A comparison of two indices of adequacy of prenatal care utilization

In a case-control study, we compared the ability of two indices of antenatal care use, the new Adequacy of Prenatal Care Utilization (APNCU) index and the Kessner index, to predict low birthweight. In crude analyses, both indices showed a linear trend with low birthweight. After controlling for confounding, however, the APNCU index was unrelated to low birthweight. To learn whether the Kessner index added explanatory information to the APNCU index (or vice versa), we regressed the APNCU index on the Kessner index (and vice versa) and computed residuals for both indices. In logistic regression analyses, the residuals of the Kessner index added meaningful information to the APNCU index, whereas the opposite did not occur.