methods computed that for a 5% false-positive rate approximatel y 60% of anomalies are detected on the basis of maternal age and serum PAPP-A. The logistic regression analysis is appropriate where the out come variable (chromosome anomaly) is binary and the detection rates r efer to the original data only. The likelihood ratio method is used to predict the outcome in the general population. The latter method depe nds on the data or some transformation of the data fitting a known fre quency distribution (Gaussian in this case). The precision of the pred icted detection rates is limited by the small sample of abnormals (30 cases). Varying the means and standard deviations (to the limits of th eir 95% confidence intervals) of the fitted log Gaussian distributions resulted in a detection rate varying between 42% and 79% for a 5% fal se-positive rate. Thus, although the likelihood ratio method is potent ially the better method in determining the usefulness of a test in the general population, larger numbers of abnormal cases are required to stabilise the means and standard deviations of the fitted log Gaussian distributions.